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+[[_ugfun_building-blocks_objects-members]]
+= Object Members
+:Notice: Licensed to the Apache Software Foundation (ASF) under one or more 
contributor license agreements. See the NOTICE file distributed with this work 
for additional information regarding copyright ownership. The ASF licenses this 
file to you under the Apache License, Version 2.0 (the "License"); you may not 
use this file except in compliance with the License. You may obtain a copy of 
the License at. http://www.apache.org/licenses/LICENSE-2.0 . Unless required by 
applicable law or agreed to in writing, software distributed under the License 
is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR  CONDITIONS OF ANY 
KIND, either express or implied. See the License for the specific language 
governing permissions and limitations under the License.
+:_basedir: ../../
+:_imagesdir: images/
+
+
+Every domain object in Apache Isis consists of (at most) three types of 
members:
+
+* properties, such as a ``Customer``'s `firstName`
+
+* collections, such as a ``Customer``'s `orders` collection of ``Order``s
+
+* actions, such as a ``Customer'``s `placeOrder(...)` method.
+
+Some domain objects -- specifically domain services and mixins -- only have 
actions.
+In the case of contributing services and mixins these actions can (depending 
upon their semantics and signatures) be represented as derived properties or 
collections on the entity/view model to which they contribute/mix-in.
+
+
+== Properties
+
+Properties follow the standard getter/setter pattern, with the return type 
being a scalar (a value object or another entity or view model).
+
+For example, with:
+
+[source,java]
+----
+public class Customer
+    private String firstName;
+    public String getFirstName() { return firstName; }
+    public void setFirstName(String firstName) { this.firstName = firstName; }
+    ...
+}
+----
+
+the framework infers the `Customer` domain entity, which in turn has a 
`firstName` string _property_.
+
+
+== Collections
+
+Collections are also represented by a getter and setter, however the return 
type is a `Collection` or subtype.
+
+For example, with:
+
+[source,java]
+----
+public class Customer
+    private SortedSet<Order> orders = new TreeSet<Order>();
+    public SortedSet<Order> getOrders() { return orders; }
+    public void setOrders(SortedSet<Order> orders) { this.orders = orders; }
+    ...
+}
+----
+
+the framework infers the `orders` _collection_.
+
+[TIP]
+====
+The most commonly used collection type is `java.util.SortedSet`; entities are 
most commonly mapped to a relational database (ie a datastore with set 
semantics) and we recommend that all entities define a natural ordering so that 
when rendered in the UI they will be ordered "meaningfully" to the end-user.
+====
+
+
+== Actions
+
+The third type of object member is actions.
+(To a first approximation), actions are all public methods that do not 
represent properties or collections.
+
+For example:
+
+[source,java]
+----
+public class Customer
+    public Customer placeOrder(Product p, int quantity) { ... }
+    ...
+}
+----
+
+corresponds to the `placeOrder` _action_.
+
+[NOTE]
+====
+The above _is_ a simplification; the Apache Isis programming model also 
recognizes a number of other supporting methods each of which has its own 
prefix such as `hide`, `disable` or `validate`.
+These can be considered as "reserved words" in Apache Isis, and do _not_ 
correspond to actions even though they have public visibility.
+====
+
+
+

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+[[_ugfun_building-blocks_types-of-domain-objects]]
+= Type of Domain Objects
+:Notice: Licensed to the Apache Software Foundation (ASF) under one or more 
contributor license agreements. See the NOTICE file distributed with this work 
for additional information regarding copyright ownership. The ASF licenses this 
file to you under the Apache License, Version 2.0 (the "License"); you may not 
use this file except in compliance with the License. You may obtain a copy of 
the License at. http://www.apache.org/licenses/LICENSE-2.0 . Unless required by 
applicable law or agreed to in writing, software distributed under the License 
is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR  CONDITIONS OF ANY 
KIND, either express or implied. See the License for the specific language 
governing permissions and limitations under the License.
+:_basedir: ../../
+:_imagesdir: images/
+
+
+
+Apache Isis supports recognises four main types of domain classes:
+
+* *domain entities* - domain objects persisted to the database using 
JDO/DataNucleus; for example `Customer`
+
+* *domain services* - generally singletons, automatically injected, and 
providing various functionality; for example `CustomerRepository`
+
+* *view models* - domain objects that are a projection of some state held by 
the database, in support a particular use case; for example `CustomerDashboard` 
(to pull together commonly accessed information about a customer).
+
+* *mixins* - allow functionality to be "contributed" in the UI by one module 
to another object, similar to traits or extension methods provided in some 
programming languages.
+This is an important capability to help keep large applications 
xref:../ugbtb/ugbtb.adoc#_ugbtb_decoupling[decoupled].
+
+From the end-user's perspective the UI displays a single domain object 
instance that has state (that is, a domain entity or a view model) per page.
+The end-user can then inspect and modify its state, and navigate to related 
objects.
+
+Domain classes are generally recognized using annotations.
+Apache Isis defines its own set of annotations, while entities are annotated 
using JDO/DataNucleus (though XML can also be used if required).
+Apache Isis recognizes some of the JDO and JAXB annotations and infers domain 
semantics from these annotations.
+Similarly, JAXB annotations are typically used for view models.
+There is a smattering of other Java/JEE annotations that are also supported, 
such as `@javax.inject.Inject` and `@javax.annotation.Nullable`.
+
+The following subsections explain this in further detail.
+
+
+include::_ugfun_building-blocks_types-of-domain-objects_domain-entities.adoc[leveloffset=+1]
+include::_ugfun_building-blocks_types-of-domain-objects_domain-services.adoc[leveloffset=+1]
+include::_ugfun_building-blocks_types-of-domain-objects_view-models.adoc[leveloffset=+1]
+include::_ugfun_building-blocks_types-of-domain-objects_mixins.adoc[leveloffset=+1]

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+[[_ugfun_building-blocks_types-of-domain-objects_domain-entities]]
+= Domain Entities
+:Notice: Licensed to the Apache Software Foundation (ASF) under one or more 
contributor license agreements. See the NOTICE file distributed with this work 
for additional information regarding copyright ownership. The ASF licenses this 
file to you under the Apache License, Version 2.0 (the "License"); you may not 
use this file except in compliance with the License. You may obtain a copy of 
the License at. http://www.apache.org/licenses/LICENSE-2.0 . Unless required by 
applicable law or agreed to in writing, software distributed under the License 
is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR  CONDITIONS OF ANY 
KIND, either express or implied. See the License for the specific language 
governing permissions and limitations under the License.
+:_basedir: ../../
+:_imagesdir: images/
+
+
+Most domain objects that the end-user interacts with are _domain entities_, 
such as `Customer`, `Order`, `Product` and so on.
+These are persistent objects and which are mapped to a database (usually 
relational), using JDO/DataNucleus annotations.
+
+Some domain entities are really aggregates, a combination of multiple objects.
+A commonly cited example of this is an `Order`, which really consists of both 
a root `Order` entity and a collection of ``OrderItem``s.
+From the end-users' perspective, when they talk of "order" they almost always 
mean the aggregate rather than just the `Order` root entity.
+
+Eric Evans' 
link:http://books.google.com/books/about/Domain_Driven_Design.html?id=hHBf4YxMnWMC[Domain
 Driven Design] has a lot to say about aggregate roots and their 
responsibilities: in particular that it is the responsibility of the aggregate 
root to maintain the invariants of its component pieces, and that roots may 
only reference other roots.
+There's good logic here: requiring only root-to-root relationships reduces the 
number of moving parts that the developer has to think about.
+
+On the other hand, this constraint can substantially complicate matters when 
mapping domain layer to the persistenec layer.
+DDD tends to de-emphasise such matters: it aims to be completely agnostic 
about the persistence layer, with the responsibilities for managing 
relationships moved (pretty much by definition) into the domain layer.
+
+As a framework though Apache Isis is less dogmatic about such things.
+Generally the domain objects are mapped to a relational database and so we can 
lean on the referential integrity capabilities of the persistence layer to 
maintain referential invariants.
+Said another way: we don't tend to require that only roots can maintain roots: 
we don't see anything wrong in an `InvoiceItem` referencing an `OrderItem`, for 
example.
+
+Nonetheless the concepts of "aggregate" and "aggregate root" are worth holding 
onto.
+You'll likely find that you'll define a repository service (discussed in more 
detail below) for each aggregate root: for example `Order` will have a 
corresponding `OrderRepository` service.
+Similarly, you may also have a factory service, for example `OrderFactory`.
+However, you are less likely to have a repository service for the parts of an 
aggregate root: the role of retrieving ``OrderItem``s should fall to the 
`Order` (typically by way of lazy loading of an "items" collection) rather than 
through an `OrderItemRepository` service.
+Again, this isn't a hard-n-fast rule, but a good rule of thumb.
+
+
+[TIP]
+====
+Details on how to actually write a domain entity (the programming model for 
domain entities) is
+xref:../ugfun/ugfun.adoc#_ugfun_programming-model_domain-entities[here].
+====
+

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+[[_ugfun_building-blocks_types-of-domain-objects_domain-services]]
+= Domain Services
+:Notice: Licensed to the Apache Software Foundation (ASF) under one or more 
contributor license agreements. See the NOTICE file distributed with this work 
for additional information regarding copyright ownership. The ASF licenses this 
file to you under the Apache License, Version 2.0 (the "License"); you may not 
use this file except in compliance with the License. You may obtain a copy of 
the License at. http://www.apache.org/licenses/LICENSE-2.0 . Unless required by 
applicable law or agreed to in writing, software distributed under the License 
is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR  CONDITIONS OF ANY 
KIND, either express or implied. See the License for the specific language 
governing permissions and limitations under the License.
+:_basedir: ../../
+:_imagesdir: images/
+
+
+_Domain services_ are (usually) singleton stateless services that provide 
additional functionality.
+Domain services consist of a set of logically grouped actions, and as such 
follow the same conventions as for entities.
+However, a service cannot have (persisted) properties, nor can it have 
(persisted) collections.
+
+A very common type of domain service is a repository, that is used to look up 
existing instances of a domain entity.
+For example, for the `Customer` entity there may be a `CustomerRepository`, 
while for `Order` entity there may be an `OrderRepository`.
+
+Similarly, entities might also have a corresponding factory service: a 
`CustomerFactory` or an `OrderFactory`; Evans' 
link:http://books.google.com/books/about/Domain_Driven_Design.html?id=hHBf4YxMnWMC[Domain
 Driven Design], draws a clear distinction is made between a factory (that 
creates object) and a repository (that is used to find existing objects).
+
+On the other hand, from an end-users' perspective the act of finding an 
existing object vs creating a new one are quite closely related.
+For this reason, in Apache Isis it's therefore quite common to have a single 
domain service that acts as both a factory and a repository (and is usually 
called just a "repository").
+
+The behaviour of these services is rendered in various ways, though the most 
obvious is as the menu actions on the top-level menu bars in the 
xref:../ugvw/ugvw.adoc#[Wicket viewer]'s UI.
+
+Domain services can also be used for a number of other purposes:
+
+* to provide additional non-UI functionality; an example being to perform an 
address geocoding lookup against the google-maps API, or to perform some 
calculation, or attach a barcode, send an email etc
+
+* to act as a subscribers to the event bus, potentially influencing events 
fired by some other module (a key technique for decoupling large applications) +
++
+This is discussed in more detail below, in the section on 
xref:../ugfun/ugfun.adoc#_ugfun_building-blocks_events[events].
+
+* to implement an 
link:https://en.wikipedia.org/wiki/Service_provider_interface[SPI] of the 
Apache Isis framework, most notably cross-cutting concerns such as security, 
command profiling, auditing and publishing.
+
+* to contribute behaviour or (derived) state to entities/view models. +
++
+
+[NOTE]
+====
+Mixins can do everything that contributed services can, and have a cleaner 
programming model.
+As such, contributed services should be considered a deprecated feature; it 
may be removed in a future release.
+====
+
+Domain objects of any type (entities, other services, view models, mixins) can 
also delegate to domain services; domain services are automatically injected 
into every other domain object.
+This injection of domain services into entities is significant: it allows 
business logic to be implemented in the domain entities, rather than have it 
"leach away" into
+supporting service layers.
+Said another way: it is the means by which Apache Isis helps you avoid the 
anaemic domain model anti-pattern.
+
+Domain services are instantiated once and once only by the framework, and are 
used to centralize any domain logic that does not logically belong in a domain 
entity or value.
+
+[TIP]
+====
+Details on how to actually write a domain service (the programming model for 
domain services) is
+xref:../ugfun/ugfun.adoc#_ugfun_programming-model_domain-services[here].
+====
+
+
+== Hexagonal Arch. + services
+
+It's worth extending the 
xref:../ugfun/ugfun.adoc#_ugfun_core-concepts_philosophy_hexagonal-architecture[Hexagonal
 Architecture] to show where domain services fit in:
+
+.The hexagonal architecture with Isis addons
+image::{_imagesdir}core-concepts/philosophy/hexagonal-architecture-addons.png[width="700px"]
+
+The (non-ASF) link:http://isisaddons.org[Isis Addons] provide SPI 
implementations of the common cross-cutting concerns.
+They also provide a number of APIs for domain objects to invoke (not shown in 
the diagram).
+You can also write your own domain services as well, for example to interface 
with some external CMS system, say.
+
+

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+[[_ugfun_building-blocks_types-of-domain-objects_mixins]]
+= Mixins
+:Notice: Licensed to the Apache Software Foundation (ASF) under one or more 
contributor license agreements. See the NOTICE file distributed with this work 
for additional information regarding copyright ownership. The ASF licenses this 
file to you under the Apache License, Version 2.0 (the "License"); you may not 
use this file except in compliance with the License. You may obtain a copy of 
the License at. http://www.apache.org/licenses/LICENSE-2.0 . Unless required by 
applicable law or agreed to in writing, software distributed under the License 
is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR  CONDITIONS OF ANY 
KIND, either express or implied. See the License for the specific language 
governing permissions and limitations under the License.
+:_basedir: ../../
+:_imagesdir: images/
+
+
+The final type of domain object is the *mixin*.
+These are similar to traits or extension methods in other programming 
languages, in that they contribute (or rather, mixin) both behaviour or 
(derived) state to entities/view models.
+
+A mixin object allows one class to contribute behaviour - actions, (derived) 
properties and (derived) collections - to another domain object, either a 
domain entity or view model.
+
+The allows the app to stay decoupled, so that it doesn't degrade into the 
proverbial link:http://www.laputan.org/mud/mud.html#BigBallOfMud["big ball of 
mud"].
+Mixins allow dependencies to be inverted, so that the dependencies between 
modules can be kept acyclic and under control.
+
+For example, the contributee (eg `Customer`, being mixed into) is in one 
module, while the contributor mixin (`DocumentHolder_documents`) is in some 
other module.
+The `customer` module knows about the `document` module, but not vice versa.
+
+Mixins are also a convenient mechanism for grouping functionality even for a 
concrete type, helping to rationalize about the dependency between the data and 
the behaviour.
+Each mixin is in effect a single behavioural "responsibility" of the domain 
object.
+
+
+There are also practical reasons for moving behaviour out of entities even 
within the same module, because structuring your application this way helps 
support hot-reloading of Java classes (so that you can modify and recompile 
your application without having to restart it).
+This can provide substantial productivity gains.
+
+The Hotspot JVM has limited support for hot reloading; generally you can 
change method implementations but you cannot introduce new methods.
+However, the link:https://dcevm.github.io/[DCEVM] open source project will 
patch the JVM to support much more complete hot reloading support.
+There are also, of course, commercial products such as JRebel.
+
+The main snag in all this is the DataNucleus enhancer... any change to 
entities is going to require the entity to be re-enhanced, and the JDO 
metamodel recreated, which invariably breaks things.
+So hot-reloading of an app whose fundamental structure is changing is likely 
to remain a no-no.
+
+However, chances are that the structure of your domain objects (the data) will 
change much less rapidly than the behaviour of those domain objects.
+Thus, it's the behaviour that you're most likely wanting to change while the 
app is still running.
+If you move that behaviour out into mixins, then these can be reloaded happily.
+(And when running in prototype mode), Apache Isis will automatically recreate 
the portion of the metamodel for any domain object as it is rendered.
+
+[TIP]
+====
+Details on how to actually write a mixin (the programming model for mixins) is
+xref:../ugfun/ugfun.adoc#_ugfun_programming-model_mixins[here].
+====
+
+
+.DCI Architecture
+****
+Mixins are an implementation of the 
link:http://www.artima.com/articles/dci_vision.html[DCI architecture] 
architecture, as formulated and described by 
link:https://en.wikipedia.org/wiki/Trygve_Reenskaug[Trygve Reenskaug] and 
link:https://en.wikipedia.org/wiki/Jim_Coplien[Jim Coplien].
+Reenskaug was the inventor of the MVC pattern (and also the external examiner 
for Richard Pawson's PhD thesis), while Coplien has a long history in 
object-orientation, C++ and patterns.
+
+DCI stands for Data-Context-Interaction and is presented as an evolution of 
object-oriented programming, but one where behaviour is bound to objects 
dynamically rather than statically in some context or other.
+The mixin pattern is Apache Isis' straightforward take on the same basic 
concept.
+
+You might also wish to check out link:http://zest.apache.org[Apache Zest] 
(formerly Qi4J), which implements a much more general purpose implementation of 
the same concepts.
+****
+
+

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+[[_ugfun_building-blocks_types-of-domain-objects_view-models]]
+= View Models
+:Notice: Licensed to the Apache Software Foundation (ASF) under one or more 
contributor license agreements. See the NOTICE file distributed with this work 
for additional information regarding copyright ownership. The ASF licenses this 
file to you under the Apache License, Version 2.0 (the "License"); you may not 
use this file except in compliance with the License. You may obtain a copy of 
the License at. http://www.apache.org/licenses/LICENSE-2.0 . Unless required by 
applicable law or agreed to in writing, software distributed under the License 
is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR  CONDITIONS OF ANY 
KIND, either express or implied. See the License for the specific language 
governing permissions and limitations under the License.
+:_basedir: ../../
+:_imagesdir: images/
+
+
+View models are similar to entities in that (unlike domain services) there can 
be many instances of any given type.
+End users interact with view models in the same way as a domain entity, indeed 
they are unlikely to distinguish one from the other.
+
+However, whereas a domain entities is mapped to a datastore, view models are 
not.
+Instead they are recreated dynamically by serializing their state, ultimately 
into the URL itself (meaning their state it is in effect implicitly managed by 
the client browser).
+You will notice that the URL for view models (as shown in 
xref:../ugvw/ugvw.adoc#[Wicket viewer] or 
xref:../ugvro/ugvro.adoc#[RestfulObjects viewer]) tends to be quite long.
+
+This capability opens up a number of more advanced use cases:
+
+* In the same way that an (RDBMS) database view can aggregate and abstract 
from multiple underlying database tables, so a view model sits on top of one or 
many underlying entities.
+
+* A view model could also be used as a proxy for some externally managed 
entity, accessed over a web service or REST API; it could even be a 
representation of state held in-memory (such as user preferences, for example).
+
+* view models can also be used to support a particular use case.
+An example that comes to mind is to expose a list of scanned PDFs to be 
processed as an "intray", showing the list of PDFs on one side of the page, and 
the current PDF being viewed on the other.
+Such view models are part of the application layer, not part of the domain 
layer (where entities live).
+
+We explore these use cases in more detail below.
+
+
+[TIP]
+====
+Details on how to actually write a view model (the programming model for view 
models) is
+xref:../ugfun/ugfun.adoc#_ugfun_programming-model_view-models[here].
+====
+
+
+
+[[__ugfun_building-blocks_view-models_externally-managed-entities]]
+== Externally-managed entities
+
+Sometimes the entities that make up your application are persisted not in the 
local JDO/DataNucleus database but reside in some other system, for example 
accessible only through a SOAP web service.
+Logically that data might still be considered a domain entity and we might 
want to associate behaviour with it, however it cannot be modelled as a domain 
entity if only because JDO/DataNucleus doesn't know about the entity nor how to 
retrieve or update it.
+
+There are a couple of ways around this: we could either replicate the data 
somehow from the external system into the Isis-managed database (in which case 
it is once again just another domain entity), or we could set up a stub/proxy 
for the externally managed entity.
+This proxy would hold the reference to the externally-managed domain entity 
(eg an external id), as well as the "smarts" to know how to interact with that 
entity (by making SOAP web service calls etc).
+
+The stub/proxy is a type of view model: a view -- if you like -- onto the 
domain entity managed by the external system.
+
+[NOTE]
+====
+DataNucleus does in fact define its own 
link:http://www.datanucleus.org/documentation/extensions/store_manager.html[Store
 Manager] extension point, so an alternative architecture would be to implement 
this interface such that DataNucleus could make the calls to the external 
system; these externally-persisted domain entities would therefore be modelled 
as regular `@PersistenceCapable` entities after all.
+For entities not persisted externally the implementation would delegate down 
to the default RDBMS-specific `StoreManager` provided by DataNucleus itself.
+
+An implementation that supported only reading from an external entity ought to 
be comparatively straight-forward, but implementing one that also supported 
updating external entities would need to carefully consider error conditions if 
the external system is unavailable; distributed transactions are most likely 
difficult/impossible to implement (and not desirable in any case).
+====
+
+
+[[__ugfun_building-blocks_view-models_in-memory-entities]]
+== In-memory entities
+
+As a variation on the above, sometimes there are domain objects that are, 
conceptually at least entities, but whose state is not actually persisted 
anywhere, merely held in-memory (eg in a hash).
+
+A simple example might be read-only configuration data that is read from a 
config file (eg log4j appender definitions) but thereafter is presented in the 
UI just like any other entity.
+
+
+[[__ugfun_building-blocks_view-models_application-layer-view-models]]
+== Application-layer view models
+
+Domain entities (whether locally persisted using JDO/DataNucleus or managed 
externally) are the bread-and-butter of Apache Isis applications: the focus 
after all, should be on the business domain concepts and ensuring that they are 
solid.
+Generally those domain entities will make sense to the business domain 
experts: they form the _ubiquitous language_ of the domain.  These domain 
entities are part of the domain layer.
+
+That said, it may not always be practical to expect end-users of the 
application to interact solely with those domain entities.
+For example, it may be useful to show a dashboard of the most significant data 
in the system to a user, often pulling in and aggregating information from 
multiple points of the app.
+Obtaining this information by hand (by querying the respective 
services/repositories) would be tedious and slow; far better to have a 
dashboard do the job for the end user.
+
+A dashboard object is a model of the most relevant state to the end-user, in 
other words it is (quite literally) a view model.
+ It is not a persisted entity, instead it belongs to the application layer.
+
+A view model need not merely aggregate data; it could also provide actions of 
its own.
+Most likely these actions will be queries and will always ultimately just 
delegate down to the appropriate domain-layer service/repository.
+But in some cases such view model actions might also modify state of 
underlying domain entities.
+
+Another common use for view models is to help co-ordinate complex business 
processes; for example to perform a quarterly invoicing run, or to upload 
annual interest rates from an Excel spreadsheet.
+In these cases the view model might have some state of its own, but in most 
cases that state does not need to be persisted per se.
+
+.Desire Lines
+****
+One way to think of application view models is as modelling the "desire line": 
the commonly-trod path that end-users must follow to get from point A to point 
B as quickly as possible.
+
+To explain: there are 
link:http://ask.metafilter.com/62599/Where-the-sidewalk-ends[documented] 
link:https://sivers.org/walkways[examples] 
link:http://www.softpanorama.org/People/Wall/larry_wall_articles_and_interviews.shtml[that]
 architects of university campus will only add in paths some while after the 
campus buildings are complete: let the pedestrians figure out the routes they 
want to take.
+The name we like best for this idea is "desire lines", though it has also been 
called a "desire path", "paving the path" or "paving the sidewalk".
+
+What that means is you should add view models _after_ having built up the 
domain layer, rather than before.
+These view models pave that commonly-trod path, automating the steps that the 
end-user would otherwise have to do by hand.
+
+It takes a little practice though, because even when building the domain layer 
"first", you should still bear in mind what the use cases are that those domain 
entities are trying to support.
+You certainly _shouldn't_ try to build out a domain layer that could support 
every conceivable use case before starting to think about view models.
+
+Instead, you should iterate.
+Identify the use case/story/end-user objective that you will deliver value to 
the business.
+Then build out the minimum domain entities to support that use case (refining 
the 
xref:../ugfun/ugfun.adoc#__ugfun_core-concepts_philosophy_domain-driven-design_ubiquitous-language[ubiquitous
 language] as you go).
+Then, identify if there any view models that could be introduced which would 
simplify the end-user interactions with the system (perhaps automating several 
related use cases together).
+****
+
+
+When developing an Apache Isis application you will most likely start off with 
the persistent domain entities: `Customer`, `Order`, `Product`, and so on.
+For some applications this may well suffice.
+
+However, if the application needs to integrate with other systems, or if the 
application needs to support reasonably complex business processes, then you 
may need to look beyond just domain entities; view models are the tool of 
choice.
+
+[WARNING]
+====
+We strongly recommend that you build your applications from the domain layer 
up, rather than from the view model down.
+====
+
+
+[[__ugfun_building-blocks_view-models_dtos]]
+== DTOs
+
+DTOs (data transfer objects) are simple classes that (according to 
link:https://en.wikipedia.org/wiki/Data_transfer_object[wikipedia]) "carry data 
between processes".
+
+If those two processes are parts of the same overall application (the same 
team builds and deploys both server and client) then there's generally no need 
to define a DTO; just access the entities using Apache Isis' 
xref:../ugvro/ugvro.adoc#[RestfulObjects viewer].
+
+On the other hand, if the client consuming the DTO is a different application 
-- by which we mean developed/deployed by a different (possible third-party) 
team -- then the DTOs act as a formal contract between the provider and the 
consumer.
+In such cases, exposing domain entities over 
xref:../ugvro/ugvro.adoc#[RestfulObjects] would be "A Bad Thing"(TM) because 
the consumer would in effect have access to implementation details that could 
then not be easily changed by the producer.
+
+To support this use case, a view model can be defined such that it can act as 
a DTO.
+This is done by annotating the class using JAXB annotations; this allows the 
consumer to obtain the DTO in XML format along with a corresponding XSD schema 
describing the structure of that XML.
+A discussion of how that might be done using an ESB such as 
link:http://camel.apache.org[Apache Camel(TM)] follows 
xref:../ugbtb/ugbtb.adoc#__ugfun_building-blocks_view-models_dtos_consumers[below].
+
+In case it's not obvious, these DTOs are still usable as "regular" view 
models; they will render in the xref:../ugvw/ugvw.adoc#[Wicket viewer] just 
like any other.
+In fact (as the 
xref:../ugbtb/ugbtb.adoc#_ugbtb_view-models_programming-model[programming 
model] section below makes clear), these JAXB-annotated view models are in many 
regards the most powerful of all the alternative ways of writing view models.
+
+It's also worth noting that it is also possible to download the XML (or XSD) 
straight from the UI, useful during development.
+The view model simply needs to implement the 
xref:../rgcms/rgcms.adoc#_rgcms_classes_mixins_Dto[`Dto`] marker interface; the 
framework has xref:../rgcms/rgcms.adoc#_rgcms_classes_mixins_Dto[mixins] that 
contribute the download actions to the view model.
+
+
+[[__ugfun_building-blocks_view-models_dtos_consumers]]
+=== DTO Consumers
+
+The actual consumers of DTOs will generally obtain the XML of the view models 
either by requesting the XML directly, eg using the 
xref:../ugvro/ugvro.adoc#[RestfulObjects viewer], or may have the XML sent to 
them asynchronously using an ESB such as Apache Camel.
+
+In the former case, the consumer requests the DTO by calling the REST API with 
the appropriate HTTP `Accept` header.
+An appropriate implementation of 
xref:../rgsvc/rgsvc.adoc#_rgsvc_spi_ContentMappingService[`ContentMappingService`]
 can then be used to return the appropriate DTO (as XML).
+
+For the latter case, one design is simply for the application to instantiate 
the view model, then call the 
xref:../rgsvc/rgsvc.adoc#_rgsvc_api_JaxbService[`JaxbService`] to obtain its 
corresponding XML.  This can then be published onto the ESB, for example using 
an http://activemq.apache.org[Apache ActiveMQ (TM)] queue.
+
+However, rather than try to push all the data that might be needed by any of 
these external systems in a single XML event (which would require anticipating 
all the requirements, likely a hopeless task), a better design is to publish 
only the fact that something of note has changed - ie, that an action on a 
domain object has been invoked - and then let the consumers call back to obtain 
other information if required.
+This can once again be done by calling the REST API with an appropriate HTTP 
`Accept` header.
+
+[TIP]
+====
+This is an example of the link:https://leanpub.com/camel-design-patterns[VETRO 
pattern] (validate, enrich, transform, route, operate).
+In our case we focus on the validation (to determine the nature of the inbound 
message, ie which action was invoked), and the enrich (callback to obtain a DTO 
with additional information required by the consumer).
+====
+
+The (non-ASF) http://github.com/isisaddons/isis-module-publishmq[Isis addons' 
publishmq] module provides an out-of-the-box solution of this design.
+It provides an implementation of the 
xref:../rgsvc/rgsvc.adoc#_rgsvc_spi_PublishingService[`PublishingService`], but 
which simply publishes instances of 
xref:../rgcms/rgcms.adoc#_rgcms_schema-aim[`ActionInvocationMemento`] to an 
ActiveMQ queue.
+Camel (or similar) can then be hooked up to consume these events from this 
queue, and use a processor to parse the action memento to determine what has 
changed on the source system.
+Thereafter, a subsequent Camel processor can then call back to the source - 
via the xref:../ugvro/ugvro.adoc[Restful Objects viewer] - to enrich the 
message with additional details using a DTO.
+
+
+
+

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-[[_ugfun_class-structure]]
-= Class Structure
-:Notice: Licensed to the Apache Software Foundation (ASF) under one or more 
contributor license agreements. See the NOTICE file distributed with this work 
for additional information regarding copyright ownership. The ASF licenses this 
file to you under the Apache License, Version 2.0 (the "License"); you may not 
use this file except in compliance with the License. You may obtain a copy of 
the License at. http://www.apache.org/licenses/LICENSE-2.0 . Unless required by 
applicable law or agreed to in writing, software distributed under the License 
is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR  CONDITIONS OF ANY 
KIND, either express or implied. See the License for the specific language 
governing permissions and limitations under the License.
-:_basedir: ../../
-:_imagesdir: images/
-
-
-Apache Isis works by building a metamodel of the domain objects: entities, 
domain services, view models and mixins.
-Dependent on the sort of domain object, the class methods represent
-both state -- (single-valued) properties and (multi-valued) collections -- and 
behaviour -- actions.
-
-More specifically, both entities and view models can have properties, 
collections and actions, while domain services have just actions.
-Mixins also define only actions, though depending on their semantics they may 
be rendered as derived properties or collections on the domain object to which 
they contribute.
-
-In the automatically generated UI a property is rendered as a field.
-This can be either of a value type (a string, number, date, boolean etc) or 
can be a reference to another entity.
-A collection is generally rendered as a table.
-
-In order for Apache Isis to build its metamodel the domain objects must follow 
some conventions: what we call the _Apache Isis Programming Model_.
-This is just an extension of the pojo / JavaBean standard of yesteryear: 
properties and collections are getters/setters, while actions are simply any 
remaining `public` methods.
-
-Additional metamodel semantics are inferred both imperatively from _supporting 
methods_ and declaratively from annotations.
-
-In this section we discuss the mechanics of writing domain objects that comply 
with Apache Isis' programming model.
-
-[TIP]
-====
-In fact, the Apache Isis programming model is extensible; you can teach Apache 
Isis new programming conventions and you can remove existing ones; ultimately 
they amount to syntax.
-The only real fundamental that can't be changed is the notion that objects 
consist of properties, collections and actions.
-
-You can learn more about extending Apache Isis programming model 
xref:../ugbtb/ugbtb.adoc#_ugbtb_programming-model[here].
-====
-
-
-include::_ugfun_class-structure_properties.adoc[leveloffset=+1]
-include::_ugfun_class-structure_collections.adoc[leveloffset=+1]
-include::_ugfun_class-structure_actions.adoc[leveloffset=+1]
-include::_ugfun_class-structure_inject-services.adoc[leveloffset=+1]
-
-include::_ugfun_class-structure_properties-vs-parameters.adoc[leveloffset=+1]
-
-include::_ugfun_class-structure_domain-services.adoc[leveloffset=+1]
-
-
-
-

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-[[_ugfun_class-structure_actions]]
-= Actions
-:Notice: Licensed to the Apache Software Foundation (ASF) under one or more 
contributor license agreements. See the NOTICE file distributed with this work 
for additional information regarding copyright ownership. The ASF licenses this 
file to you under the Apache License, Version 2.0 (the "License"); you may not 
use this file except in compliance with the License. You may obtain a copy of 
the License at. http://www.apache.org/licenses/LICENSE-2.0 . Unless required by 
applicable law or agreed to in writing, software distributed under the License 
is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR  CONDITIONS OF ANY 
KIND, either express or implied. See the License for the specific language 
governing permissions and limitations under the License.
-:_basedir: ../../
-:_imagesdir: images/
-
-
-
-While xref:../ugfun/ugfun.adoc#_ugfun_class-structure_properties[properties] 
and xref:../ugfun/ugfun.adoc#_ugfun_class-structure_collections[collections] 
define the state held by a domain object (its "know what" responsibilities), 
actions define the object's behaviour (its "know how-to" responsibilities).
-
-An application whose domain objects have only/mostly "know-what" 
responsibilities is pretty dumb: it requires that the end-user know the 
business rules and doesn't modify the state of the domain objects such that 
they are invalid (for example, an "end date" being before a "start date").
-Such applications are often called CRUD applications 
(create/read/update/delete).
-
-In more complex domains, it's not realistic/feasible to expect the end-user to 
have to remember all the different business rules that govern the valid states 
for each domain object.
-So instead actions allow those business rules to be encoded programmatically.
-An Apache Isis application doesn't try to constrain the end-user as to way in 
which they interact with the user (it doesn't attempt to define a rigid 
business process) but it does aim to ensure that business rule invariants are 
maintained, that is that business objects aren't allowed to go into an invalid 
state.
-
-For simple domain applications, you may want to start prototyping only with 
properties, and only later introduce actions (representing the most common 
business operations).
-But an alternative approach, recommended for more complex applications, is 
actually to start the application with all properties non-editable.
-Then, as the end-user requires the ability to modify some state, there is a 
context in which to ask the question "why does this state need to change?" and 
"are their any side-effects?" (ie, other state that changes at the same time, 
or other behaviour that should occur).
-If the state change is simple, for example just being able to correct an 
invalid address, or adding a note or comment, then that can probably be 
modelled as a simple editable property.
-But if the state change is more complex, then most likely an action should be 
used instead.
-
-
-[[__ugfun_class-structure_actions_defining-actions]]
-== Defining actions
-
-Broadly speaking, actions are all the `public` methods that are not getters or 
setters which represent properties or collections.
-This is a slight simplification; there are a number of other method prefixes 
(such as `hide` or `validate`) that represent 
xref:../ugfun/ugfun.adoc#_ugfun_business-rules[business rules]); these also not 
treated as actions.
-And, any method that are annotated with `@Programmatic` will also be excluded.
-But by and large, all other methods such as `placeOrder(...)` or 
`approveInvoice(...)` will be treated as actions.
-
-For example:
-
-[source,java]
-----
-@Action(semantics=SemanticsOf.IDEMPOTENT)       // <1>
-public ShoppingBasket addToBasket(
-        Product product,
-        @ParameterLayout(named="Quantity")      // <2>
-        int quantity
-        ) {
-    ...
-    return this;
-}
-----
-<1> `@Action` annotation is optional but used to specify additional domain 
semantics (such as being idempotent).
-<2> The names of action parameters (as rendered in the UI) will by default be 
the parameter types, not the paramter names.
-For the `product` parameter this is reasonable, but not so for the `quantity` 
parameter (which would by default show up with a name of "int".
-The `@ParameterLayout` annotation provides a UI hint to the framework.
-
-[TIP]
-====
-The (non-ASF) Isis addons' 
http://github.com/isisaddons/isis-metamodel-paraname8[paraname8] metamodel 
extension allows the parameter name to be used in the UI, rather than the type.
-====
-
-
-[[__ugfun_class-structure_actions_reference-parameter-types]]
-== (Reference) Parameter types
-
-Parameter types can be value types or reference types.
-In the case of primitive types, the end-user can just enter the value directly 
through the parameter field.
-In the case of reference types however (such as `Product`), a drop-down must 
be provided from which the end-user to select.
-This is done using either a supporting 
xref:../rgcms/rgcms.adoc#_rgcms_methods_prefixes_choices[`choices`] or 
xref:../rgcms/rgcms.adoc#_rgcms_methods_prefixes_autoComplete[`autoComplete`] 
method.
-The "choices" is used when there is a limited set of options, while 
"autoComplete" is used when there are large set of options such that the 
end-user must provide some characters to use for a search.
-
-For example, the `addToBasket(...)` action shown above might well have a :
-
-[source,java]
-----
-@Action(semantics=SemanticsOf.IDEMPOTENT)
-public ShoppingBasket addToBasket(
-        Product product,
-        @ParameterLayout(named="Quantity")
-        int quantity
-        ) {
-    ...
-    return this;
-}
-public List<Product> autoComplete0AddToBasket(              // <1>
-    @MinLength(3)                                           // <2>
-    String searchTerm) {
-    return productRepository.find(searchTerm);              // <3>
-}
-@javax.inject.Inject
-ProductRepository productRepository;
-----
-<1> Supporting `autoComplete` method.
-The "0" in the name means that this corresponds to parameter 0 of the 
"addToBasket" action (ie `Product`).
-It is also required to return a Collection of that type.
-<2> The xref:../rgant/rgant.adoc#_rgant_MinLength[`@MinLength`] annotation 
defines how many characters the end-user must enter before performing a search.
-<3> The implementation delegates to an injected repository service.  This is 
typical.
-
-Note that it is also valid to define "choices" and "autoComplete" for value 
types (such as `quantity`, above); it just isn't as common to do so.
-
-[[__ugfun_class-structure_actions_reference-parameter-types_removing-boilerplate]]
-=== Removing boilerplate
-
-To save having to define an `autoCompleteNXxx(...)` method everywhere that a 
reference to a particular type (such as `Product`) appears as an action 
parameter, it is also possible to use the `@DomainObject` annotation on 
`Product` itself:
-
-[source,java]
-----
-@DomainObject(
-    autoCompleteRepository=ProductRepository.class          // <1>
-    autoCompleteAction="find"                               // <2>
-)
-public class Product ... {
-    ...
-}
-----
-<1> Whenever an action parameter requiring a `Product` is defined, provide an 
autoComplete drop-down automatically
-<2> Use the "find" method of `ProductRepository` (rather than the default name 
of "autoComplete")
-
-(As noted above), if the number of available instances of the reference type 
is a small number (in other words, all of which could comfortably be shown in a 
drop-down) then instead the `choicesNXxx()` supporting method can be used.
-This too can be avoided by annotating the referenced class.
-
-For example, suppose we have an action to specify the `PaymentMethodType`, 
where there are only 10 or so such (Visa, Mastercard, Amex, Paypal etc).
-We could define this as:
-
-[source,java]
-----
-public Order payUsing(PaymentMethodType type) {
-    ...
-}
-----
-
-where `PaymentMethodType` would be annotated using:
-
-[source,java]
-----
-@DomainObject(
-    bounded=true                            // <1>
-)
-public class PaymentMethodType ... {
-    ...
-}
-----
-<1> only a small (ie "bounded") number of instances available, meaning that 
the framework should render all in a drop-down.
-
-
-[[__ugfun_class-structure_actions_collection-parameter-types]]
-== Collection Parameter types
-
-Action parameters can also be collections of values (for example 
`List<String>`), or can be collections of references (such as `List<Customer>`).
-
-For example:
-
-[source,java]
-----
-@Action(semantics=SemanticsOf.IDEMPOTENT)
-public ShoppingBasket addToBasket(
-        List<Product> products,
-        @ParameterLayout(named="Quantity") int quantity
-        ) {
-    ...
-    return this;
-}
-public List<Product> autoComplete0AddToBasket(@MinLength(3) String searchTerm) 
{
-    return ...
-}
-----
-
-As the example suggests, any collection parameter type must provide a way to 
select items, either by way of a "choices" or "autoComplete" supporting method 
or alternatively defined globally using 
xref:../rgant/rgant.adoc#_rgant_DomainObject[`@DomainObject`] on the referenced 
type (described 
xref:../ugfun/ugfun.adoc#__ugfun_class-structure_actions_reference-parameter-types_removing-boilerplate[above]).
-
-
-[[__ugfun_class-structure_actions_optional-parameters]]
-== Optional Parameters
-
-Whereas the 
xref:../ugfun/ugfun.adoc#__ugfun_class-structure_properties_optional-properties[optionality
 of properties] is defined using 
xref:../rgant/rgant.adoc#_rgant_Column_allowsNull[`@javax.jdo.annotations.Column#allowsNull()`],
 that JDO annotation cannot be applied to parameter types.
-Instead, either the xref:../rgant/rgant.adoc#_rgant_Nullable[`@Nullable`] 
annotation or the 
xref:../rgant/rgant.adoc#_rgant_Parameter_optionality[`@Parameter#optionality()`]
  annotation/attribute is used.
-
-For example:
-
-[source,java]
-----
-@javax.jdo.annotations.Column(allowsNull="true")                // <1>
-@lombok.Getter @lombok.Setter
-private LocalDate shipBy;
-
-public Order invoice(
-                PaymentMethodType paymentMethodType,
-                @Nullable                                       // <2>
-                @ParameterLayout(named="Ship no later than")
-                LocalDate shipBy) {
-    ...
-    setShipBy(shipBy)
-    return this;
-}
-----
-<1> Specifies the property is optional.
-<2> Specifies the corresponding parameter is optional.
-
-See also 
xref:../ugfun/ugfun.adoc#_ugfun_class-structure_properties-vs-parameters[properties
 vs parameters].
-
-[[__ugfun_class-structure_actions_string-parameters]]
-== ``String`` Parameters (Length)
-
-Whereas the 
xref:../ugfun/ugfun.adoc#__ugfun_class-structure_properties_datatypes_strings[length
 of string properties] is defined using 
xref:../rgant/rgant.adoc#_rgant_Column_length[`@javax.jdo.annotations.Column#length()`],
 that JDO annotation cannot be applied to parameter types.
-Instead, the 
xref:../rgant/rgant.adoc#_rgant_Parameter_maxLength[`@Parameter#maxLength()`] 
annotation/attribute is used.
-
-For example:
-
-[source,java]
-----
-@javax.jdo.annotations.Column(length=50)                // <1>
-@lombok.Getter @lombok.Setter
-private String firstName;
-
-@javax.jdo.annotations.Column(length=50)
-@lombok.Getter @lombok.Setter
-private String lastName;
-
-public Customer updateName(
-                @Parameter(maxLength=50)                // <2>
-                @ParameterLayout(named="First name")
-                String firstName,
-                @Parameter(maxLength=50)
-                @ParameterLayout(named="Last name")
-                String lastName) {
-    setFirstName(firstName);
-    setLastName(lastName);
-    return this;
-}
-----
-<1> Specifies the property length using the JDO 
xref:../rgant/rgant.adoc#_rgant_Column_length[`@Column#length()`] annotation
-<2> Specifies the parameter length using the (Apache Isis) 
xref:../rgant/rgant.adoc#_rgant_Parameter_maxLength[`@Parameter#maxLength()`] 
annotation
-
-[IMPORTANT]
-====
-Incidentally, note in the above example that the new value is assigned to the 
properties using the setter methods; the action does not simply set the 
instance field directly.
-This is important, because it allows JDO/DataNucleus to keep track that this 
instance variable is "dirty" and so needs flushing to the database table before 
the transaction completes.
-====
-
-See also 
xref:../ugfun/ugfun.adoc#_ugfun_class-structure_properties-vs-parameters[properties
 vs parameters].
-
-[[__ugfun_class-structure_actions_bigdecimal-parameters]]
-== ``BigDecimal``s (Precision)
-
-Whereas the 
xref:../ugfun/ugfun.adoc#__ugfun_class-structure_properties_datatypes_bigdecimals[precision
 of BigDecimal properties] is defined using 
xref:../rgant/rgant.adoc#_rgant_Column_scale[`@javax.jdo.annotations.Column#scale()`],
 that JDO annotation cannot be applied to parameter types.
-Instead, the 
xref:../rgant/rgant.adoc#_rgant_Digits_fraction[`@javax.validation.constraints.Digits#fraction()`]
 annotation/attribute is used.
-
-For example:
-
-[source,java]
-----
-@javax.jdo.annotations.Column(scale=2)                              // <1>
-@lombok.Getter @lombok.Setter
-private BigDecimal discountRate;
-
-public Order updateDiscount(
-                @javax.validation.constraints.Digits(fraction=2)    // <2>
-                @ParameterLayout(named="Discount rate")
-                String discountRate) {
-    setDiscountRate(discountRate);
-    return this;
-}
-----
-<1> Specifies the property precision using 
xref:../rgant/rgant.adoc#_rgant_Column_scale[`@Column#scale()`]
-<2> Specifies the corresponding parameter precision using 
xref:../rgant/rgant.adoc#_rgant_Digits_fraction[`@Digits#fraction()`].
-
-See also 
xref:../ugfun/ugfun.adoc#_ugfun_class-structure_properties-vs-parameters[properties
 vs parameters].
-
-
-

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-[[_ugfun_class-structure_collections]]
-= Collections
-:Notice: Licensed to the Apache Software Foundation (ASF) under one or more 
contributor license agreements. See the NOTICE file distributed with this work 
for additional information regarding copyright ownership. The ASF licenses this 
file to you under the Apache License, Version 2.0 (the "License"); you may not 
use this file except in compliance with the License. You may obtain a copy of 
the License at. http://www.apache.org/licenses/LICENSE-2.0 . Unless required by 
applicable law or agreed to in writing, software distributed under the License 
is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR  CONDITIONS OF ANY 
KIND, either express or implied. See the License for the specific language 
governing permissions and limitations under the License.
-:_basedir: ../../
-:_imagesdir: images/
-
-
-A collection is an instance variable of a domain object, of a collection type 
that holds references to other domain objects.
-For example, a `Customer` may have a collection of ``Order``s).
-
-It's ok for a 
xref:../ugfun/ugfun.adoc#__ugfun_class-structure_class-definition_entities[domain
 entity] to reference another domain entity, and for a 
xref:../ugfun/ugfun.adoc#__ugfun_class-structure_class-definition_view-models[view
 model] to reference both view model and domain entities.
-However, it isn't valid for a domain entity to hold a persisted reference to 
view model (DataNucleus will not know how to persist that view model).
-
-Formally speaking, a collection is simply a regular JavaBean getter, returning 
a collection type (subtype of `java.util.Collection`).
-Most collections (those that are modifiable) will also have a setter and (if 
persisted) a backing instance field.
-And collections properties will also have a number of annotations:
-
-* Apache Isis defines its own set own `@Collection` annotation for capturing 
domain semantics.
-It also provides a `@CollectionLayout` for UI hints (though the information in 
this annotation may instead be provided by a supplementary 
xref:../ugvw/ugvw.adoc#_ugvw_layout[`.layout.xml`] file
-
-* the collections of domain entities are often annotated with various 
JDO/DataNucleus annotations, most notable `javax.jdo.annotations.Persistent`.
-This and other annotations can be used to specify if the association is 
bidirectional, and whether to define a link table or not to hold foreign key 
columns.
-
-* for the collections of view models, then JAXB annotations such as 
`@javax.xml.bind.annotation.XmlElementWrapper` and 
`@javax.xml.bind.annotation.XmlElement` will be present
-
-Apache Isis recognises some of these annotations for JDO/DataNucleus and JAXB 
and infers some domain semantics from them (for example, the maximum allowable 
length of a string property).
-
-Unlike xref:../ugfun/ugfun.adoc#_ugfun_class-structure_properties[properties], 
the framework (at least, the xref:../ugvw/ugvw.adoc[Wicket viewer]) does not 
allow collections to be "edited".
-Instead, xref:../ugfun/ugfun.adoc#_ugfun_class-structure_actions[action]s can 
be written that will modify the contents of the collection as a side-effect.
-For example, a `placeOrder(...)` action will likely add an `Order` to the 
`Customer#orders` collection.
-
-Since writing getter and setter methods adds quite a bit of boilerplate, it's 
common to use link:https://projectlombok.org/[Project Lombok] to code generate 
these methods at compile time (using Java's annotation processor) simply by 
adding the `@lombok.Getter` and `@lombok.Setter` annotations to the field.
-
-
-
-[[__ugfun_class-structure_collections_mapping-bidir-1m]]
-== Mapping bidir 1:m
-
-Bidirectional one-to-many collections are one of the most common types of 
associations between two entities.
-In the parent object, the collection can be defined as:
-
-[source,java]
-----
-public class ParentObject
-        implements Comparable<ParentObject>{
-
-    @javax.jdo.annotations.Persistent(
-        mappedBy = "parent",                                                // 
<1>
-        dependentElement = "false"                                          // 
<2>
-    )
-    @Collection                                                             // 
<3>
-    @lombok.Getter @lombok.Setter
-    private SortedSet<ChildObject> children = new TreeSet<ChildObject>();   // 
<4>
-
-}
-----
-<1> indicates a bidirectional association; the foreign key pointing back to 
the `Parent` will be in the table for `ChildObject`
-<2> disable cascade delete
-<3> (not actually required in this case, because no attributes are set, but 
acts as a useful reminder that this collection will be rendered in the UI by 
Apache Isis)
-<4> uses a `SortedSet` (as opposed to some other collection type; discussion 
below)
-
-while in the child object you will have:
-
-[source,java]
-----
-public class ChildObject
-        implements Comparable<ChildObject> {    // <1>
-
-    @javax.jdo.annotations.Column(
-        allowsNull = "false"                    // <2>
-    )
-    @Property(editing = Editing.DISABLED)       // <3>
-    @lombok.Getter @lombok.Setter
-    private ParentObject parent;
-}
-----
-<1> implements `Comparable` because is mapped using a `SortedSet`
-<2> mandatory; every child must reference its parent
-<3> cannot be edited directly
-
-Generally speaking you should use `SortedSet` for collection types (as opposed 
to `Set`, `List` or `Collection`).
-JDO/Datanucleus does support the mapping of these other types, but RDBMS are 
set-oriented, so using this type introduces the least friction.
-
-[NOTE]
-====
-For further details on mapping associations, see the JDO/DataNucleus 
documentation for 
link:http://www.datanucleus.org/products/accessplatform_4_1/jdo/orm/one_to_many.html[one-to-many]
 associations, 
link:http://www.datanucleus.org/products/accessplatform_4_1/jdo/orm/many_to_one.html[many-to-one]
 associations, 
link:http://www.datanucleus.org/products/accessplatform_4_1/jdo/orm/many_to_many.html[many-to-many]
 associations, and so on.
-
-Also, while JDO/DataNucleus itself supports `java.util.Map` as a collection 
type, this is not supported by Apache Isis.
-If you do wish to use this collection type, then annotate the getter with 
`@Programmatic` so that it is ignored by the Apache Isis framework.
-====
-
-
-
-== Value vs Reference Types
-
-Apache Isis can (currently) only provide a UI for collections of references.
-While you can use DataNucleus to persist collections/arrays of value types, 
such properties must be annotated as `@Programmatic` so that they are ignored 
by Apache Isis.
-
-If you want to visualize an array of value types in Apache Isis, then one 
option is to wrap value in a view model, as explained 
xref:../ugfun/ugfun.adoc#_ugbtb_hints-and-tips_simulating-collections-of-values[elsewhere].
-
-
-
-[[__ugfun_class-structure_collections_derived-collections]]
-== Derived Collections
-
-A derived collection is simply a getter (no setter) that returns a 
`java.util.Collection` (or subtype).
-
-While derived properties and derived collections typically "walk the graph" to 
associated objects, there is nothing to prevent the returned value being the 
result of invoking a repository (domain service) action.
-
-For example:
-
-[source,java]
-----
-public class Customer {
-    ...
-    public List<Order> getMostRecentOrders() {
-        return orderRepo.findMostRecentOrders(this, 5);
-    }
-}
-----
-

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-[[_ugfun_class-structure_domain-services]]
-= Domain Services
-:Notice: Licensed to the Apache Software Foundation (ASF) under one or more 
contributor license agreements. See the NOTICE file distributed with this work 
for additional information regarding copyright ownership. The ASF licenses this 
file to you under the Apache License, Version 2.0 (the "License"); you may not 
use this file except in compliance with the License. You may obtain a copy of 
the License at. http://www.apache.org/licenses/LICENSE-2.0 . Unless required by 
applicable law or agreed to in writing, software distributed under the License 
is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR  CONDITIONS OF ANY 
KIND, either express or implied. See the License for the specific language 
governing permissions and limitations under the License.
-:_basedir: ../../
-:_imagesdir: images/
-
-
-This section looks at the programming conventions of writing your own domain 
services.
-
-
-== Typical Implementation
-
-Domain services are generally singletons that are automatically injected into 
other domain services.
-A very common usage is as a repository (to find/locate existing entities) or 
as a factory (to create new instances of entities).
-But services can also be exposed in the UI as top-level menus; and services 
are also used as a bridge to access technical resources (eg rendering a 
document object as a PDF).
-
-The Apache Isis framework itself also provides a large number of number of 
domain services, catalogued in the xref:../rgsvc/rgsvc.adoc#[Domain Services 
Reference Guide].
-Some of these are APIs (intended to be called by your application's own domain 
objects) and some are SPIs (implemented by your application and called by the 
framework, customising the way it works).
-
-The following is a typical menu service:
-
-[source,java]
-----
-@DomainService(                                                 // <1>
-        nature = NatureOfService.VIEW_MENU_ONLY
-)
-@DomainServiceLayout(                                           // <2>
-        named = "Simple Objects",
-        menuOrder = "10"
-)
-public class SimpleObjectMenu {
-
-    ...
-
-    @Action(semantics = SemanticsOf.SAFE)
-    @ActionLayout(bookmarking = BookmarkPolicy.AS_ROOT)
-    @MemberOrder(sequence = "2")
-    public List<SimpleObject> findByName(                       // <3>
-            @ParameterLayout(named="Name")
-            final String name
-    ) {
-        return simpleObjectRepository.findByName(name);
-    }
-
-    @javax.inject.Inject
-    SimpleObjectRepository simpleObjectRepository;              // <4>
-}
-----
-<1> The (Apache Isis) `@DomainService` annotation is used to identify the 
class as a domain service.
-Apache Isis scans the classpath looking for classes with this annotation, so 
there very little configuration other than to tell the framework which packages 
to scan underneath.
-The `VIEW_MENU_ONLY` nature indicates that this service's actions should be 
exposed as menu items.
-<2> The (Apache Isis) `@DomainServiceLayout` annotation provides UI hints.
-In the example above the menu is named "Simple Objects" (otherwise it would 
have defaulted to "Simple Object Menu", based on the class name, while the 
`menuOrder` attribute determines the order of the menu with respect to other 
menu services.
-<3> The `findByName` method is annotated with various Apache Isis annotations 
(`@Action`, `@ActionLayout` and `@MemberOrder`) and is itself rendered in the 
UI as a "Find By Name" menu item underneath the "Simple Objects" menu.
-The implementation delegates to an `SimpleObjectRepository` service, which is 
injected.
-<4> The `javax.inject.Inject` annotation instructs Apache Isis framework to 
inject the `SimpleObjectRepository` service into this domain object.
-The framework can inject into not just other domain services but will also 
automatically into domain entities and view models.
-There is further discussion of service injection 
xref:../ugfun/ugfun.adoc#_ugfun_class-structure_inject-services[below].
-
-
-
-
-== Scoped services
-
-By default all domain services are considered to be singletons, and 
thread-safe.
-
-Sometimes though a service's lifetime is applicable only to a single request; 
in other words it is request-scoped.
-
-The CDI annotation 
xref:../rgant/rgant.adoc#_rgant-RequestScoped[`@javax.enterprise.context.RequestScoped`]
 is used to indicate this fact:
-
-[source,java]
-----
-@javax.enterprise.context.RequestScoped
-public class MyService extends AbstractService {
-    ...
-}
-----
-
-The framework provides a number of request-scoped services, include a 
xref:../rgsvc/rgsvc.adoc#_rgsvc_api_Scratchpad[`Scratchpad`] service query 
results caching through the 
xref:../rgsvc/rgsvc.adoc#_rgsvc_api_QueryResultsCache[`QueryResultsCache`], and 
support for co-ordinating bulk actions through the 
xref:../rgsvc/rgsvc.adoc#_rgsvc_api_ActionInvocationContext[`ActionInvocationContext`]
 service.  See the xref:../rgsvc/rgsvc.adoc[domain services] reference guide 
for further details.
-
-
-
-
-== Registering domain services
-
-The easiest way to register domain services is using 
xref:../rgcms/rgcms.adoc#_rgcms_classes_AppManifest-bootstrapping[`AppManifest`]
 to specify the modules
-which contain 
xref:../rgant/rgant.adoc#_rgant-DomainService[`@DomainService`]-annotated 
classes.
-
-For example:
-
-[source,ini]
-----
-public class MyAppManifest implements AppManifest {
-    public List<Class<?>> getModules() {
-        return Arrays.asList(
-                ToDoAppDomainModule.class,
-                ToDoAppFixtureModule.class,
-                ToDoAppAppModule.class,
-                org.isisaddons.module.audit.AuditModule.class);
-    }
-    ...
-}
-----
-
-will load all services in the packages underneath the four modules listed.
-
-An alternative (older) mechanism is to registered domain services in the 
`isis.properties` configuration file, under `isis.services` key (a 
comma-separated list); for example:
-
-[source,ini]
-----
-isis.services = com.mycompany.myapp.employee.Employees\,
-                com.mycompany.myapp.claim.Claims\,
-                ...
-----
-
-This will then result in the framework instantiating a single instance of each 
of the services listed.
-
-If all services reside under a common package, then the `isis.services.prefix` 
can specify this prefix:
-
-[source,ini]
-----
-isis.services.prefix = com.mycompany.myapp
-isis.services = employee.Employees,\
-                claim.Claims,\
-                ...
-----
-
-This is quite rare, however; you will often want to use default 
implementations of domain services that are provided by the framework and so 
will not reside under this prefix.
-
-Examples of framework-provided services (as defined in the applib) include 
clock, auditing, publishing, exception handling, view model support, 
snapshots/mementos, and user/application settings management; see the 
xref:../rgsvc/rgsvc.adoc[domain services] reference guide for further details.
-
-
-
-== Initialization
-
-Services can optionally declare lifecycle callbacks to initialize them (when 
the app is deployed) and to shut them down (when the app is undeployed).
-
-An Apache Isis session _is_ available when initialization occurs (so services 
can interact with the object store, for example).
-
-
-The framework will call any `public` method annotated with 
xref:../rgant/rgant.adoc#_rgant-PostConstruct[`@PostConstruct`] with either no 
arguments of an argument of type `Map<String,String>`
-
-or
-
-In the latter case, the framework passes in the configuration 
(`isis.properties` and any other component-specific configuration files).
-
-
-Shutdown is similar; the framework will call any method annotated with 
xref:../rgant/rgant.adoc#_rgant-PreDestroy[`@PreDestroy`].
-
-
-
-== The getId() method
-
-Optionally, a service may provide a 
xref:../rgcms/rgcms.adoc#_rgcms_methods_reserved_getId[`getId()`] method.  This 
method returns a logical identifier for a service, independent of its 
implementation.
-

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-[[_ugfun_class-structure_inject-services]]
-= Injecting services
-:Notice: Licensed to the Apache Software Foundation (ASF) under one or more 
contributor license agreements. See the NOTICE file distributed with this work 
for additional information regarding copyright ownership. The ASF licenses this 
file to you under the Apache License, Version 2.0 (the "License"); you may not 
use this file except in compliance with the License. You may obtain a copy of 
the License at. http://www.apache.org/licenses/LICENSE-2.0 . Unless required by 
applicable law or agreed to in writing, software distributed under the License 
is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR  CONDITIONS OF ANY 
KIND, either express or implied. See the License for the specific language 
governing permissions and limitations under the License.
-:_basedir: ../../
-:_imagesdir: images/
-
-
-Apache Isis autowires (automatically injects) domain services into each 
entity, as well as into the domain services themselves, using either method 
injection or field injection.
-The framework defines many additional services (such as 
xref:../rgsvc/rgsvc.adoc#_rgsvc_api_RepositoryService[`RepositoryService`]); 
these are injected in exactly the same manner.
-
-Sometimes there may be multiple services that implement a single type.
-This is common for example for SPI service, whereby one module defines an SPI 
service, and other module(s) in the application implement that service.
-To support this, the framework also allows lists of services to be injected.
-
-When there are multiple service implementations of a given type, the framework 
will inject the service with highest priority, as defined through 
xref:../rgant/rgant.adoc#_rgant_DomainService_menuOrder[`@DomainService#menuOrder()`]
 (even for domain services that are not menus), lowest first.
-If a list of services is injected, then that list will be ordered according to 
`menuOrder`, again lowest first.
-
-
-
-[NOTE]
-====
-Isis currently does _not_ support qualified injection of services; the domain 
service of each type must be distinct from any other.
-
-If you find a requirement to inject two instances of type `SomeService`, say, 
then the work-around is to create trivial subclasses `SomeServiceA` and 
`SomeServiceB` and inject these instead.
-====
-
-
-== Field Injection
-
-Field injection is recommended, using the `@javax.inject.Inject` annotation.
-For example:
-
-[source,java]
-----
-public class Customer {
-    ...
-    @javax.inject.Inject
-    OrderRepository orderRepository;
-}
-----
-
-To inject a list of services, use:
-
-[source,java]
-----
-public class DocumentService {
-    ...
-    @javax.inject.Inject
-    List<PaperclipFactory> paperclipFactories;
-}
-----
-
-We recommend using default rather than `private` visibility so that the field 
can be mocked out within unit tests (placed in the same package as the code 
under test).
-
-
-
-
-== Method Injection
-
-The framework also supports two forms of method injection.
-All that is required to inject a service into a entity/service is to provide 
an appropriate method or field.
-The name of the method does not matter, only that it is prefixed either `set` 
or `inject`, is public, and has a single parameter of the correct type.
-
-For example:
-
-[source,java]
-----
-public class Customer {
-    private OrderRepository orderRepository;
-    public void setOrderRepository(OrderRepository orderRepository) {
-        this.orderRepository = orderRepository;
-    }
-    ...
-}
-----
-
-or alternatively, using 'inject' as the prefix:
-
-[source,java]
-----
-public class Customer {
-    private OrderRepository orderRepository;
-    public void injectOrderRepository(OrderRepository orderRepository) {
-        this.orderRepository = orderRepository;
-    }
-    ...
-}
-----
-
-Lists of services can be injected in a similar manner.
-
-Note that the method name can be anything; it doesn't need to be related to 
the type being injected.
-
-
-== Constructor injection
-
-Simply to note that constructor injection is _not_ supported by Apache Isis 
(and is unlikely to be, because the JDO specification for entities requires a 
no-arg constructor).
-
-
-
-
-
-

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-[[_ugfun_class-structure_properties-vs-parameters]]
-= Properties vs Parameters
-:Notice: Licensed to the Apache Software Foundation (ASF) under one or more 
contributor license agreements. See the NOTICE file distributed with this work 
for additional information regarding copyright ownership. The ASF licenses this 
file to you under the Apache License, Version 2.0 (the "License"); you may not 
use this file except in compliance with the License. You may obtain a copy of 
the License at. http://www.apache.org/licenses/LICENSE-2.0 . Unless required by 
applicable law or agreed to in writing, software distributed under the License 
is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR  CONDITIONS OF ANY 
KIND, either express or implied. See the License for the specific language 
governing permissions and limitations under the License.
-:_basedir: ../../
-:_imagesdir: images/
-
-
-In many cases the value types of properties and of action parameters align.
-For example, a `Customer` entity might have a `surname` property, and there 
might also be corresponding `changeSurname`.
-Ideally we want the surname property and surname action parameter to use the 
same value type.
-
-Since JDO/DataNucleus handles persistence, its annotations are requiredto 
specify semantics such as optionality or maximum length on properties.
-However, they cannot be applied to action parameters.
-It is therefore necessary to use Apache Isis' equivalent annotations for 
action parameters.
-
-The table below summarises the equivalence of some of the most common cases.
-
-.Comparing annotations of Properties vs Action Parameters
-[cols="2,3,3", options="header"]
-|===
-|value type/semantic
-|(JDO) property
-|action parameter
-
-|string (length)
-|`@javax.jdo.annotations.Column(length=50)`
-|`@javax.jdo.annotations.Parameter(maxLength=50)`
-
-|big decimal (precision)
-|`@javax.jdo.annotations.Column(scale=2)`
-|`@javax.validation.constraints.Digits(fraction=2)`
-
-|optionality
-|`@Column(allowsNull="true")`
-|`@Nullable` or `ParameterLayout(optionality=Optionality.OPTIONAL`) (also 
`@Optional`, now deprecated)
-|===
-
-

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