On 12/09/2021 02:05, Nir Lisker wrote:
I've gotten back to look at this.
For now I'm dealing only with the nullableMapping method in Bindings so
we can limit the amount of new classes to LazyObjectBinding
(FlatMapBinding and ConditionalBinding can come later). This method is
used by map, orElse and orElseGet in ObservableValue. Of these, map is
the only fundamental one since the other 2 can be represented by it. I
don't mind keeping them in the discussion, though I will be centered on
the map method.
The implementation of these methods rely on Bindings, LazyObjectBinding,
and Subscription in the current implementation. I think that we can
introduce these internally for now. The biggest hurdle left are the
public changes to ObjectBinding. If we add protected methods, we need to
be sure that by the end of this large task they would have been the
right ones to add and at the right place. This is why I recommend adding
them at the package visibility level and add LazyObjectBinding (and
friends) in its package so they can extend it. I understand that this
can look ugly, but moving internal implementation is cheap, and in this
case, since the coupling involves about 3 classes, is very cheap. This
will lower the initial integration barrier and let the community get
used to- and give feedback on the new changes.
I think that's a good idea, there is no direct need to make those
protected methods part of the public API as the usefulness of those
methods will be limited and the major use case will basically be
provided by LazyObjectBinding already.
This will leave only 1 change that we are committed to, and that's the
new API on ObservableValue (which is the map method in this case). The
method looks good; the only question, which has arisen in a few places,
is how to handle null. As we discussed here, this method works like its
ReactFX counterpart, ignoring null. My questions would be:
1. Is there a good reason to allow null? If so, do we add a new method
for it, or do we pass some parameter to the current method to indicate
that?
In JavaFX, null is something we have to deal in some fashion as
properties can easily be null. For the "primitive" properties, null (if
encountered) is translated to a default value. For StringProperty it
could be an empty string although JavaFX doesn't do this. For
ObjectProperty there is no sensible default possible.
In ReactFX, nulls are indeed skipped when mapping as it considers null
to be an empty value, and empty values are skipped according to the
documentation. The code below will not throw an NPE in the mapping
function and will simply result in null:
Var.newSimpleVar(null).map(x -> x + "2").getValue();
This is similar to the PoC implementation:
new SimpleStringProperty().map(x -> x + "2").getValue());
Having worked with ReactFX and also the PoC, I think it would be very
cumbersome to have to deal with nulls in mapping functions, as many
simple mappings expressed with a short lambda would need to deal with
the null case with a ternary or an if/else block.
In the PoC any mapping you could need that requires mapping null
explicitely can be expressed in another form:
.map(x -> x == null ? "empty!" : x + "2")
becomes:
.map(x -> x + "2").orElse("empty!)
which is not only more concise, but allows to delay dealing with null
until the very end:
.map(x -> x == null ? "empty!" : fetchDataWhichCouldBeNull(x))
.map(x -> x == null ? "empty!" : x + "2")
versus:
.map(Helper::fetchDataThatCouldBeNull)
.map(x -> x + "2")
.orElse("empty!")
You don't have to delay it though, if for some reason you would want to
map the 2nd null case differently, you could use "orElse" after each
mapping still.
Although for mapping this may seem somewhat contrived, for selecting (or
flatMapping) where you go through a chain of properties (like
Node->Scene->Window) being able to delay dealing with nulls leads to
more concise and IMHO more expressive code.
So in summary, and to answer your first question, I don't think there is
a good reason to allow null to be passed to mapping functions. We do
need to deal with nulls though, and that's what "orElse" is for. This
could also be done with an additional parameter to "map" (a
"mapOrDefault" similar to "getOrDefault" from the collections API) but I
think we'd be better served with multiple methods that take a single
argument as the resulting code is easier to understand especially when
one of the arguments is a lambda.
2. If we want to replace the Bindings.select (non-type safe) API, can we
do it with our current way of treating null?
In the current Bindings.select API, null is skipped when encountered and
the resulting value of the chain will be null. This is exactly what
"flatMap" in the PoC does as well, in other words:
Bindings.select(nodeProperty, "scene", "window", "showing")
is exactly equivalent to:
nodeProperty.flatMap(Node::sceneProperty)
.flatMap(Scene::windowProperty)
.flatMap(Window::showingProperty)
No null checks are needed, and the binding will be null if any of the
selected properties contain null. Note that both versions return null
despite the fact that the last property selected is a primitive boolean
which normally cannot hold null.
However, Bindings also offers selectBoolean. In this case it does
indeed return false when any of the properties contains null, but it
also logs this uncomfortable warning then:
WARNING: Value of select-binding has wrong type, returning default value
(+ stack trace omitted)
The warning is somewhat deceptive as the binding does refer to a boolean
so it is not strictly of the wrong type, but an intermediate value
encountered was null and this cannot be cast to a primitive boolean. It
should probably just convert to the default value without any warning.
So, to answer your question, I think we can indeed replace
Bindings.select with the PoC's typesafe equivalent. There is almost no
need to create specific primitive versions of this mechanism as users
can use "orElse" to map to a suitable primitive value as the last step
if null is undesired.
Do you think that this is a valid approach?
Yes, I think our messages may have crossed paths as I suggested limiting
the API (based on your earlier recommendations) in a post on the
"Enhancements for JavaFX 18" thread in a reply to Kevin Rushforth.
Your suggestion takes this a bit further by leaving the new methods in
ObjectBinding package protected and reducing the new API in
ObservableValue to its bare essentials. I think that's a fine approach
as it keeps the API that we're commiting to small and allows the highest
flexibility for future extensions.
If an agreement can be reached on the initial API, I can rework the PoC
and also add the unit tests (I'll need to convert the JUnit 5 tests I
have to JUnit 4).
--John
- Nir
On Wed, Apr 7, 2021 at 11:30 PM John Hendrikx <hj...@xs4all.nl
<mailto:hj...@xs4all.nl>> wrote:
On 07/04/2021 03:41, Nir Lisker wrote:
> In the PoC I made I specifically also disallowed 'null' as an
input
>
>
> I like the way ReactFX does it where the property is empty. I
think that
> this is also what you mean by disallowing `null` (in other
contexts,
> "disallowing null" would mean throwing an exception).
Yes, it is the same concept as ReactFX calling a property "empty",
but I
was hesitant to call this as `null` is a valid value for many JavaFX
properties (a Scene can be null, a String can be null, etc.) which I
don't think means the same as it being empty (in the Optional sense).
But as long as the documentation is clear, I don't mind calling it
either.
>
> Not entirely sure what you mean by this.
>
>
> Basically, what you said. My point was that this is a different API
> section. The first deals with expanding the observables/properties
> methods. The second with listeners methods. Even if mapping a
property
> requires a new listening model, like subscriptions, this is done
under
> the hood. Exposing this API should be a separate step. At least
that's
> how I see it.
Yes, I think it is good to limit new API as much as possible to
reduce
scope and increase the chances of its acceptance. The subscription
parts
can be designed separately and do not need to be public at this
point.
They can be moved to a helper, or the implementation can take the
extra
effort to use standard listeners.
>
> I'd be happy to spend more time and work on this. Perhaps it
would be
> possible to collaborate on this?
>
>
> That would be good. I will need to re-review the ReactFX internals
and
> see how your proposal differs exactly.
Yes, I think that would be good to do.
I've done some comparisons myself and didn't find a difference in
functionality with `Val` (so far). It is a new implementation
though, I
didn't really look at how `Val` was done internally as implementing
it
directly into JavaFX is quite different (I had to make a few minor
changes in `ObjectBinding` to allow for the choice of lazy
binding). I
was also initially more focused on Streams only to realize at a later
point that having a Stream implement ObservableValue was not going
to be
pretty (I suspect this also happened when ReactFX was created, which
is
why Val/Var were later introduced in 2.x).
Both the PoC and Val do lazy binding and are null safe and provide
methods to deal with null/empty.
The main thing I didn't do yet is provide a `filter` method.
Filtering
properties that you want to use for bindings seems awkard as a
binding
should always have some kind of value. The `filter` method in ReactFX
basically maps the value to `null` when it doesn't match the filter.
I've left this out as you can easily achieve this with `map` and
`filter` seems to be too easy to misunderstand.
Aside from that, ReactFX's Val offers a lot of other methods that
are I
think a bit too specialized to consider at this point, like the
`animate`, `pin`, `mapDynamic` and `suspendable` methods.
Val also has all the other `Optional` methods (ifPresent, isPresent,
isEmpty) but I think they may make the API a bit confusing (an
observable value is not the same as an optional). I've also not had a
need for these so far in practice and you can easily convert the
current
value to get this functionality with `Optional.ofNullable`.
Finally Val offers a few methods to convert to ReactFX's streams.
While
convenient, I think static methods like `Values.of`,
`Invalidations.of`
or `Changes.of` would make for a less cluttered API to do stream
conversions -- this would also make it possible to leave this part of
the API up to a 3rd party.
> By the way, do you make a distinction between ReactFX's Val and
Var in
> your proposal (one being read-only)?
No, `ObservableValue` is basically the same as `Val`, and the
equivalent
to `Var` is `ObjectProperty`. Aside from it being a good companion
to
`Val` (and less typing), I don't see a reason to implement `Var`.
--John
>
> On Sun, Apr 4, 2021 at 12:43 PM John Hendrikx <hj...@xs4all.nl
<mailto:hj...@xs4all.nl>
> <mailto:hj...@xs4all.nl <mailto:hj...@xs4all.nl>>> wrote:
>
>
>
> On 02/04/2021 08:47, Nir Lisker wrote:
> > Hi John,
> >
> > I've had my eyes set on ReactFX enhancements for a while too,
> especially as
> > a replacement for the unsafe "select" mechanism. One of the
things
> that
> > kept me from going forward with this is seeing what Valhalla
will
> bring.
> > Generic specialization might save a lot of duplication work
on
> something
> > like this, and Tomas touched another related issue [1], but
since
> it could
> > be a long time before that happens, it's worth planning what
we
> can extract
> > from ReactFX currently.
>
> Agreed, Valhalla is certainly a highly anticipated feature but
I
> fear it
> is still a couple of years away.
>
> Even without any initial support for dealing with "? extends
Number"
> from the various ObservableValue specializations I think
looking into
> this can already be tremendous help.
>
> The proof of concept mainly requires you convert the Number to
a
> suitable type when reading the property but has no problems in
the
> other
> direction:
>
> label.widthProperty().map(Number::doubleValue).map(x -> x
+ 1);
>
> Not pretty, but certainly workable. Specific methods could be
> introduced
> (even at a later time) to make this more streamlined, similar
to what
> the Stream API offers with 'mapToDouble' etc.
>
> > I think that we should break the enhancements into parts.
> > The first that I would advise to look at are the additions to
> > properties/observables. Tomas had to create Val and Var
because he
> couldn't
> > change the core interfaces, but we can. Fitting them with
the Optional
> > methods like `isPresent`, `isEmpty`, `ifPresent`, `map`.
`flatMap`
> etc.;
> > and `select` and friends, is already a good start that will
> address many
> > common requirements.
>
> Yes, Val/Var had to be created for that reason, and also
because
> properties don't quite behave the same as streams -- streams
with a
> "toBinding" method results in things people didn't quite
expect.
>
> As far as the Optional methods go, I'm not entirely sure
properties
> would benefit from all of them. Properties are not immutable
like
> Optional and it may make less sense to fit them with
'isPresent',
> 'isEmpty' and 'ifPresent' ('ifPresent' would I think need to
behave
> similar to 'addListener' or 'subscribe').
>
> In the PoC I made I specifically also disallowed 'null' as an
input for
> functions like 'map' and 'flatMap' (opting to use 'orElse'
semantics
> for
> 'null'), as this for allows much cleaner mapping (and
especially flat
> mapping when selecting nested properties). If 'null' were to be
> allowed,
> I think at a minimum we'd need to add another method to allow
for easy
> selecting of nested properties to avoid:
>
> obs.flatMap(x -> x == null ? null : x.otherProperty())
>
> > The second part is related to listeners. The subscription
model
> and event
> > streams try to solve the memory issues with hard and weak
> references, and
> > allow better composition.
>
> Not entirely sure what you mean by this. JavaFX's current
model uses
> weak references which was I think an unfortunate decision as
it can
> result in huge confusion. For example, a direct binding will
work, but
> with an indirection step a binding stops working:
>
> button.textProperty()
> .concat("World") // weak binding used here
> .addListener((obs, old, cur) ->
System.out.println(cur));
>
> The above stops working, but without the 'concat' it keeps
working.
>
> I think the use of weak listeners should be avoided and
instead other
> mechanisms should be provided to make cleaning up easier. This
is the
> main reason for 'conditionOn' and why ReactFX even had a
specialized
> version of it: 'conditionOnShowing(Node)'.
>
> > The third part is for collections - things like
transformation lists
> > (LiveList) and for other collections.
>
> This is indeed best saved for last. The problems there I think
are less
> of an issue for now.
>
> > Since these share behavior under the hood, we need to look
ahead,
> but in
> > terms of functionality, I think we should take smaller
steps. It
> will also
> > be easier to propose these then.
>
> I've for this reason kept the PoC small with only the most
basic
> functionality. I did however add some work for a different
> subscription
> model, mainly because the internals of this code benefits
greatly from
> it. It is however kept to a minimum.
>
> I'd be happy to spend more time and work on this. Perhaps it
would be
> possible to collaborate on this?
>
> --John
>
> >
> > - Nir
> >
> > [1]
> >
>
https://github.com/TomasMikula/ReactFX/wiki/Creating-a-Val-or-Var-Instance#the-javafx-propertynumber-implementation-issue
> >
> > On Wed, Mar 24, 2021 at 11:49 PM John Hendrikx
<hj...@xs4all.nl <mailto:hj...@xs4all.nl>
> <mailto:hj...@xs4all.nl <mailto:hj...@xs4all.nl>>> wrote:
> >
> >> I just wanted to draw some attention to a recent proof of
concept
> I made
> >> in this pull request:
https://github.com/openjdk/jfx/pull/434
> >>
> >> It is based on the work I did in
> >> https://github.com/hjohn/hs.jfx.eventstream which is in
part based on
> >> work done in ReactFX by Tomas Mikula. The PR itself however
shares no
> >> code with ReactFX and is
> >> completely written by me.
> >>
> >> If there is interest, I'm willing to invest more time in
> smoothing out
> >> the API and documentation, investigating further how this
would
> interact
> >> with the primitive types and adding unit test coverage (I
have
> extensive
> >> tests, but thesea are written in JUnit 5, so they would
require
> >> conversion or JavaFX could move to support JUnit 5).
> >>
> >> What follows below is the text of the PR for easy reading.
> Feedback is
> >> appreciated.
> >>
> >> ================
> >>
> >> This is a proof of concept of how fluent bindings could be
> introduced to
> >> JavaFX. The main benefit of fluent bindings are ease of
use, type
> safety
> >> and less surprises. Features:
> >>
> >> Flexible Mappings
> >> Map the contents of a property any way you like with map,
or map
> nested
> >> properties with flatMap.
> >>
> >> Lazy
> >> The bindings created are lazy, which means they are always
> invalid when
> >> not themselves observed. This allows for easier garbage
> collection (once
> >> the last observer is removed, a chain of bindings will stop
observing
> >> their parents) and less listener management when dealing
with nested
> >> properties. Furthermore, this allows inclusion of such
bindings in
> >> classes such as Node without listeners being created when
the binding
> >> itself is not used (this would allow for the inclusion of a
> >> treeShowingProperty in Node without creating excessive
listeners, see
> >> this fix I did in an earlier PR: #185)
> >>
> >> Null Safe
> >> The map and flatMap methods are skipped, similar to
> java.util.Optional
> >> when the value they would be mapping is null. This makes
mapping
> nested
> >> properties with flatMap trivial as the null case does not
need to be
> >> taken into account in a chain like this:
> >>
>
node.sceneProperty().flatMap(Scene::windowProperty).flatMap(Window::showingProperty).
> >>
> >> Instead a default can be provided with orElse or orElseGet.
> >>
> >> Conditional Bindings
> >> Bindings can be made conditional using the conditionOn
method. A
> >> conditional binding retains its last value when its
condition is
> false.
> >> Conditional bindings donot observe their source when the
condition is
> >> false, allowing developers to automatically stop listening
to
> properties
> >> when a certain condition is met. A major use of this
feature is
> to have
> >> UI components that need to keep models updated which may
outlive
> the UI
> >> conditionally update the long lived model only when the UI
is
> showing.
> >>
> >> Some examples:
> >>
> >> void mapProperty() {
> >> // Standard JavaFX:
> >>
label.textProperty().bind(Bindings.createStringBinding(() ->
> >> text.getValueSafe().toUpperCase(), text));
> >>
> >> // Fluent: much more compact, no need to handle null
> >> label.textProperty().bind(text.map(String::toUpperCase));
> >> }
> >>
> >> void calculateCharactersLeft() {
> >> // Standard JavaFX:
> >>
> >>
>
label.textProperty().bind(text.length().negate().add(100).asString().concat("
> >>
> >> characters left"));
> >>
> >> // Fluent: slightly more compact and more clear (no
negate needed)
> >> label.textProperty().bind(text.orElse("").map(v -> 100 -
> v.length() +
> >> " characters left"));
> >> }
> >>
> >> void mapNestedValue() {
> >> // Standard JavaFX:
> >> label.textProperty().bind(Bindings.createStringBinding(
> >> () -> employee.get() == null ? ""
> >> : employee.get().getCompany() == null ? ""
> >> : employee.get().getCompany().getName(),
> >> employee
> >> ));
> >>
> >> // Fluent: no need to handle nulls everywhere
> >> label.textProperty().bind(
> >> employee.map(Employee::getCompany)
> >> .map(Company::getName)
> >> .orElse("")
> >> );
> >> }
> >>
> >> void mapNestedProperty() {
> >> // Standard JavaFX:
> >> label.textProperty().bind(
> >>
Bindings.when(Bindings.selectBoolean(label.sceneProperty(),
> >> "window", "showing"))
> >> .then("Visible")
> >> .otherwise("Not Visible")
> >> );
> >>
> >> // Fluent: type safe
> >> label.textProperty().bind(label.sceneProperty()
> >> .flatMap(Scene::windowProperty)
> >> .flatMap(Window::showingProperty)
> >> .orElse(false)
> >> .map(showing -> showing ? "Visible" : "Not Visible")
> >> );
> >> }
> >>
> >> void updateLongLivedModelWhileAvoidingMemoryLeaks() {
> >> // Standard JavaFX: naive, memory leak; UI won't get
garbage
> collected
> >>
listView.getSelectionModel().selectedItemProperty().addListener(
> >> (obs, old, current) ->
> >> longLivedModel.lastSelectedProperty().set(current)
> >> );
> >>
> >> // Standard JavaFX: no leak, but stops updating after a
while
> >>
listView.getSelectionModel().selectedItemProperty().addListener(
> >> new WeakChangeListener<>(
> >> (obs, old, current) ->
> >> longLivedModel.lastSelectedProperty().set(current)
> >> )
> >> );
> >>
> >> // Standard JavaFX: fixed version
> >> listenerReference = (obs, old, current) ->
> >> longLivedModel.lastSelectedProperty().set(current);
> >>
> >>
listView.getSelectionModel().selectedItemProperty().addListener(
> >> new WeakChangeListener<>(listenerReference)
> >> );
> >>
> >> // Fluent: naive, memory leak... fluent won't solve
this...
> >> listView.getSelectionModel().selectedItemProperty()
> >>
.subscribe(longLivedModel.lastSelectedProperty()::set);
> >>
> >> // Fluent: conditional update when control visible
> >>
> >> // Create a property which is only true when the UI is
visible:
> >> ObservableValue<Boolean> showing =
listView.sceneProperty()
> >> .flatMap(Scene::windowProperty)
> >> .flatMap(Window::showingProperty)
> >> .orElse(false);
> >>
> >> // Use showing property to automatically disconnect long
lived
> model
> >> // allowing garbage collection of the UI:
> >> listView.getSelectionModel().selectedItemProperty()
> >> .conditionOn(showing)
> >> .subscribe(longLivedModel.lastSelectedProperty()::set);
> >>
> >> // Note that the 'showing' property can be provided in
> multiple ways:
> >> // - create manually (can be re-used for multiple
bindings though)
> >> // - create with a helper: Nodes.showing(Node node) ->
> >> ObservableValue<Boolean>
> >> // - make it part of the Node class; as the fluent
bindings
> only bind
> >> themselves
> >> // to their source when needed (lazy binding), this
won't create
> >> overhead
> >> // for each node in the scene
> >> }
> >> Note that this is based on ideas in ReactFX and my own
experiments in
> >> https://github.com/hjohn/hs.jfx.eventstream. I've come to
the
> conclusion
> >> that this is much better directly integrated into JavaFX,
and I'm
> hoping
> >> this proof of concept will be able to move such an effort
forward.
> >>
> >> --John
> >>
> >
>
