[Forwarded from email discussion with Marc.]
I will try to explain the static and dynamic aspects that
were added to cforms with class/new and union. Please let
me know any areas this does not clear up for you.
Class/new in the form model:
A *general* utility to allow for widget definition reuse.
It is not at all specific to the dynamic stuff in the form
model, but just happens to also be useful for that application.
Union in the form model:
Please see the sample form model design GUI for an example
of its use. The widget definitions are handled in the normal
static way at definition building time. The difference is the
widget instances are created on demand at runtime (this is the
"dynamic stuff in the form model"). Note that this non-recursive
use does not require class/new.
Recursive form models:
If we combine the features of class/new and union we can
statically specify theoretically infinitely recursive forms,
where the actual recursive depth is controlled by the data
that is present, such as required to create form-based GUI's.
Problem:
A class that contains a "new" reference to itself would
specify an unlimited recursive definition. A smart builder
would notice that the class contains a reference to itself
and would substitute a simple Java object reference to the
first (and only) definition it built to represent the class.
With this trick we do not run out of memory attempting to
re-create the class definition inside itself over and over.
So we have no problems until we try to create an instance
from this class definition. The instance of the class
cannot use the object reference trick to handle the infinite
recursion, because each nested instance needs to have its
own data. Therefore the simple recursive class definition
cannot be instantiated on finite physical hardware.
Solution:
Since we only have trouble with the instance, we only need
to introduce dynamic behavior in the instance. Notice that
the definitions are never dynamically created or modified.
The only dynamic activity is the act of choosing when to
create instances from the definitions.
We need a way to delay creation of the nested instances
until they are actually required, which implies we need a
way to indicate or determine when the nested instances
become required. This indicator could be stored in a normal
widget to give us all the things a widget gives, such as
binding, validation, and user interaction. While we could
use a boolean (required/not-required), why not make it a more
general selector that indicates which nested instance from
a set of options is currently required. This is what was
implemented as the Union widget. Maybe it should have been
called the "Selector" widget instead?
The rest of the interaction between class/new and union is
just to catch non-terminating recursion caused by form design
errors where class/new and union were not used appropriately
as described above. No use allowing the server to fall over
with out-of-memory errors if we can catch the problem early.
Dynamic behavior in form bindings and templates:
The recursive forms described above still need bindings to
load and save their data and templates to present their data
to the user. This forces us to mirror the class/new and union
concept to the binding and template implementations.
Like the definition handling above, the bindings and templates
can be statically specified just as they normally are, with
just the object reference trick added to the building process
to prevent infinitely recursive building.
The dynamic selection of bindings or templates to process is
controlled by "union" bindings or templates which are driven by
the actual data being loaded or the data in the form being
saved or templated. Notice that the bindings and templates
are never dynamically created or modified. The only dynamic
behavior is the choice of when to invoke the bindings or
templates to perform their binding or templating.
The "static final" issue with the model, binding, and template:
The way class/new resolution currently works prevents the use
of the "final" modifier because the last build step, resolve(),
acts like a linker, replacing each "new" references to a
"class" with references to the definitions, bindings, or
templates contained in the referenced class. Note that this
all occurs at build time, and the definitions, bindings, and
templates are are never modified after the build is completed.
Also note that the definitions, bindings, and templates are
created only once (when their containing class is built) and
they are then only *referenced* with Java object references
into each place where the class is referenced by "new".
As described above, the only places were there is dynamic
behavior is in the choice of when to create widget *instances*,
when to call bindings, and when to perform templating. The
definitions, bindings, and templates themselves are completely
*static* (never modified) after they are first built.
There are various techniques we could use, such as the DOM
tree processing you described, to allow us to use the "final"
modifier. I am not totally convinced (yet) that it is worth
the effort to satisfy the compiler, or that the subtle resolving
issues will be easy to address.
Hope this adds clarity,
--Tim Larson
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