Proof of concept for fluent bindings for ObservableValue

[John Hendrikx]

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 <hjohn at 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
>>
>