Proof of concept for fluent bindings for ObservableValue

[John Hendrikx]

On 04/10/2021 20:37, Michael Strauß wrote:
> A little bit of bikeshedding here: I think this feature should be
> thought of as a type-safe version of `Bindings.select`, which should
> also be reflected in the terminology used (i.e. not map/flatMap). The
> terminology, combined with the frequent comparisons to reactive
> libraries might make it seem like an attempt to bring reactive
> programming to JavaFX. That will certainly not happen, as JavaFX is
> not (and never will be) a reactive framework. Naming it `select` will
> convey the purpose and its place within JavaFX in a better and clearer
> way.

The PoC is much more than that though. It allows arbitrary conversion of 
property values and creating bindings for these that are automatically 
kept updated. It not only removes the need for Bindings#select but also 
sort of obsoletes many of the functions offered by 
Object/String/IntegerExpression as for most of these #map can be used.

The terms #map and #flatMap (flatten + map) come from functional 
programming which predates reactive programming, and do not necessarily 
imply a stream as used by reactive. Java 8's Optional is a good example.

Both Optional and Stream are part of Java 8 and I contend are far better 
known within the Java community than reactive programming. Therefore the 
terms map and flatMap should be quite well understood by now.

--John

> On Wed, Mar 24, 2021 at 10:49 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
>