RFR: 8274771: Map, FlatMap and OrElse fluent bindings for ObservableValue [v8]

[Nir Lisker]

On Thu, 27 Jan 2022 21:49:07 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

>> This is an implementation of the proposal in https://bugs.openjdk.java.net/browse/JDK-8274771 that me and Nir Lisker (@nlisker) have been working on.  It's a complete implementation including good test coverage.  
>> 
>> This was based on https://github.com/openjdk/jfx/pull/434 but with a smaller API footprint.  Compared to the PoC this is lacking public API for subscriptions, and does not include `orElseGet` or the `conditionOn` conditional mapping.
>> 
>> **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: https://github.com/openjdk/jfx/pull/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`.
>> 
>> 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")
>>       );
>>     }
>> 
>> 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 Hendrikx has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix wrong test values

The tests look good. I'm happy with the coverage and added one comment about a missing case. My own sanity checks also work as I expect.

Will approve when these are addressed as I have already reviewed the API and implementation.

modules/javafx.base/src/test/java/test/javafx/beans/value/ObservableValueFluentBindingsTest.java line 42:

> 40: 
> 41: import javafx.beans.property.ObjectProperty;
> 42: import javafx.beans.property.SimpleObjectProperty;

Unused imports

modules/javafx.base/src/test/java/test/javafx/beans/value/ObservableValueFluentBindingsTest.java line 58:

> 56:         @Nested
> 57:         class WithNull {
> 58:             @Test

Maybe some line separation between the classes is helpful. Same for other places in this class.

modules/javafx.base/src/test/java/test/javafx/beans/value/ObservableValueFluentBindingsTest.java line 356:

> 354:             }
> 355:         }
> 356:         // TODO test for when something is flatMapped to null in getValue call

Is this still relevant? I think that you tested it in line 407 `shouldIgnoreFlatMapsToNull`.

What I would like to see is a test when `left`, `right` or `unknown`'s value is set to `null`. What I see is `property`'s value being set to `null`, or the reference of one of the above being set to `null`, but not the value itself. Only when you compose `orElse` on `flatMap` this case is tested (line 604).
Is this what you meant?

modules/javafx.base/src/test/java/test/javafx/beans/value/ObservableValueFluentBindingsTest.java line 363:

> 361:             private StringProperty right = new SimpleStringProperty("RIGHT");
> 362:             private StringProperty unknown = new SimpleStringProperty("UNKNOWN");
> 363:             private ObservableValue<String> observableValue = property.flatMap(v -> "Left".equals(v) ? left : "Right".equals(v) ? right : unknown);

Maybe break this line since it's too long. I think that 120 characters is the maximum length.

modules/javafx.base/src/test/java/test/javafx/beans/value/ObservableValueFluentBindingsTest.java line 379:

> 377:                 @Test
> 378:                 void shouldReturnPropertyValuesWithOperationApplied() {
> 379:                     assertEquals("UNKNOWN", observableValue.getValue());  // initially it is not left or right, so unknown

Maybe these comments should be in the `String message` argument? I don't mind either way.

modules/javafx.base/src/test/java/test/javafx/beans/value/ObservableValueFluentBindingsTest.java line 457:

> 455:                     right = null;
> 456: 
> 457:                     property.set("Right");

Isn't `unknown = null;` enough like you did previously? It doesn't really matter, just for consistency.

modules/javafx.base/src/test/java/test/javafx/beans/value/ObservableValueFluentBindingsTest.java line 648:

> 646: 
> 647:     /**
> 648:      * Ensures nothing has been observed.

"Ensures nothing has been observed since the last check" or something like that because the values get cleared.

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PR: https://git.openjdk.java.net/jfx/pull/675