RFR: 8274771: Map, FlatMap and OrElse fluent bindings for ObservableValue [v18]

[Kevin Rushforth]

On Wed, 6 Jul 2022 07:07:20 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

>> I have yet another question. The following test passes for `Bindings.select`, but fails for `ObservableValue.flatMap`:
>> 
>> 
>> JMemoryBuddy.memoryTest(test -> {
>>     class ValueHolder {
>>         final StringProperty value = new SimpleStringProperty(this, "value");
>>         StringProperty valueProperty() { return value; }
>>     }
>> 
>>     ObjectProperty<ValueHolder> valueHolderProperty = new SimpleObjectProperty<>();
>>     valueHolderProperty.set(new ValueHolder());
>> 
>>     // Map the nested property value
>>     ObservableValue<String> mapped = valueHolderProperty.flatMap(ValueHolder::valueProperty);
>>     
>>     // Note: the test passes when using the following alternative to flatMap:
>>     // ObservableValue<String> mapped = Bindings.selectString(valueHolderProperty, "value");
>> 
>>     // Bind the mapped value to a property that will soon be GC'ed.
>>     ObjectProperty<String> otherProperty = new SimpleObjectProperty<>();
>>     otherProperty.bind(mapped);
>> 
>>     test.setAsReferenced(valueHolderProperty);
>>     test.assertCollectable(otherProperty);
>>     test.assertCollectable(mapped); // expectation: the mapped value is eligible for GC
>> });
>> 
>> 
>> My observation is that a flat-mapped value that was once observed is not eligible for garbage-collection even when the observer itself is collected. This seems to be quite unexpected to me, because it means that a bound property that is collected without being manually unbound will cause a memory leak in the mapped binding.
>> 
>> Is this by design? If so, I think this can lead to subtle and hard to diagnose bugs, and should be documented at the very least.
>
> Some more about the GC problem discovered by @mstr2 
> 
> ### How to deal with this when using Fluent bindings (`conditionOn`)
> 
> In the initial proposal, there was a `conditionOn` mechanism and `Subscription` mechanism.  `conditionOn` can be used to make your bindings conditional on some external factor to ensure they are automatically cleaned up. The Fluent bindings only require their final result to be unsubscribed, as all intermediate steps will unsubscribe themselves from their source as soon as they themselves become unobserved:
> 
>> a listens to b, listens to c
> 
> If `a` becomes unobserved, it unsubscribes itself from `b`, which unsubscribes itself from `c`. `c` is now eligible for GC. With standard JavaFX listeners, such a chain must be unsubscribed at each step making it almost impossible to use in practice.
> 
> Using `conditionOn` the chain of mappings can be automatically unsubscribed:
> 
>       ObservableValue<Boolean> condition = ... ;
> 
>       longLivedProperty.conditionOn(condition)
>            .map(x -> x + "%")
>            .addListener((obs, old, current) -> ... );
> 
> The condition can be anything, like a `Skinnable` reference becoming `null`, a piece of UI becoming invisible, etc.
> 
> Note that even though `conditionOn` is currently not available as a nice short-cut, you can still do this with the current implementation:
> 
>       ObservableValue<Boolean> condition = ... ;
> 
>       condition.flatMap(c -> c ? longLivedProperty : null)
>            .map(x -> x + "%")
>            .addListener((obs, old, current) -> ... );
> 
> `longLivedProperty` will be unsubscribed as soon as `condition` becomes false.
> 
> ### How to deal with this when using Fluent bindings (`Subscription`)
> 
> Although `conditionOn` is IMHO by far the preferred mechanism to handle clean-up, `Subscription` also could be very useful. It is less awkward to use than `addListener` / `removeListener` because the `Subscription` is returned:
> 
>      ChangeListener<ObservableValue<String>, String, String> listener = (obs, old, current) -> ... ;
>      x.addListener(listener);
>      x.removeListener(listener);
> 
> vs:
> 
>      Subscription s = x.subscribe((obs, old, current) -> ... );
>      s.unsubscribe();
> 
> Subscriptions can also be combined:
> 
>      Subscription s = x.subscribe((obs, old, current) -> ... )
>          .and(y.subscribe( ... ))
>          .and(z.subscribe( ... ));
>     
>      s.unsubscribe();   // releases listeners on x, y and z
> 
> ### Dealing with "stub" memory leak in current JavaFX
> 
> Relying on `invalidated` or `changed` being called to clean up dead listeners is perhaps not ideal. It may be an idea to start using a `ReferenceQueue` where all such stubs are registered when they get GC'd. As JavaFX is already continuously running its UI and render threads, it is no great leap to check this `ReferenceQueue` each tick and pro-actively clean up these stubs.  Alternatively, a single daemon thread could be used specifically for this purpose.  The FX thread would be more suitable however as listener clean-up must be done on that thread anyway.
> 
> This would solve the issue found by @mstr2 in any normal JavaFX application (one where the FX thread is running), and would also solve the issue I highlighted with stubs not being cleaned up in my test program.

@hjohn Thanks for your detailed analysis of the bindings GC situation. The conclusion I take from all of this is that the current implementation is fine, but we might consider a follow-up issue to clean up dead listener stubs. If so, my initial take on that later cleanup issue is that we might use the Disposer mechanism on a thread other than the JavaFX application thread, but that can be a topic for later discussion.

@mstr2 What are your thoughts on this?

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PR: https://git.openjdk.org/jfx/pull/675