RFR: 8294809: ListenerHelper for managing and disconnecting listeners

[Andy Goryachev]

On Fri, 7 Oct 2022 20:50:55 GMT, Andy Goryachev <angorya at openjdk.org> wrote:

> Introduction
> 
> There is a number of places where various listeners (strong as well as weak) are added, to be later disconnected in one go. For example, Skin implementations use dispose() method to clean up the listeners installed in the corresponding Control (sometimes using LambdaMultiplePropertyChangeListenerHandler class).
> 
> This pattern is also used by app developers, but there is no public utility class to do that, so every one must invent their own, see for instance
> https://github.com/andy-goryachev/FxTextEditor/blob/master/src/goryachev/fx/FxDisconnector.java
> 
> Proposal
> 
> It might be beneficial to create a class (ListenerHelper, feel free to suggest a better name) which:
> 
> - provides convenient methods like addChangeListener, addInvalidationListener, addWeakChangeListener, etc.
> - keeps track of the listeners and the corresponding ObservableValues
> - provides a single disconnect() method to remove all the listeners in one go.
> - optionally, it should be possible to add a lambda (Runnable) to a group of properties
> - optionally, there should be a boolean flag to fire the lambda immediately
> - strongly suggest implementing an IDisconnectable functional interface with a single disconnect() method
> 
> Make it Public Later
> 
> Initially, I think we could place the new class in package com.sun.javafx.scene.control; to be made publicly accessible later.
> 
> Where It Will Be Useful
> 
> [JDK-8294589](https://bugs.openjdk.org/browse/JDK-8294589) "MenuBarSkin: memory leak when changing skin"
> and other skins, as a replacement for LambdaMultiplePropertyChangeListenerHandler.
> 
> https://github.com/openjdk/jfx/pull/908#:~:text=19%20hours%20ago-,8295175%3A%20SplitPaneSkinSkin%3A%20memory%20leak%20when%20changing%20skin%20%23911,-Draft
> 
> https://github.com/openjdk/jfx/pull/914

Thank you @nlisker and @hjohn for providing the link and explanations.  

You are right, ListenerHelper is somewhat similar to Subscription in that it allows for quick disconnect().

It is, however, designed for a different purpose, that goes beyond what is needed from a simple listener management.  Specifically,

- it assumes multiple listeners/handlers/callbacks from the start
- it allows for invoking listeners/callbacks immediately as it is a frequent pattern not only in Skins but also in application code (I do want to make this class a part of public API eventually)
- unlike Subscription, we can addEventHandlers and addEventFilters
- unlike Subscription, we can add Weak listeners/callbacks, though I admit it might be a design decision to use or not to use that

to summarize, I see some minor overlap between Subscription and ListenerHelper, but mostly these are two different beasts designed for two different purposes.

I do agree with the call to cooperate!  Would love to.

what do you think?

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PR: https://git.openjdk.org/jfx/pull/908