RFR: 8294809: ListenerHelper for managing and disconnecting listeners

[Andy Goryachev]

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

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Commit messages:
 - 8294809: use weak reference correctly this time
 - 8294809: tests
 - 8294809: remove
 - 8294809: change listener with callback
 - 8294809: skin base
 - 8294809: whitespace
 - 8294809: event handlers and filters
 - 8294809: cleanup
 - 8294809: override
 - 8294809: null checks
 - ... and 4 more: https://git.openjdk.org/jfx/compare/9768b5e4...af77693c

Changes: https://git.openjdk.org/jfx/pull/908/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=908&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294809
  Stats: 1498 lines in 4 files changed: 1495 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/908.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/908/head:pull/908

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