Hi, I'd like to propose an API change in Skin interface (details below). Your feedback will be greatly appreciated!
Thank you, -andy Summary ------- Introduce a new Skin.install() method with an empty default implementation. Modify Control.setSkin(Skin) implementation to invoke install() on the new skin after the old skin has been removed with dispose(). Problem ------- Presently, switching skins is a two-step process: first, a new skin is constructed against the target Control instance, and is attached (i.s. listeners added, child nodes added) to that instance in the constructor. Then, Control.setSkin() is invoked with a new skin - and inside, the old skin is detached via its dispose() method. This creates two problems: 1. if the new skin instance is discarded before setSkin(), it remains attached, leaving the control in a weird state with two skins attached, causing memory leaks and performance degradation. 2. if, in addition to adding listeners and child nodes, the skin sets a property, such as an event listener, or a handler, it overwrites the current value irreversibly. As a result, either the old skin would not be able to cleanly remove itself, or the new skin would not be able to set the new values, as it does not know whether it should overwrite or keep a handler installed earlier (possibly by design). Unsurprisingly, this also might cause memory leaks. We can see the damage caused by looking at JDK-8241364<https://bugs.openjdk.org/browse/JDK-8241364> ☂ Cleanup skin implementations to allow switching, which refers a number of bugs: JDK-8245145 Spinner: throws IllegalArgumentException when replacing skin JDK-8245303 InputMap: memory leak due to incomplete cleanup on remove mapping JDK-8268877 TextInputControlSkin: incorrect inputMethod event handler after switching skin JDK-8236840 Memory leak when switching ButtonSkin JDK-8240506 TextFieldSkin/Behavior: misbehavior on switching skin JDK-8242621 TabPane: Memory leak when switching skin JDK-8244657 ChoiceBox/ToolBarSkin: misbehavior on switching skin JDK-8245282 Button/Combo Behavior: memory leak on dispose JDK-8246195 ListViewSkin/Behavior: misbehavior on switching skin JDK-8246202 ChoiceBoxSkin: misbehavior on switching skin, part 2 JDK-8246745 ListCell/Skin: misbehavior on switching skin JDK-8247576 Labeled/SkinBase: misbehavior on switching skin JDK-8253634 TreeCell/Skin: misbehavior on switching skin JDK-8256821 TreeViewSkin/Behavior: misbehavior on switching skin JDK-8269081 Tree/ListViewSkin: must remove flow on dispose JDK-8273071 SeparatorSkin: must remove child on dispose JDK-8274061 Tree-/TableRowSkin: misbehavior on switching skin JDK-8244419 TextAreaSkin: throws UnsupportedOperation on dispose JDK-8244531 Tests: add support to identify recurring issues with controls et al Solution -------- This problem does not exist in e.g. Swing because the steps of instantiation, uninstalling the old ComponentUI ("skin"), and installing a new skin are cleanly separated. ComponentUI constructor does not alter the component itself, ComponentUI.uninstallUI(JComponent) cleanly removes the old skin, ComponentUI.installUI(JComponent) installs the new skin. We should follow the same model in javafx. Specifically, I'd like to propose the following changes: 1. Add Skin.install() with a default no-op implementation. 2. Clarify skin creation-attachment-detachment sequence in Skin and Skin.install() javadoc 3. Modify Control.setSkin(Skin) method (== invalidate listener in skin property) to call oldSkin.dispose() followed by newSkin.install() 4. Many existing skins that do not set properties in the corresponding control may remain unchanged. The skins that do, such as TextInputControlSkin (JDK-8268877), must be refactored to utilize the new install() method. I think the refactoring would simply move all the code that accesses its control instance away from the constructor to install(). Impact Analysis ------------- The changes should be fairly trivial. Only a subset of skins needs to be refactored, and the refactoring itself is trivial. The new API is backwards compatible with the existing code, the customer-developed skins can remain unchanged (thanks to default implementation). In case where customers could benefit from the new API, the change is trivial. The change will require CSR as it modifies a public API.
