Proposal: Add Skin.install() method

[John Hendrikx]

Hi Andy,

Was a single step install process considered, something like:

      control.installSkin(MySkin::new);

This would also make it much more clear that Skins are single use only, 
where the API currently has to bend over backwards to make that clear 
and enforce it.

Other than that, I think your suggestion would be a definite improvement 
over the current situation. Something never felt quite right about how 
skins where set up and attached to controls, it felt fragile and 
cumbersome -- possibly as the result of relying on a writable property 
as the means to install a skin.

--John

On 20/07/2022 23:39, Andy Goryachev wrote:
>
> 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.
>
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