Re: [External] : Aw: Proposal: Add Skin.install() method

[Andy Goryachev]

You do bring a good point!  I don't know the rationale behind passing control pointer to the Skin constructor.

I think Swing got it right, clearly separating


  *   instantiation (using either a no-arg constructor, or any other constructor that does not require component pointer)
  *   configuration (optional step, possibly widely separated in time and space)
  *   uninstallation of the old skin
  *   installation of the new skin

What you are proposing - moving to a default constructor makes the most sense.  It comes with a high price though - everyone with a custom skin implementation would need to change their code.

At the very minimum, we should explain in Skin javadoc that creating a skin for one control and setting it in the other is a no-no.  Or, perhaps we should explicitly check for this condition in setSkin().

Thank you
-andy


From: Marius Hanl <mariushanl at web.de>
Date: Friday, 2022/07/22 at 05:06
To: openjfx-dev at openjdk.org <openjfx-dev at openjdk.org>, Andy Goryachev <andy.goryachev at oracle.com>
Subject: [External] : Aw: Proposal: Add Skin.install() method
I had a similar idea in the past and like the idea.
Ideally, setting/switching a skin is a one step process. Currently you can construct a skin for a control and set it after to a different control.

Your approach sounds good, if you can set a skin by creating a new skin (with a default constructor) and then the setSkin() method will actually trigger the install process on the control (this), this will work and solve the problem above. But for backward compatibilty we still need to keep the skin constructor with the control as parameter and think about deprecating it.

-- Marius
Am 20.07.22, 23:40 schrieb Andy Goryachev <andy.goryachev at oracle.com>:
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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