[gtk] Make Fonts GC disposable

[laeubi]

Current State

Currently handling fonts in SWT is hard and cumbersome, because one can't use the usual java techniques users are forced to either explicitly manage fonts (what maybe retain them longer as needed) or even create and dispose them for individual widgets (what hurts performance). This also leads to a lot of boilerplate code e.g. installing dispose handlers or having managed caches or local resource managers as well as requires documentation who 'owns' a font for example set on a label. Especially for newcommers or seldom users of SWT this is hard to get right and hinders them to use SWT correctly as java is usually managing objects automatically.

What this changes

This now introduces the concept of Cleaners added to Java 9+ that allows automatic management of external resources that get automatically disposed when the JVM decides that an object is only phantom reachable and eligible for garbage collection.

I also used this to have a Handle act as a proxy to the native method calls so these are much more readable and the grouped together even related calls or required transformations to reduce code duplication.

Further considerations

This itself is just meant as an initial starting point, we actually should extends this concept to any native resource handled by SWT (e.g. Images) so it become much more convenient for users to use these. I just choosing a font here as the only managed resource is a long pointer.

I also like to encourage users of other platforms (Win/Mac) to use this as a hint to prototype something similar like this as well so we can evaluate how it works out.

Also this will not replace dispose() or make it completely redundant as of course a user can always better dispose a resource directly if it is known that it is no longer in use. Also it might not be suitable for any "root objects" like a Display that is like a Thread in java some kind of UI root that always needs to be disposed explicitly.

[github-actions]

Test Results

   299 files  ±0     299 suites  ±0   5m 58s ⏱️ -57s
 4 099 tests ±0   4 091 ✅ ±0   8 💤 ±0  0 ❌ ±0 
12 209 runs  ±0  12 134 ✅ ±0  75 💤 ±0  0 ❌ ±0 

Results for commit f534d57. ± Comparison against base commit 7b18570.

♻️ This comment has been updated with latest results.

[laeubi]

I adjusted the title a bit now to make clear I don't want to replace the current disposal of resources, but make them GC disposable (that is for example it would be possible to hold Weak/Soft/PhantomReferences of items).

[mickaelistria]

That seems very good and can probably prevent some "no more handles" error from happening.
Just one question (not a change request), could this apply to any resource, eg also Colors, Cursor, Image...?

[laeubi]

could this apply to any resource, eg also Colors, Cursor, Image...?

Yes this basically could (and should) be applied everywhere we are holding a native resource that must be disposed when the object is disposed. (Colors don't need to be disposed anyways but thats a different topic).

[laeubi]

One example where it is useful to only hold a SoftReference is for example here:

• OutOfMemoryError in Plug-in Image Browser eclipse-pde/eclipse.pde#1179

Currently with SWT I only have two choices:

1. I hold a hard reference and only free that on request, e.g. when the control is disposed, or even when the bundle is stopped, e.g. by using ResourceManagers
2. I only hold a SoftReference to the ImageData, but this means I need to create a new Image each time I want to paint that and instantly dispose it, what makes it quite cumbersome and maybe not very performant.

A common usage for this would be some kind of a custom drawn control, where I want to store the last drawn state as an Image and repaint just paint the image.