Add keepalive cache to keep objects stored as void* from being gc'ed.

[mCodingLLC]

Fixes: #248

Here is an alternative fix for #248. This approach keeps a cache of objects stored as void pointers so that the corresponding Python objects are not garbage collected before they are needed. The cache is cleared every new_frame(). The cache is not threadsafe, but I assume pyimgui does not intend to be threadsafe. The cache could cause memory use to increase without bound if the user does something nonsensical, e.g.

atlas = ... #some _FontAtlas object
while True:
    atlas.texture_id = 0 # copies of 0 will keep getting added to the cache until new_frame() is called.

I chose the cache to be a list because it would be the fastest and easiest option.
Another option would be to use a dict of id(object) -> object, which would be slower but not suffer from the memory issue above.

[KinoxKlark]

Indeed, pyimgui does not intent to be thread-safe. DearImgui itself is not thread-safe. It is the responsibility of the user to take care of that if it needs it. So I don't think this is an issue here.

Thank you very much for your help!

I added test cases to reproduce issue #248. I also changed _keepalive_cache to be inside _ImGuiContext, this leads to some refactoring of context management. Unfortunately, it seems that I can't push my changes to your branch. Can you grant me access please ? (https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/allowing-changes-to-a-pull-request-branch-created-from-a-fork)

[mCodingLLC]

Hmm, I have (and had) "Allow edits by maintainers" checked, so you should have access.

[learn-more]

Indeed, pyimgui does not intent to be thread-safe. DearImgui itself is not thread-safe. It is the responsibility of the user to take care of that if it needs it. So I don't think this is an issue here.

Thank you very much for your help!

I added test cases to reproduce issue #248. I also changed _keepalive_cache to be inside _ImGuiContext, this leads to some refactoring of context management. Unfortunately, it seems that I can't push my changes to your branch. Can you grant me access please ? (https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/allowing-changes-to-a-pull-request-branch-created-from-a-fork)

afaik editing a contributors branch only work with https, not over ssh.

[KinoxKlark]

Hmm, I have (and had) "Allow edits by maintainers" checked, so you should have access.

Yes indeed, my bad. Apparently, this was a bug with the push command.

Here is my proposal for inserting _keepalive_cache inside _ImGuiContext. I had to refactor a bit _ImGuiContext to be able to retrieve an instance of its python version. Originally we created a new instance from cpp pointer every time we wanted to work with it. I hope this is not too much of an overload.

What do you think about it?

[KinoxKlark]

Thanks for your help!