Remove List<IObserver<T>>.ToArray() allocations in LightweightObservableBase

[xoofx]

Hello folks, ☺️

What does the pull request do?

This PR is removing the List<IObserver<T>>.ToArray() allocations happening in LightweightObservableBase when Routing events are fired (e.g. whenever you move the mouse)

When profiling the memory, I noticed that when generating lots of routing events (e.g. just moving the mouse over a window) several MB of IObserver<ValueTuple<Object, RoutedEventArgs>>[] were created.

See for example here with the ControlDialog application by just moving the mouse on the Window for a few seconds:

What is the current behavior?

I noticed 2 problems:

1. The case of array 0 was not covered and always allocating an empty array (at minimum 32 bytes per alloc on x64)
2. Several other smaller allocs could happen for size 2/3
3. Depending on the number of controls, the size of the allocation can grow quickly (several controls having to be notified)

For example, here are some statistics about these array allocations:

Array size to count:

• Array size: 0 bytes, Count: 31926
• Array size: 1 bytes, Count: 29254
• Array size: 2 bytes, Count: 278919
• Array size: 3 bytes, Count: 5567
• Array size: 4 bytes, Count: 2141

Total array count: 347807
Total array size in bytes: 13246224 bytes

What is the updated/expected behavior with this PR?

This PR is introducing the case for 0/2/3 elements and pooling the array for other cases with ArrayPool.

After this PR, the allocation is removed:

A direct side effect is less GC collections:

Before this PR, a trigger of the GC happens after 4.6s

After this PR, a trigger of the GC happens after 7s.

How was the solution implemented (if it's not obvious)?

Cases of 0/2/3 and ArrayPool.

I have left the code that can print some statistics commented. If it is a problem, I can remove it from the PR.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

None

Obsoletions / Deprecations

None

Fixed issues

None

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0055151-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0055153-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

LGTM, thank you! Reducing allocations is always nice :)

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0055161-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[grokys]

Thanks for this! This is one of those small things that I intended to fix every time I stepped through this code but never got round to!