[BUG] (Performance) SkSwapChaninPanel makes unnecessary per-frame heap allocations

[akater320]

SkSwapChainPanel::OnRenderFrame(Rect) makes 2 heap allocations. It would be preferable to not have any allocations in the hot path.

1. SkAutoCanvasRestore(...), The RAII semantics are neat, but maybe this could be changed to one of the new disposable stack-scoped structs?
2. Can SKPaintGLSurfaceEventArgs be reused rather than allocated each frame?

SkiaSharp/source/SkiaSharp.Views/SkiaSharp.Views.UWP/SKSwapChainPanel.cs

Line 85 in 354d85d

using (new SKAutoCanvasRestore(canvas, true))

[mattleibow]

We can most definitely have a look at the disposable structs - or actually not use it at all. All it does is a Restore

We can't reuse the args as this is a breaking change. For some reason, the developer may store a reference to the object and changing it may be unexpected. You shouldn't, but someone might...

[akater320]

Fair enough.
Is there any option get a no-garbage path? (Maybe it already exists somewhere else?)
Certainly not the most elegant, but maybe add a second PaintSuface type event to SKSwapChainPanel with a new signature. At the same time make it possible to get a reference to the SKCanvas without using SKPaintGLSurfaceEventArgs.Surface.Canvas{get;} (which, rather surprisingly, makes a managed heap allocation).
I'd be happy to submit a PR if it fits the project.

[mattleibow]

I see the two allocations per frame... We can totally replace the SKAutoCanvasRestore with a canvas.RestoreToCount(0); after the draw. We just need to do this to "reset" the canvas each frame.

We could potentially add a property to the view to actually opt-into a zero allocation loop. If you are really looking for this perf, then set the prop and the args will be re-used....

I am thinking that SKPaintGLSurfaceEventArgs.Surface.Canvas allocation is now fixed on the later previews. I now cache everything since they stay valid. If you are not able to upgrade, then I think you might be fine to just cache the value of that property and make sure to only use it in the draw frame.

[akater320]

Awesome. Thanks!