Allocations in gesture-related code for Windows

[MartyIX]

@jonathanpeppers I think that this method

maui/src/Controls/src/Core/EnumerableExtensions.cs

Line 46 in c754463

public static IEnumerable<T> GetGesturesFor<T>(this IEnumerable<IGestureRecognizer> gestures, Func<T, bool> predicate = null) where T : GestureRecognizer

allocates needlessly¹ in some cases.

The allocation is here:

maui/src/Controls/src/Core/EnumerableExtensions.cs

Line 54 in c754463

foreach (IGestureRecognizer item in new List<IGestureRecognizer>(gestures))

- it does a defensive copy so that one can safely add/remove a gesture later on.

The use cases are

• maui/src/Controls/src/Core/Platform/GestureManager/GesturePlatformManager.Windows.cs

  Lines 717 to 718 in c754463

  	if (gestures.GetGesturesFor<TapGestureRecognizer>(g => g.NumberOfTapsRequired == 1).Any()
  	|| children?.GetChildGesturesFor<TapGestureRecognizer>(g => g.NumberOfTapsRequired == 1).Any() == true)

  - there should be imo a new method (e.g. CountGestures<T>(predicate)?) to just answer the query (e.g. CountGestures<T>(predicate) or AnyGesture<T>(predicate)).

• maui/src/Controls/src/Core/Platform/GestureManager/GesturePlatformManager.Windows.cs

  Lines 750 to 754 in c754463

  	bool hasSwipeGesture = gestures.GetGesturesFor<SwipeGestureRecognizer>().GetEnumerator().MoveNext();
  	bool hasPinchGesture = gestures.GetGesturesFor<PinchGestureRecognizer>().GetEnumerator().MoveNext();
  	bool hasPanGesture = gestures.GetGesturesFor<PanGestureRecognizer>().GetEnumerator().MoveNext();
  	if (!hasSwipeGesture && !hasPinchGesture && !hasPanGesture)
  	return;

  - this is also a bit weird and if it were in the if statement then less work would be done.
• children?.GetChildGesturesFor<TapGestureRecognizer> appears multiple times in the UpdatingGestureRecognizers method (

  maui/src/Controls/src/Core/Platform/GestureManager/GesturePlatformManager.Windows.cs

  Line 702 in c754463

  void UpdatingGestureRecognizers()

  ) but seemingly gestures cannot change in that method.

I'm writing this writeup because I can see in my speedscope:

Would it make sense to you to make some modifications in this area?

Footnotes

1. Added in 2018, here https://github.com/xamarin/Xamarin.Forms/pull/2828. ↩

[MartyIX]

I opened #21450 with these changes.