Android Shadow performance is poor

[albyrock87]

Description

The speedscope

Apparently DrawShadow has an absurd impact on the performance.
Here's a speedscope from our app sorted by self time descending, also look at total time %!!!

Why is this happening?

This is what happened while drawing the Android.Views.View every time you enable a Shadow somewhere:

1. It creates a shadow Bitmap in memory
2. It draws the descendants on top of that Bitmap
3. It creates another Bitmap to extract Alpha channel (extractAlpha)
4. It clears the shadow Bitmap
5. It draws the alpha bitmap to the shadow bitmap with blur and coloring
6. Shadow bitmap is cached on a field to be reused on subsequent draw
7. It draws the shadow bitmap to the rendering pipeline canvas
8. It draws the descendants once again, but this time on the rendering pipeline canvas

You can imagine the kind of slow down this generates when a shadow is applied to a node which has a lot of descendants.
The cache at field level is really a poor one, in the sense that invalidation happens every time something changes on the descendants via requestLayout.

Steps to Reproduce

Use a real-world application containing shadows, profile it, and see how drawShadows consumes a lot of CPU.

Link to public reproduction project repository

No response

Version with bug

9.0.30 SR3

Is this a regression from previous behavior?

No, this is something new

Last version that worked well

Unknown/Other

Affected platforms

Android

Affected platform versions

No response

Did you find any workaround?

No workaround available besides not using shadows.

[StephaneDelcroix]

/cc @jonathanpeppers

[jonathanpeppers]

Android has always been slow (literally for decade+) as soon as:

• You use Android.Graphics.Bitmap: C# GC doesn't know the true size of these, and so OOM can result.
• You use Android.Graphics.Canvas: if I remember correctly, is never hardware accelerated.

So, if there is a way to implement shadow with Drawable instead, that will perform much better.