[Android] Prevent ObjectDisposedException in ButtonBackgroundTracker

[velocitysystems]

Description of Change

Fixes an ObjectDisposedException thrown in the ButtonDrawable.Draw method. The drawable itself is held by the new class ButtonBackgroundTracker introduced in the refactoring PR #941 in XF 2.3.5.255-pre5.

Inside the UpdateDrawableMethod(), the field _backgroundDrawable is used to hold a reference to the drawable. Previously, we only recreate the drawable if it was null however this does not account for edge cases where the object has been disposed but is not null.

To ensure we have a valid instance of ButtonDrawable, we initialise a new instance.
Note: I experimented with simply checking the handle for IntPtr.Zero at this point, but this is not 100% effective. It appears there are certain cases whereby the object can be disposed between _nativeButton.SetBackground(_backgroundDrawable); and the native `Draw()' method being invoked.

This is particular noticeable when using advanced layout mechanisms such as Android's RecyclerView.

Bugs Fixed

https://bugzilla.xamarin.com/show_bug.cgi?id=57789

API Changes

None

Behavioral Changes

None

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of master at time of PR
• Changes adhere to coding standard
• Consolidate commits as makes sense

[dnfclas]

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[hartez]

@mattrichnz I believe that PR #1033 fixes this problem - if not, please re-open this PR and we'll take another look at it.

[velocitysystems]

Thanks @hartez, we have tested #1033 but this does not resolve this particular issue.

As mentioned, there appear to be edge cases where "_backgroundDrawable" is getting disposed but is not null so throws an ObjectDisposedException when it is accessed. Prior to #941, this bug did not occur.

We tested a number of different solutions and without rewriting the entire implementation found that ensuring the drawable is recreated each time resolved the issue. Please feel free to improve on this however.