[android] fix memory leak in TabbedPage

[jonathanpeppers]

Fixes: #17959
Context: https://github.com/VoznichkaSviatoslav1/TabbedPageMemoryLeak

In the sample app, I added some GC.Collect() calls to force the GC to run, but the following log message never prints:

public partial class MyTabbedPage : TabbedPage
{
    public MyTabbedPage()
    {
        InitializeComponent();

        Console.WriteLine("Constructor was called");
    }

    // Doesn't print!!!
    ~MyTabbedPage() => Console.WriteLine("Destructor was called");

I took a memory snapshot with dotnet-gcdump and found a tree like:

SampleApp.MyTabbedPage ->
    Microsoft.Maui.Controls.Platform.MultiPageFragmentStateAdapter<Microsoft.Maui.Controls.Page> ->
        Action<Microsoft.Maui.Controls.Platform.AdapterItemKey>

I didn't find an explanation of what was holding
MultiPageFragmentStateAdapter. I tried eliminating the Action<T>, but that didn't solve the underlying issue; it just made the object tree shorter:

SampleApp.MyTabbedPage ->
    Microsoft.Maui.Controls.Platform.MultiPageFragmentStateAdapter<Microsoft.Maui.Controls.Page>

I could also reproduce the problem in a new device test.

Reviewing the code, I found we were doing:

_viewPager.Adapter = new MultiPageFragmentStateAdapter<Page>(tabbedPage, FragmentManager, _context) { CountOverride = tabbedPage.Children.Count };

But then in the cleanup code, we never set Adapter to null:

_viewPager.Adapter = null;

After this change, the memory is released as expected. I am not exactly sure why this leaked, but the change does seem appropriate and fixes the problem.

Now the new device test passes & the sample app prints:

03-14 15:06:32.394  6055  6055 I DOTNET  : Constructor was called
03-14 15:06:37.008  6055  6089 I DOTNET  : Destructor was called

[jonathanpeppers]

So I fixed Android, but the new test discovered it leaks on Windows?

[jsuarezruiz]

Weird, I have triggered the UITests again.

[jonathanpeppers]

No Windows is actually leaking when you use TabbedPage:

I haven't figured it out yet.

[jonathanpeppers]

I disabled the test on Windows for now, since the original issue is Android. I spent 2 hours commenting out code, and can't seem to figure it out.

I don't understand what the [Dependent Handle] entries in the above screenshot are on Windows. Is there a doc somewhere on internals of WindowsAppSDK?

[jonathanpeppers]

WindowsAppSDK looks like it makes extensive use of DependentHandle:

• https://learn.microsoft.com/dotnet/api/system.runtime.dependenthandle

Probably worth me learning about it, might be useful for other things.