[controls] fix memory leak in Window

Fixes: #10029

I could reproduce a memory leak by doing:

    App.Current.OpenWindow(new Window { Page = new ContentPage() });

I found that `App.Current._requestedWindows` just held onto every
`Window` object forever.

Note that we *can't* use `ConditionalWeakTable` here:

https://learn.microsoft.com/dotnet/api/system.runtime.compilerservices.conditionalweaktable-2

After initially trying this, I added a test showing not to use it. The
problem is if the `string` is GC'd the `Window` will be lost.

We can use `Dictionary<string, WeakReference<Window>>` instead.

The only other change I made was to use `Guid.ToString("n")` as it
removes `{`, `}`, and `-` characters from the string.

After these changes, I still found `Window` objects hanging around
that appeared to be held onto by many other objects.

Then I reviewed:

    void IWindow.Destroying()
    {
        SendWindowDisppearing();
        Destroying?.Invoke(this, EventArgs.Empty);
        OnDestroying();

        Application?.RemoveWindow(this);
        // This wasn't here!
        Handler?.DisconnectHandler();
    }

It appears that upon `Window`'s closing, we didn't have any code that
"disconnected" the MAUI handler. After this change, I could see
`Window` objects properly go away.

I added a unit test as well.

src/Controls/src/Core/HandlerImpl/Application/Application.Impl.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Runtime.CompilerServices;
 using Microsoft.Extensions.Logging;
 using Microsoft.Maui.ApplicationModel;
 using Microsoft.Maui.Devices;
@@ -9,10 +10,10 @@ namespace Microsoft.Maui.Controls
 {
  public partial class Application : IApplication
  {
- const string MauiWindowIdKey = "__MAUI_WINDOW_ID__";
+ internal const string MauiWindowIdKey = "__MAUI_WINDOW_ID__";
 
  readonly List<Window> _windows = new();
- readonly Dictionary<string, Window> _requestedWindows = new();
+ readonly Dictionary<string, WeakReference<Window>> _requestedWindows = new();
  ILogger<Application>? _logger;
 
  ILogger<Application>? Logger =>
@@ -29,8 +30,14 @@ IWindow IApplication.CreateWindow(IActivationState? activationState)
  // try get the window that is pending
  if (activationState?.State?.TryGetValue(MauiWindowIdKey, out var requestedWindowId) ?? false)
  {
- if (requestedWindowId != null && _requestedWindows.TryGetValue(requestedWindowId, out var w))
- window = w;
+ if (requestedWindowId != null && _requestedWindows.TryGetValue(requestedWindowId, out var r))
+ {
+ if (r.TryGetTarget(out var w))
+ {
+ window = w;
+ }
+ _requestedWindows.Remove(requestedWindowId);
+ }
  }
 
  // create a new one if there is no pending windows
@@ -86,9 +93,8 @@ internal void RemoveWindow(Window window)
 
  public virtual void OpenWindow(Window window)
  {
- var id = Guid.NewGuid().ToString();
-
- _requestedWindows[id] = window;
+ var id = Guid.NewGuid().ToString("n");
+ _requestedWindows.Add(id, new WeakReference<Window>(window));
 
  var state = new PersistedState
  {

src/Controls/src/Core/HandlerImpl/Window/Window.Impl.cs

@@ -504,6 +504,7 @@ void IWindow.Destroying()
  OnDestroying();
 
  Application?.RemoveWindow(this);
+ Handler?.DisconnectHandler();
  }
 
  void IWindow.Resumed()

src/Controls/tests/Core.UnitTests/WindowsTests.cs

@@ -1,5 +1,8 @@
-using System.Collections.Generic;
+using System;
+using System.Collections;
+using System.Collections.Generic;
 using System.Linq;
+using System.Reflection;
 using System.Threading.Tasks;
 using Microsoft.Maui.Graphics;
 using Xunit;
@@ -621,5 +624,63 @@ public void MinDimensionsArePassedToCoreCorrectly(double inW, double inH, double
  Assert.Equal(outW, coreWindow.MinimumWidth);
  Assert.Equal(outH, coreWindow.MinimumHeight);
  }
+
+ [Fact]
+ public async Task WindowDoesNotLeak()
+ {
+ var application = new Application();
+ WeakReference reference;
+
+ // Scope for window
+ {
+ var window = new Window { Page = new ContentPage() };
+ reference = new WeakReference(window);
+ application.OpenWindow(window);
+ ((IWindow)window).Destroying();
+ }
+
+ // GC
+ await Task.Yield();
+ GC.Collect();
+ GC.WaitForPendingFinalizers();
+
+ Assert.False(reference.IsAlive, "Window should not be alive!");
+ }
+
+ // NOTE: this test is here to show `ConditionalWeakTable _requestedWindows` was a bad idea
+ [Fact]
+ public async Task TwoKeysSameWindow()
+ {
+ var application = new Application();
+ var window = new Window { Page = new ContentPage() };
+
+ application.OpenWindow(window);
+
+ // Access Application._requestedWindows
+ var flags = BindingFlags.NonPublic | BindingFlags.Instance;
+ var table = typeof(Application).GetField("_requestedWindows", flags).GetValue(application) as IDictionary;
+ Assert.NotNull(table);
+
+ // A "cloned" key should still work
+ string key;
+ {
+ var originalKey = table.Keys.OfType<string>().Single();
+ key = new string(originalKey);
+ Assert.NotSame(originalKey, key);
+ }
+
+ // GC collect the original key
+ await Task.Yield();
+ GC.Collect();
+ GC.WaitForPendingFinalizers();
+
+ // Same window, doesn't create a new one
+ var actual = ((IApplication)application).CreateWindow(new ActivationState(new MockMauiContext(), new PersistedState
+ {
+ { Application.MauiWindowIdKey, key }
+ }));
+ Assert.Same(window, actual);
+ Assert.Empty(table);
+ }
  }
 }