Commits on Feb 13, 2023

1. [controls] fix memory leak in Style and AttachedCollection …

   Context: https://github.com/Vroomer/MAUI-master-detail-memory-leak
   Context: https://github.com/symbiogenesis/Maui.DataGrid/tree/memory-leak
   Context: dotnet#12039
   
   Using the Visual Studio's `.NET Object Allocation Tracking` profiler
   on the sample above I noticed after using the app and taking snapshots:
   
   Snapshot 2:
   
       WeakReference<Microsoft.Maui.Controls.BindableObject>
       Count: 1,545
       Bytes: 37,080
   
   Snapshot 10:
   
       WeakReference<Microsoft.Maui.Controls.BindableObject>
       Count: 1,657
       Bytes: 39,768
   
   It appears memory usage is slowly growing by about ~2.45KB each time I
   navigate forward a page then return.
   
   f8171b4 introduced a `CleanupTrigger`, but the code has a slight issue:
   
       void CleanUpWeakReferences()
       {
           if (_targets.Count < _cleanupThreshold)
           {
               return;
           }
   
           _targets.RemoveAll(t => !t.TryGetTarget(out _));
           _cleanupThreshold = _targets.Count + CleanupTrigger;
       }
   
   Because `_cleanupThreshold` is modified at the bottom of this method,
   the `_targets` collection increases by 128 in size over time. The
   result being lots of tiny `WeakReference<T>` objects in memory, but
   not the actual underlying `T` target.
   
   I did a little research to try to find a better collection than
   `List<WeakReference<T>>`:
   
   * https://learn.microsoft.com/dotnet/api/system.runtime.compilerservices.conditionalweaktable-2
   * https://source.dot.net/#Microsoft.Extensions.Caching.Memory/MemoryCache.cs
   * https://source.dot.net/#PresentationFramework/System/Windows/Input/KeyboardNavigation.cs,08271c29723aa10a
   
   I did not find one that exactly met our needs, so I wrote a very
   simple `WeakList<T>`. It cleans up `WeakReference<T>` that are no
   longer alive after N operations. I set this threshold to 32 to start,
   we can modify this later if needed. This makes the above samples call
   it a couple times for large `CollectionView`'s, which seems reasonable.
   
   After these changes, I'm able to navigate in the app over and over
   with stable memory usage:
   
   | ID | Objects | Heap Size     |
   | -- | --:     | --:           |
   | 1  | 107,885 | 407,645.87 KB |
   | 2  |   2,267 | 401,126.22 KB |
   
   This was after navigating through the app *many* times, and it
   actually went *down*.
   
   Then I found `Microsoft.Maui.Internals.WeakList<T>`, which I did not
   like quite as much as my implementation. I deleted/replaced its usage
   adding new unit tests for `WeakList<T>`.
   
   Note that I believe I found one other leak, so this doesn't fully
   solve dotnet#12039 yet.

   

   jonathanpeppers committed Feb 13, 2023
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