Commits on Jun 1, 2023

1. [controls] fix performance issue in {AppThemeBinding} …

   Context: #12130
   Context: https://github.com/angelru/CvSlowJittering
   
   Profiling a customer sample app, I noticed a lot of time spent in
   `{AppThemeBinding}` and `WeakEventManager` while scrolling:
   
       2.08s (17%) microsoft.maui.controls!Microsoft.Maui.Controls.AppThemeBinding.Apply(object,Microsoft.Maui.Controls.BindableObject,Micr...
       2.05s (16%) microsoft.maui.controls!Microsoft.Maui.Controls.AppThemeBinding.AttachEvents()
       2.04s (16%) microsoft.maui!Microsoft.Maui.WeakEventManager.RemoveEventHandler(System.EventHandler`1<TEventArgs_REF>,string)
   
   16% is a *lot* to notice while scrolling. Sometimes I've made
   improvements where I only shaved off 3% of the total time.
   
   What is going on here is:
   
   * Default `maui` template has lots of `{AppThemeBinding}` in the
     default `Styles.xaml`. This supports Light vs Dark theming.
   
   * `{AppThemeBinding}` subscribes to `Application.RequestedThemeChanged`
   
   * Making every MAUI view subscribe to this event -- potentially
     multiple times.
   
   * Subscribers are a `Dictionary<string, List<Subscriber>>`, where
     there is a dictionary lookup followed by a O(N) search for
     unsubscribe operations.
   
   I spent a little time investigating if we can make a faster
   `WeakEventManager`, in general:
   
   dotnet/runtime#61517
   
   I did not immediately see a way to make "weak events" fast, but I did
   see a way to make this scenario fast.
   
   Before:
   * For any `{AppThemeBinding}`, it calls both:
       * `RequestedThemeChanged -= OnRequestedThemeChanged` O(N) time
       * `RequestedThemeChanged += OnRequestedThemeChanged` constant time
   * Where the `-=` is notably slower, due to possibly 100s of subscribers.
   
   After:
   * Create an `_attached` boolean, so we know know the "state" if it is
     attached or not.
   * New bindings only call `+=`, where `-=` will now only be called by
     `{AppThemeBinding}` in *rare* cases.
   * Most .NET MAUI apps do not "unapply" bindings, but `-=` would only
     be used in that case.
   
   After this change, the following method disappeared from `dotnet-trace`
   output completely:
   
       2.05s (16%) microsoft.maui.controls!Microsoft.Maui.Controls.AppThemeBinding.AttachEvents()
   
   Meaning that `AppThemeBinding.AttachEvents()` is now so fast that 0%
   (basically no time) is spent inside this method.
   
   I also could notice a difference in general startup time of the sample
   app. An average of 10 runs on a Pixel 5:
   
       Before:
       Average(ms): 967.7
       Std Err(ms): 4.62132737064436
       Std Dev(ms): 14.6139203045133
       After:
       Average(ms): 958.9
       Std Err(ms): 3.22645316098034
       Std Dev(ms): 10.2029407525478
   
   So I could notice a ~10ms improvement to startup in this app, and
   scrolling seemed a bit better as well.
   
   Note that I don't think this completely solves #12130, as things still
   seem sluggish to me when scrolling. But it is a reasonable improvement
   to start with that benefits all .NET MAUI apps on all platforms.

   

   jonathanpeppers authored and github-actions committed Jun 1, 2023
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   jonathanpeppers authored and github-actions committed Jun 1, 2023
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