Add better WeakEventListener sample

[michael-hawker]

We need to summarize the discussion from this thread about WeakEventListener into a proper example for our document on it.

[michael-hawker]

Think this is the example we need to provide:

    var inpc = rowGroupInfo.CollectionViewGroup as INotifyPropertyChanged;
    var weakPropertyChangedListener = new WeakEventListener<DataGrid, object, PropertyChangedEventArgs>(this) {
        OnEventAction = (instance, source, eventArgs) => instance.CollectionViewGroup_PropertyChanged(source, eventArgs),
        OnDetachAction = (weakEventListener) => inpc.PropertyChanged -= weakEventListener.OnEvent // Use Local References Only
    }
    inpc.PropertyChanged += weakPropertyChangedListener.OnEvent;

With a note about not referencing this within the lambdas.

[Marv51]

I would suggest adding a comment to the sample, with the corresponding native event syntax, for the example above:

inpc.PropertyChanged += CollectionViewGroup_PropertyChanged;

Maybe the names could be a bit clarified? Maybe it was just me, but the correct way to use WeakEventListener took me a long time to figure out.

Also the toolkit provides lots of examples using WeakEventListener differently, pretty sure some don't actually work:

• https://github.com/CommunityToolkit/WindowsCommunityToolkit/blob/63cbb4a51bdef59083ccfc86bbcba41f966d0027/Microsoft.Toolkit.Uwp.UI/Triggers/IsNullOrEmptyStateTrigger.cs#L50

• https://github.com/CommunityToolkit/WindowsCommunityToolkit/blob/63cbb4a51bdef59083ccfc86bbcba41f966d0027/Microsoft.Toolkit.Uwp.UI.Controls.DataGrid/DataGrid/DataGridDataConnection.cs#L762

• https://github.com/CommunityToolkit/WindowsCommunityToolkit/blob/63cbb4a51bdef59083ccfc86bbcba41f966d0027/UnitTests/UnitTests.UWP/Helpers/Test_WeakEventListener.cs#L38

[Marv51]

In c# 9 and later the OnEventAction should be a static anonymous function, that prevents the this problem:

var inpc = rowGroupInfo.CollectionViewGroup as INotifyPropertyChanged;
var weakPropertyChangedListener = new WeakEventListener<DataGrid, object, PropertyChangedEventArgs>(this) {
    OnEventAction = static (instance, source, eventArgs) => instance.CollectionViewGroup_PropertyChanged(source, eventArgs),
    OnDetachAction = (weakEventListener) => inpc.PropertyChanged -= weakEventListener.OnEvent // Use Local References Only
}
inpc.PropertyChanged += weakPropertyChangedListener.OnEvent;