[UWP Migration] Dispatcher to DispatcherQueue

[michael-hawker]

Related to #6

See:

• Dispatcher is null in WinUI Desktop in WinUI 3 Preview 4 microsoft/microsoft-ui-xaml#4219
• https://www.reddit.com/r/csharp/comments/12233os/converting_from_uwp_to_winui_dispatcherqueue_etc/
• https://learn.microsoft.com/windows/apps/windows-app-sdk/migrate-to-windows-app-sdk/guides/threading

It's not widely known that Dispatcher is effectively deprecated (not sure why it hasn't been marked as such) and that it's better to use DispatcherQueue instead. We made this change in the Windows Community Toolkit a while ago now. It's well supported and it means a seemlesss transition for those pieces to the Windows App SDK and WinUI 3.

We should have an analyzer and codefix which detects the old usage and swaps it out for DispatcherQueue instead.

We'd probably need to inject a private variable and set in the constructor to ensure the most compatible fix.

// MainPage.xaml.cs in a UWP app
public void NotifyUser(string strMessage)
{
    if (this.Dispatcher.HasThreadAccess)
    {
        StatusBlock.Text = strMessage;
    }
    else
    {
        var task = this.Dispatcher.RunAsync(
            Windows.UI.Core.CoreDispatcherPriority.Normal,
            () => StatusBlock.Text = strMessage);
    }
}

Expected:

// MainPage.xaml.cs in a Windows App SDK app
public void NotifyUser(string strMessage)
{
    if (this.DispatcherQueue.HasThreadAccess)
    {
        StatusBlock.Text = strMessage;
    }
    else
    {
        bool isQueued = this.DispatcherQueue.TryEnqueue(
        Microsoft.UI.Dispatching.DispatcherQueuePriority.Normal,
        () => StatusBlock.Text = strMessage);
    }
}

Should handle both the DP usage of Dispatcher as well as the CoreWindow general one.

Should also handle more complex cases where the containing type doesn't have a DispatcherQueue?