[WinRT] Use a queue to prevent multiple MessageDialogs from causing a crash (#347)

* [WinRT] Use a queue to prevent multiple MessageDialogs from causing a crash

* Make change to use Device.IsInvokeRequired

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla43469.cs

@@ -0,0 +1,37 @@
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+using System;
+using System.Threading.Tasks;
+
+#if UITEST
+using Xamarin.UITest;
+using NUnit.Framework;
+#endif
+
+namespace Xamarin.Forms.Controls
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Bugzilla, 43469, "Calling DisplayAlert twice in WinRT causes a crash", PlatformAffected.WinRT)]
+	public class Bugzilla43469 : TestContentPage
+	{
+		protected override void Init()
+		{
+			var button = new Button { Text = "Click to call DisplayAlert twice" };
+
+			button.Clicked += (sender, args) =>
+			{
+				Device.BeginInvokeOnMainThread(new Action(async () =>
+				{
+					await DisplayAlert("First", "Text", "Cancel");
+				}));
+
+				Device.BeginInvokeOnMainThread(new Action(async () =>
+				{
+					await DisplayAlert("Second", "Text", "Cancel");
+				}));
+			};
+
+			Content = button;
+		}
+	}
+}

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems

@@ -131,6 +131,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla42364.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla42519.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla43313.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Bugzilla43469.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla43516.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla43663.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla44944.cs" />

Xamarin.Forms.Platform.WinRT/Platform.cs

@@ -761,8 +761,41 @@ namespace Xamarin.Forms.Platform.WinRT
 				dialog.CancelCommandIndex = (uint)dialog.Commands.Count - 1;
 			}
 
-			IUICommand command = await dialog.ShowAsync();
-			options.SetResult(command.Label == options.Accept);
+			if (Device.IsInvokeRequired)
+			{
+				Device.BeginInvokeOnMainThread(async () =>
+				{
+					IUICommand command = await dialog.ShowAsyncQueue();
+					options.SetResult(command.Label == options.Accept);
+				});
+			}
+			else
+			{
+				IUICommand command = await dialog.ShowAsyncQueue();
+				options.SetResult(command.Label == options.Accept);
+			}
+		}
+	}
+	
+	// refer to http://stackoverflow.com/questions/29209954/multiple-messagedialog-app-crash for why this is used
+	// in order to allow for multiple MessageDialogs, or a crash occurs otherwise
+	public static class MessageDialogExtensions
+	{
+		static TaskCompletionSource<MessageDialog> _currentDialogShowRequest;
+
+		public static async Task<IUICommand> ShowAsyncQueue(this MessageDialog dialog)
+		{
+			while (_currentDialogShowRequest != null)
+			{
+				await _currentDialogShowRequest.Task;
+			}
+
+			var request = _currentDialogShowRequest = new TaskCompletionSource<MessageDialog>();
+			var result = await dialog.ShowAsync();
+			_currentDialogShowRequest = null;
+			request.SetResult(dialog);
+
+			return result;
 		}
 	}
 }