Add the "ShowAsync<T>():T" method and IDialogResult interface to the Dialog control.

HandyControl/Controls/Other/Dialog.cs

@@ -1,4 +1,7 @@
-using System.Windows.Controls;
+using System;
+using System.Threading.Tasks;
+using System.Windows;
+using System.Windows.Controls;
 using System.Windows.Documents;
 using System.Windows.Input;
 using HandyControl.Interactivity;
@@ -6,6 +9,11 @@ using HandyControl.Tools;
 
 namespace HandyControl.Controls
 {
+    public interface IDialogResult
+    {
+        object Result { get; set; }
+    }
+
     public class Dialog : ContentControl
     {
         private Adorner _container;
@@ -15,6 +23,50 @@ namespace HandyControl.Controls
             CommandBindings.Add(new CommandBinding(ControlCommands.Close, (s, e) => Close()));
         }
 
+        public static Task<TResult> ShowAsync<TView, TResult>() where TView : class, new() =>
+            ShowAsync<TResult>(new TView());
+
+        public static Task<TResult> ShowAsync<TResult>(object content)
+        {
+            var tcs = new TaskCompletionSource<TResult>();
+
+            var dialog = Show(content);
+            dialog.Unloaded += OnUnloaded;
+
+            return tcs.Task;
+
+            void OnUnloaded(object sender, RoutedEventArgs args)
+            {
+                try
+                {
+                    dialog.Unloaded -= OnUnloaded;
+                    tcs.SetResult(GetResult(dialog));
+                }
+                catch (Exception e)
+                {
+                    tcs.SetException(e);
+                }
+            }
+
+            TResult GetResult(Dialog embeddedDialog)
+            {
+                if (!(embeddedDialog.Content is FrameworkElement frameworkElement))
+                    throw new InvalidOperationException("The dialog is not a derived class of the FrameworkElement. ");
+
+                if (!(frameworkElement.DataContext is IDialogResult dialogResult))
+                    throw new InvalidOperationException("The view model of the dialog is not implement the IDialogResult interface. ");
+
+                if (!(dialogResult.Result is TResult result))
+                    throw new InvalidCastException("Could not cast " +
+                                                   $"the {(dialogResult.Result != null ? $"{dialogResult.Result.GetType()} type" : "null value")} " +
+                                                   $"to the {typeof(TResult)} type. ");
+
+                return result;
+            }
+        }
+
+        public static Dialog Show<T>() where T : class, new() => Show(new T());
+
         public static Dialog Show(object content)
         {
             var dialog = new Dialog
@@ -64,6 +116,6 @@ namespace HandyControl.Controls
                     layer?.Remove(_container);
                 }
             }
-        } 
+        }
     }
 }