Fix #342: better async thumbnail loading

QuickLook.Plugin/QuickLook.Plugin.PDFViewer/NotifyTaskCompletion.cs

@@ -0,0 +1,89 @@
+// Copyright © 2018 Paddy Xu
+// 
+// This file is part of QuickLook program.
+// 
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+// 
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+// 
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+using System;
+using System.ComponentModel;
+using System.Threading.Tasks;
+
+namespace QuickLook.Plugin.PDFViewer
+{
+    public sealed class NotifyTaskCompletion<TResult> : INotifyPropertyChanged
+    {
+        private readonly TResult _loading;
+
+        public NotifyTaskCompletion(Task<TResult> task, TResult loading = default(TResult))
+        {
+            Task = task;
+            _loading = loading;
+            if (!task.IsCompleted)
+            {
+                var _ = WatchTaskAsync(task);
+            }
+        }
+
+        public Task<TResult> Task { get; }
+        public TResult Result => Task.Status == TaskStatus.RanToCompletion ? Task.Result : _loading;
+        public TaskStatus Status => Task.Status;
+        public bool IsCompleted => Task.IsCompleted;
+        public bool IsNotCompleted => !Task.IsCompleted;
+        public bool IsSuccessfullyCompleted => Task.Status ==
+                                               TaskStatus.RanToCompletion;
+        public bool IsCanceled => Task.IsCanceled;
+        public bool IsFaulted => Task.IsFaulted;
+        public AggregateException Exception => Task.Exception;
+        public Exception InnerException => Exception?.InnerException;
+        public string ErrorMessage => InnerException?.Message;
+        public event PropertyChangedEventHandler PropertyChanged;
+
+        private async Task WatchTaskAsync(Task task)
+        {
+            try
+            {
+                await task;
+            }
+            catch
+            {
+                // ignored
+            }
+
+            var propertyChanged = PropertyChanged;
+            if (propertyChanged == null)
+                return;
+            propertyChanged(this, new PropertyChangedEventArgs("Status"));
+            propertyChanged(this, new PropertyChangedEventArgs("IsCompleted"));
+            propertyChanged(this, new PropertyChangedEventArgs("IsNotCompleted"));
+            if (task.IsCanceled)
+            {
+                propertyChanged(this, new PropertyChangedEventArgs("IsCanceled"));
+            }
+            else if (task.IsFaulted)
+            {
+                propertyChanged(this, new PropertyChangedEventArgs("IsFaulted"));
+                propertyChanged(this, new PropertyChangedEventArgs("Exception"));
+                propertyChanged(this,
+                    new PropertyChangedEventArgs("InnerException"));
+                propertyChanged(this, new PropertyChangedEventArgs("ErrorMessage"));
+            }
+            else
+            {
+                propertyChanged(this,
+                    new PropertyChangedEventArgs("IsSuccessfullyCompleted"));
+                propertyChanged(this, new PropertyChangedEventArgs("Result"));
+            }
+        }
+    }
+}