Fix fatal error in mass downloaders when Twitch API lies about having…

… a next page (#1235)

* Prevent cursorIndex from leaving the of bounds of cursorList.Count

* Fix crash caused by Twitch API returning pageInfo.hasNextPage but no video edges

* Extract common reset code

TwitchDownloaderWPF/WindowMassDownload.xaml.cs

@@ -25,7 +25,7 @@ public partial class WindowMassDownload : Window
         public ObservableCollection<TaskData> videoList { get; set; } = new ObservableCollection<TaskData>();
         public readonly List<TaskData> selectedItems = new List<TaskData>();
         public readonly List<string> cursorList = new List<string>();
-        public int cursorIndex = -1;
+        public int cursorIndex = 0;
         public string currentChannel = "";
         public string period = "";
         public int videoCount = 50;
@@ -49,12 +49,18 @@ private async void btnChannel_Click(object sender, RoutedEventArgs e)
             await ChangeCurrentChannel();
         }
 
-        private Task ChangeCurrentChannel()
+        private void ResetLists()
         {
-            currentChannel = textChannel.Text;
             videoList.Clear();
             cursorList.Clear();
-            cursorIndex = -1;
+            cursorList.Add("");
+            cursorIndex = 0;
+        }
+
+        private Task ChangeCurrentChannel()
+        {
+            currentChannel = textChannel.Text;
+            ResetLists();
             return UpdateList();
         }
 
@@ -69,9 +75,7 @@ private async Task UpdateList()
             {
                 // Pretend we are doing something so the status icon has time to show
                 await Task.Delay(50);
-                videoList.Clear();
-                cursorList.Clear();
-                cursorIndex = -1;
+                ResetLists();
                 StatusImage.Visibility = Visibility.Hidden;
                 return;
             }
@@ -125,13 +129,18 @@ private async Task UpdateList()
                         });
                     }
 
-                    btnNext.IsEnabled = res.data.user.videos.pageInfo.hasNextPage;
-                    btnPrev.IsEnabled = res.data.user.videos.pageInfo.hasPreviousPage;
+                    btnPrev.IsEnabled = cursorIndex > 0;
                     if (res.data.user.videos.pageInfo.hasNextPage)
                     {
-                        string newCursor = res.data.user.videos.edges[0].cursor;
-                        if (!cursorList.Contains(newCursor))
-                            cursorList.Add(newCursor);
+                        string newCursor = res.data.user.videos.edges.FirstOrDefault()?.cursor;
+                        if (newCursor is not null)
+                        {
+                            btnNext.IsEnabled = true;
+                            if (!cursorList.Contains(newCursor))
+                            {
+                                cursorList.Add(newCursor);
+                            }
+                        }
                     }
                 }
             }
@@ -184,13 +193,18 @@ private async Task UpdateList()
                         });
                     }
 
-                    btnNext.IsEnabled = res.data.user.clips.pageInfo.hasNextPage;
-                    btnPrev.IsEnabled = cursorIndex >= 0;
+                    btnPrev.IsEnabled = cursorIndex > 0;
                     if (res.data.user.clips.pageInfo.hasNextPage)
                     {
-                        string newCursor = res.data.user.clips.edges.First(x => x.cursor != null).cursor;
-                        if (!cursorList.Contains(newCursor))
-                            cursorList.Add(newCursor);
+                        string newCursor = res.data.user.clips.edges.FirstOrDefault(x => x.cursor != null)?.cursor;
+                        if (newCursor is not null)
+                        {
+                            btnNext.IsEnabled = true;
+                            if (!cursorList.Contains(newCursor))
+                            {
+                                cursorList.Add(newCursor);
+                            }
+                        }
                     }
                 }
             }
@@ -220,15 +234,23 @@ private async void btnNext_Click(object sender, RoutedEventArgs e)
         {
             btnNext.IsEnabled = false;
             btnPrev.IsEnabled = false;
-            cursorIndex++;
+            if (cursorIndex < cursorList.Count - 1)
+            {
+                cursorIndex++;
+            }
+
             await UpdateList();
         }
 
         private async void btnPrev_Click(object sender, RoutedEventArgs e)
         {
             btnNext.IsEnabled = false;
             btnPrev.IsEnabled = false;
-            cursorIndex--;
+            if (cursorIndex > 0)
+            {
+                cursorIndex--;
+            }
+
             await UpdateList();
         }
 
@@ -260,9 +282,7 @@ private void btnQueue_Click(object sender, RoutedEventArgs e)
         private async void ComboSortByDate_SelectionChanged(object sender, SelectionChangedEventArgs e)
         {
             period = ((ComboBoxItem)ComboSortByDate.SelectedItem).Tag.ToString();
-            videoList.Clear();
-            cursorList.Clear();
-            cursorIndex = -1;
+            ResetLists();
             await UpdateList();
         }
 
@@ -312,9 +332,7 @@ private async void TextChannel_OnKeyDown(object sender, KeyEventArgs e)
         private async void ComboVideoCount_SelectionChanged(object sender, SelectionChangedEventArgs e)
         {
             videoCount = int.Parse((string)((ComboBoxItem)ComboVideoCount.SelectedValue).Content);
-            videoList.Clear();
-            cursorList.Clear();
-            cursorIndex = -1;
+            ResetLists();
             await UpdateList();
         }