Cancel previous load on refresh.

src/GitHub.App/Collections/SequentialListSource.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading;
 using System.Threading.Tasks;
 using System.Windows;
 using System.Windows.Threading;
@@ -14,6 +15,7 @@ public abstract class SequentialListSource<TModel, TViewModel> : IVirtualizingLi
     {
         static readonly ILogger log = LogManager.ForContext<SequentialListSource<TModel, TViewModel>>();
 
+        readonly CancellationToken cancel;
         readonly Dispatcher dispatcher;
         readonly object loadLock = new object();
         Dictionary<int, Page<TModel>> pages = new Dictionary<int, Page<TModel>>();
@@ -23,8 +25,9 @@ public abstract class SequentialListSource<TModel, TViewModel> : IVirtualizingLi
         int loadTo;
         string after;
 
-        public SequentialListSource()
+        public SequentialListSource(CancellationToken cancel)
         {
+            this.cancel = cancel;
             dispatcher = Application.Current.Dispatcher;
         }
 
@@ -48,6 +51,12 @@ public async Task<IReadOnlyList<TViewModel>> GetPage(int pageNumber)
             dispatcher.VerifyAccess();
 
             var page = await EnsureLoaded(pageNumber);
+
+            if (page == null)
+            {
+                return null;
+            }
+
             var result = page.Items
                 .Select(CreateViewModel)
                 .ToList();
@@ -76,7 +85,7 @@ async Task<Page<TModel>> EnsureLoaded(int pageNumber)
             var pageLoaded = WaitPageLoaded(pageNumber);
             loadTo = Math.Max(loadTo, pageNumber);
 
-            while (true)
+            while (!cancel.IsCancellationRequested)
             {
                 lock (loadLock)
                 {
@@ -93,6 +102,8 @@ async Task<Page<TModel>> EnsureLoaded(int pageNumber)
                     return pages[pageNumber];
                 }
             }
+
+            return null;
         }
 
         Task WaitPageLoaded(int page)
@@ -115,7 +126,7 @@ async Task Load()
         {
             OnBeginLoading();
 
-            while (nextPage <= loadTo)
+            while (nextPage <= loadTo && !cancel.IsCancellationRequested)
             {
                 await LoadNextPage().ConfigureAwait(false);
                 PageLoaded?.Invoke(this, EventArgs.Empty);

src/GitHub.App/Collections/VirtualizingList.cs

@@ -172,8 +172,12 @@ async void LoadPage(int number)
             {
                 pages.Add(number, placeholderPage);
                 var page = await source.GetPage(number);
-                pages[number] = page;
-                SendReset();
+
+                if (page != null)
+                {
+                    pages[number] = page;
+                    SendReset();
+                }
             }
             catch (Exception)
             {

src/GitHub.App/ViewModels/GitHubPane/IssueListViewModelBase.cs

@@ -15,6 +15,7 @@ namespace GitHub.ViewModels.GitHubPane
 {
     public abstract class IssueListViewModelBase : PanePageViewModelBase, IIssueListViewModelBase
     {
+        CancellationTokenSource cancel;
         IReadOnlyList<IViewModel> items;
         ICollectionView itemsView;
         string searchQuery;
@@ -65,13 +66,17 @@ public async Task InitializeAsync(ILocalRepositoryModel repository, IConnection
 
         public override Task Refresh()
         {
-            var items = new VirtualizingList<IViewModel>(CreateItemSource(), null);
+            cancel?.Cancel();
+            cancel?.Dispose();
+            cancel = new CancellationTokenSource();
+
+            var items = new VirtualizingList<IViewModel>(CreateItemSource(cancel.Token), null);
             Items = items;
             ItemsView = new VirtualizingListCollectionView<IViewModel>(items);
             return Task.CompletedTask;
         }
 
-        protected abstract IVirtualizingListSource<IViewModel> CreateItemSource();
+        protected abstract IVirtualizingListSource<IViewModel> CreateItemSource(CancellationToken cancel);
         protected abstract Task DoOpenItem(IViewModel item);
 
         async Task OpenItemImpl(object i)

src/GitHub.App/ViewModels/GitHubPane/PullRequestListViewModel.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.ComponentModel.Composition;
 using System.Diagnostics;
+using System.Threading;
 using System.Threading.Tasks;
 using GitHub.Collections;
 using GitHub.Models;
@@ -26,9 +27,9 @@ public PullRequestListViewModel(IPullRequestService service)
 
         public override IReadOnlyList<string> States => states;
 
-        protected override IVirtualizingListSource<IViewModel> CreateItemSource()
+        protected override IVirtualizingListSource<IViewModel> CreateItemSource(CancellationToken cancel)
         {
-            return new ItemSource(this);
+            return new ItemSource(this, cancel);
         }
 
         protected override Task DoOpenItem(IViewModel item)
@@ -42,7 +43,8 @@ class ItemSource : SequentialListSource<PullRequestListItemModel, IViewModel>
         {
             readonly PullRequestListViewModel owner;
 
-            public ItemSource(PullRequestListViewModel owner)
+            public ItemSource(PullRequestListViewModel owner, CancellationToken cancel)
+                : base(cancel)
             {
                 this.owner = owner;
             }