enhance: generate commit graph off the UI thread

src/Models/CommitGraph.cs

@@ -174,7 +174,6 @@ public static CommitGraph Generate(List<Commit> commits, bool firstParentOnlyEna
                             break;
                     }
                 }
-                commit.IsHighlightedInGraph = isHighlighted;
 
                 // If no path found, create new curve for branch head
                 // Otherwise, create new curve for new merged commit

src/ViewModels/Histories.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Threading;
 using System.Threading.Tasks;
 
 using Avalonia.Collections;
@@ -110,12 +111,6 @@ public bool IsCommitTimeColumnVisible
         public List<Models.Commit> Commits
         {
             get => _commits;
-            set
-            {
-                GenerateGraph(value);
-                if (SetProperty(ref _commits, value))
-                    PostCommitsChanged();
-            }
         }
 
         public Models.CommitGraph Graph
@@ -132,7 +127,7 @@ public Models.CommitGraphHighlighting GraphHighlighting
                 if (_repo.UIStates.GraphHighlighting != value)
                 {
                     _repo.UIStates.GraphHighlighting = value;
-                    GenerateGraph(_commits);
+                    RefreshGraph();
                 }
             }
         }
@@ -254,6 +249,46 @@ public void ToggleShowFlag(Models.HistoryShowFlags flag)
             _repo.RefreshCommits();
         }
 
+        public void SetCommits(List<Models.Commit> commits, Models.CommitGraph graph, HashSet<string> graphExtraHeads)
+        {
+            if (_cancellationRefreshGraph is { IsCancellationRequested: false })
+                _cancellationRefreshGraph.Cancel();
+
+            ApplyGraph(commits, graph);
+
+            if (SetProperty(ref _commits, commits, nameof(Commits)))
+                PostCommitsChanged();
+
+            // The selection may have changed while the graph was generated in background.
+            if (!graphExtraHeads.SetEquals(CollectGraphExtraHeads()))
+                RefreshGraph();
+        }
+
+        /// <summary>
+        ///     Must be called on the UI thread: `_selectedCommits` is only modified there.
+        /// </summary>
+        public HashSet<string> CollectGraphExtraHeads()
+        {
+            var extraHeads = new HashSet<string>();
+            if (_repo.UIStates.GraphHighlighting >= Models.CommitGraphHighlighting.SelectedCommitsOnly)
+            {
+                foreach (var c in _selectedCommits)
+                    extraHeads.Add(c.SHA);
+            }
+
+            return extraHeads;
+        }
+
+        /// <summary>
+        ///     Thread-safe as long as `extraHeads` is not shared with the UI thread.
+        /// </summary>
+        public Models.CommitGraph GenerateGraph(List<Models.Commit> commits, HashSet<string> extraHeads)
+        {
+            var firstParentOnly = _repo.UIStates.HistoryShowFlags.HasFlag(Models.HistoryShowFlags.FirstParentOnly);
+            var highlighting = _repo.UIStates.GraphHighlighting;
+            return Models.CommitGraph.Generate(commits, firstParentOnly, highlighting, extraHeads);
+        }
+
         public Models.BisectState UpdateBisectInfo()
         {
             var test = Path.Combine(_repo.GitDir, "BISECT_START");
@@ -564,22 +599,38 @@ private void PostSelectedCommitsChanged()
             }
 
             if (_repo.UIStates.GraphHighlighting >= Models.CommitGraphHighlighting.SelectedCommitsOnly)
-                GenerateGraph(_commits);
+                RefreshGraph();
         }
 
-        private void GenerateGraph(List<Models.Commit> commits)
+        private void RefreshGraph()
         {
-            var firstParentOnly = _repo.UIStates.HistoryShowFlags.HasFlag(Models.HistoryShowFlags.FirstParentOnly);
-            var highlighting = _repo.UIStates.GraphHighlighting;
-            var extraHeads = new HashSet<string>();
+            if (_cancellationRefreshGraph is { IsCancellationRequested: false })
+                _cancellationRefreshGraph.Cancel();
+
+            _cancellationRefreshGraph = new CancellationTokenSource();
+            var token = _cancellationRefreshGraph.Token;
+            var commits = _commits;
+            var extraHeads = CollectGraphExtraHeads();
 
-            if (highlighting >= Models.CommitGraphHighlighting.SelectedCommitsOnly)
+            Task.Run(() =>
             {
-                foreach (var c in _selectedCommits)
-                    extraHeads.Add(c.SHA);
-            }
+                var graph = GenerateGraph(commits, extraHeads);
+                Dispatcher.UIThread.Post(() =>
+                {
+                    if (!token.IsCancellationRequested)
+                        ApplyGraph(commits, graph);
+                });
+            }, token);
+        }
+
+        private void ApplyGraph(List<Models.Commit> commits, Models.CommitGraph graph)
+        {
+            // There is exactly one dot for each commit (in the same order). Update `IsHighlightedInGraph` here
+            // so that the property-changed notifications of displayed commits are raised on the UI thread.
+            for (var i = 0; i < commits.Count; i++)
+                commits[i].IsHighlightedInGraph = graph.Dots[i].IsHighlighted;
 
-            Graph = Models.CommitGraph.Generate(commits, firstParentOnly, highlighting, extraHeads);
+            Graph = graph;
         }
 
         private Repository _repo = null;
@@ -589,6 +640,7 @@ private void GenerateGraph(List<Models.Commit> commits)
         private bool _isLoading = true;
         private List<Models.Commit> _commits = [];
         private Models.CommitGraph _graph = null;
+        private CancellationTokenSource _cancellationRefreshGraph = null;
         private List<Models.Commit> _selectedCommits = [];
         private Models.Bisect _bisect = null;
         private object _detailContext = Models.Null.Instance;

src/ViewModels/Repository.cs

@@ -1255,7 +1255,15 @@ public void RefreshCommits()
 
             Task.Run(async () =>
             {
-                await Dispatcher.UIThread.InvokeAsync(() => _histories.IsLoading = true);
+                var graphExtraHeads = new HashSet<string>();
+                await Dispatcher.UIThread.InvokeAsync(() =>
+                {
+                    if (_histories == null)
+                        return;
+
+                    _histories.IsLoading = true;
+                    graphExtraHeads = _histories.CollectGraphExtraHeads();
+                });
 
                 var builder = new StringBuilder();
                 builder
@@ -1292,6 +1300,12 @@ public void RefreshCommits()
                     }
                 }
 
+                var histories = _histories;
+                if (token.IsCancellationRequested || histories == null)
+                    return;
+
+                var graph = histories.GenerateGraph(commits, graphExtraHeads);
+
                 Dispatcher.UIThread.Invoke(() =>
                 {
                     if (token.IsCancellationRequested)
@@ -1300,7 +1314,7 @@ public void RefreshCommits()
                     if (_histories != null)
                     {
                         _histories.IsLoading = false;
-                        _histories.Commits = commits;
+                        _histories.SetCommits(commits, graph, graphExtraHeads);
                         BisectState = _histories.UpdateBisectInfo();
 
                         if (!string.IsNullOrEmpty(_navigateToCommitDelayed))