code_review: PR #703

* change the name of this feature to `Enable Block-Navigation`
* change the icon of the toggle button used to enable this feature
* use a new class `BlockNavigation` to hold all the data about this feature
* create `BlockNavigation` data only when it is enabled

Signed-off-by: leo <longshuang@msn.cn>

src/ViewModels/BlockNavigation.cs

@@ -0,0 +1,122 @@
+using System.Collections.Generic;
+using Avalonia.Collections;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace SourceGit.ViewModels
+{
+    public class BlockNavigation : ObservableObject
+    {
+        public class Block
+        {
+            public int Start { get; set; } = 0;
+            public int End { get; set; } = 0;
+
+            public Block(int start, int end)
+            {
+                Start = start;
+                End = end;
+            }
+
+            public bool IsInRange(int line)
+            {
+                return line >= Start && line <= End;
+            }
+        }
+
+        public AvaloniaList<Block> Blocks
+        {
+            get;
+        } = [];
+
+        public int Current
+        {
+            get => _current;
+            private set => SetProperty(ref _current, value);
+        }
+
+        public string Indicator
+        {
+            get
+            {
+                if (Blocks.Count == 0)
+                    return "-/-";
+                
+                if (_current >= 0 && _current < Blocks.Count)
+                    return $"{_current+1}/{Blocks.Count}";
+
+                return $"-/{Blocks.Count}";
+            }
+        }
+        
+        public BlockNavigation(object context)
+        {
+            Blocks.Clear();
+            Current = -1;
+            
+            var lines = new List<Models.TextDiffLine>();
+            if (context is Models.TextDiff combined)
+                lines = combined.Lines;
+            else if (context is TwoSideTextDiff twoSide)
+                lines = twoSide.Old;
+
+            if (lines.Count == 0)
+                return;
+            
+            var lineIdx = 0;
+            var blockStartIdx = 0;
+            var isNewBlock = true;
+            var blocks = new List<Block>();
+                
+            foreach (var line in lines)
+            {
+                lineIdx++;
+                if (line.Type == Models.TextDiffLineType.Added ||
+                    line.Type == Models.TextDiffLineType.Deleted ||
+                    line.Type == Models.TextDiffLineType.None)
+                {
+                    if (isNewBlock)
+                    {
+                        isNewBlock = false;
+                        blockStartIdx = lineIdx;
+                    }
+                }
+                else
+                {
+                    if (!isNewBlock)
+                    {
+                        blocks.Add(new Block(blockStartIdx, lineIdx - 1));
+                        isNewBlock = true;
+                    }
+                }
+            }
+            
+            if (!isNewBlock)
+                blocks.Add(new Block(blockStartIdx, lines.Count - 1));
+
+            Blocks.AddRange(blocks);
+        }
+
+        public Block GetCurrentBlock()
+        {
+            return (_current >= 0 && _current < Blocks.Count) ? Blocks[_current] : null;
+        }
+
+        public Block GotoNext()
+        {
+            if (Blocks.Count == 0) return null;
+            
+            Current = (_current + 1) % Blocks.Count;
+            return Blocks[_current];
+        }
+
+        public Block GotoPrev()
+        {
+            if (Blocks.Count == 0) return null;
+            
+            Current = _current == -1 ? Blocks.Count - 1 : (_current - 1 + Blocks.Count) % Blocks.Count;
+            return Blocks[_current];
+        }
+
+        private int _current = -1;
+    }
+}