Fix cache rebuild losing image, fix unbounded memIndex, update README

- Save TileSource BEFORE RebuildControllerCache so image is preserved
  after cache type/size changes
- Fix double-dispose: _delayCache.Dispose() cascades to inner cache;
  _browserCache no longer disposed separately
- Add _innerCache field to clearly track ownership in cache chain
- Cap BrowserStorageTileCache._memIndex at 512 entries (tiles beyond
  the cap remain in sessionStorage, accessible via TryGetAsync)
- PutAsync only populates memIndex on successful sessionStorage write
- Remove ShowDebugStats reference from README (feature removed)
- Rename SKDeepZoomTileCache → SKDeepZoomMemoryTileCache in README

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: mattleibow

samples/SkiaSharpDemo.Blazor/BrowserStorageTileCache.cs

@@ -21,7 +21,9 @@ namespace SkiaSharpDemo;
 public sealed class BrowserStorageTileCache : ISKDeepZoomTileCache
 {
     private readonly IJSRuntime _js;
-    // In-memory index so Contains/TryGet (sync) can return fast on hits without round-tripping JS.
+    // In-memory index so Contains/TryGet (sync) can return fast without JS interop round-trips.
+    // Capped to prevent unbounded growth (tiles still persist in sessionStorage beyond this limit).
+    private const int MaxMemoryEntries = 512;
     private readonly ConcurrentDictionary<SKDeepZoomTileId, SKBitmap> _memIndex = new();
     private int _storageCount;
 
@@ -59,7 +61,7 @@ public bool TryGet(SKDeepZoomTileId id, out SKBitmap? bitmap)
 
             var bytes = Convert.FromBase64String(base64);
             var bitmap = SKBitmap.Decode(bytes);
-            if (bitmap is not null)
+            if (bitmap is not null && _memIndex.Count < MaxMemoryEntries)
                 _memIndex.TryAdd(id, bitmap);
             return bitmap;
         }
@@ -71,19 +73,22 @@ public bool TryGet(SKDeepZoomTileId id, out SKBitmap? bitmap)
     public void Put(SKDeepZoomTileId id, SKBitmap? bitmap)
     {
         if (bitmap is null) return;
-        _memIndex[id] = bitmap;
+        if (_memIndex.Count < MaxMemoryEntries)
+            _memIndex[id] = bitmap;
         // Fire-and-forget write to storage (best-effort, no await)
         _ = WriteToBrowserAsync(id, bitmap, CancellationToken.None);
     }
 
     public async Task PutAsync(SKDeepZoomTileId id, SKBitmap? bitmap, CancellationToken ct = default)
     {
         if (bitmap is null) return;
-        _memIndex[id] = bitmap;
-        await WriteToBrowserAsync(id, bitmap, ct).ConfigureAwait(false);
+        bool stored = await WriteToBrowserAsync(id, bitmap, ct).ConfigureAwait(false);
+        // Only cache in memory if the storage write succeeded and we have room.
+        if (stored && _memIndex.Count < MaxMemoryEntries)
+            _memIndex.TryAdd(id, bitmap);
     }
 
-    private async Task WriteToBrowserAsync(SKDeepZoomTileId id, SKBitmap bitmap, CancellationToken ct)
+    private async Task<bool> WriteToBrowserAsync(SKDeepZoomTileId id, SKBitmap bitmap, CancellationToken ct)
     {
         try
         {
@@ -93,8 +98,9 @@ private async Task WriteToBrowserAsync(SKDeepZoomTileId id, SKBitmap bitmap, Can
             await _js.InvokeVoidAsync("deepZoomCacheSet", ct, TileKey(id), base64)
                      .ConfigureAwait(false);
             Interlocked.Increment(ref _storageCount);
+            return true;
         }
-        catch { /* quota exceeded or interop unavailable */ }
+        catch { return false; /* quota exceeded or interop unavailable */ }
     }
 
     public bool Remove(SKDeepZoomTileId id)

samples/SkiaSharpDemo.Blazor/Pages/DeepZoom.razor

@@ -367,7 +367,9 @@
     private bool _useBrowserCache;
     private int  _cacheCapacity = 1024;
     private DelayTileCache? _delayCache;
-    private BrowserStorageTileCache? _browserCache;
+    // _innerCache is the cache directly wrapped by _delayCache (owned; dispose via _delayCache)
+    private ISKDeepZoomTileCache? _innerCache;
+    private BrowserStorageTileCache? _browserCache; // same ref as _innerCache when _useBrowserCache=true
 
     // URL input
     private string _urlInput = DefaultDziUrl;
@@ -402,8 +404,8 @@
         _thisRef = DotNetObjectReference.Create(this);
         await JS.InvokeVoidAsync("deepZoomRegisterResize", _thisRef);
 
-        _browserCache = new BrowserStorageTileCache(JS);
-        _delayCache = new DelayTileCache(new SkiaSharp.Extended.DeepZoom.SKDeepZoomMemoryTileCache(_cacheCapacity));
+        _innerCache = new SkiaSharp.Extended.DeepZoom.SKDeepZoomMemoryTileCache(_cacheCapacity);
+        _delayCache = new DelayTileCache(_innerCache);
         _controller = new SKDeepZoomController(_delayCache);
         _controller.InvalidateRequired += OnInvalidateRequired;
 
@@ -603,18 +605,20 @@
     private void OnCacheTypeChanged()
     {
         if (_controller == null) return;
-        // Reload with new cache type
+        var source = _controller.TileSource;  // save BEFORE dispose
         RebuildControllerCache();
-        var source = _controller.TileSource;
-        var fetcher = new SkiaSharp.Extended.DeepZoom.SKDeepZoomHttpTileFetcher();
         if (source != null)
-            _controller.Load(source, fetcher);
+            _controller.Load(source, new SkiaSharp.Extended.DeepZoom.SKDeepZoomHttpTileFetcher());
         InvokeAsync(() => _canvas?.Invalidate());
     }
 
     private void OnCacheSizeChanged()
     {
+        if (_controller == null) return;
+        var source = _controller.TileSource;  // save BEFORE dispose
         RebuildControllerCache();
+        if (source != null)
+            _controller.Load(source, new SkiaSharp.Extended.DeepZoom.SKDeepZoomHttpTileFetcher());
         InvokeAsync(() => _canvas?.Invalidate());
     }
 
@@ -627,17 +631,26 @@
     private void RebuildControllerCache()
     {
         if (_controller == null) return;
-        // Dispose old caches
-        _delayCache?.Dispose();
-        _browserCache?.Dispose();
 
-        _browserCache = new BrowserStorageTileCache(JS);
+        // _delayCache.Dispose() cascades to _innerCache (and transitively _browserCache)
+        // so we must NOT dispose _innerCache or _browserCache separately.
+        _delayCache?.Dispose();
+        _delayCache = null;
+        _innerCache = null;    // already disposed via _delayCache
+        _browserCache = null;  // same object as _innerCache when useBrowserCache=true
 
-        ISKDeepZoomTileCache inner = _useBrowserCache
-            ? _browserCache
-            : new SkiaSharp.Extended.DeepZoom.SKDeepZoomMemoryTileCache(_cacheCapacity);
+        // Build new cache chain
+        if (_useBrowserCache)
+        {
+            _browserCache = new BrowserStorageTileCache(JS);
+            _innerCache = _browserCache;
+        }
+        else
+        {
+            _innerCache = new SkiaSharp.Extended.DeepZoom.SKDeepZoomMemoryTileCache(_cacheCapacity);
+        }
 
-        _delayCache = new DelayTileCache(inner)
+        _delayCache = new DelayTileCache(_innerCache)
         {
             IsEnabled  = _cacheDelayEnabled,
             MinDelayMs = _cacheDelayMin,
@@ -665,7 +678,8 @@
             _controller.InvalidateRequired -= OnInvalidateRequired;
             _controller.Dispose();
         }
-        _browserCache?.Dispose();
+        // _delayCache owns _innerCache (and transitively _browserCache); dispose cascades.
+        _delayCache?.Dispose();
     }
 }
 

source/SkiaSharp.Extended/DeepZoom/README.md

@@ -25,7 +25,7 @@ flowchart TD
     Controller["SKDeepZoomController\n(orchestrator)"]
     Viewport["SKDeepZoomViewport\n(geometry)"]
     Scheduler["SKDeepZoomTileScheduler\n(tile selection)"]
-    Cache["SKDeepZoomTileCache\n(LRU bitmap store)"]
+    Cache["SKDeepZoomMemoryTileCache\n(LRU bitmap store)"]
     Renderer["SKDeepZoomRenderer\n(draws tiles)"]
     Fetcher["ISKDeepZoomTileFetcher\n(HTTP or file)"]
     TileSource["SKDeepZoomImageSource\n(DZI metadata)"]
@@ -145,7 +145,7 @@ flowchart LR
     end
 
     subgraph "Caching"
-        C["SKDeepZoomTileCache\n(LRU, configurable capacity)"]
+        C["SKDeepZoomMemoryTileCache\n(LRU, configurable capacity)"]
     end
 
     IF --> C
@@ -154,7 +154,7 @@ flowchart LR
 
 **`SKDeepZoomTileId`** — uniquely identifies a tile as `(level, col, row)`.  
 **`SKDeepZoomTileRequest`** — adds fallback context: if the exact tile isn't cached, a parent tile at a lower level is used to fill the space while the tile loads.  
-**`SKDeepZoomTileCache`** — thread-safe LRU cache of `SKBitmap` objects. Evicts least-recently-used tiles when capacity is reached.
+**`SKDeepZoomMemoryTileCache`** — thread-safe LRU cache of `SKBitmap` objects. Evicts least-recently-used tiles when capacity is reached.
 
 ### 3 — Viewport & Geometry
 
@@ -207,7 +207,7 @@ classDiagram
 ```mermaid
 flowchart TD
     Renderer["SKDeepZoomRenderer"]
-    Cache["SKDeepZoomTileCache"]
+    Cache["SKDeepZoomMemoryTileCache"]
     Viewport["SKDeepZoomViewport"]
     TileSource["SKDeepZoomImageSource"]
     Scheduler["SKDeepZoomTileScheduler"]
@@ -223,7 +223,7 @@ flowchart TD
     Cache --> note1
 ```
 
-The renderer always falls back to a lower-level tile while the correct tile is loading, so the display is never blank. Tile borders and a debug overlay can be enabled via `ShowTileBorders` / `ShowDebugStats`.
+The renderer always falls back to a lower-level tile while the correct tile is loading, so the display is never blank. Tile borders and a debug overlay can be enabled via `ShowTileBorders`.
 
 ---
 
@@ -233,7 +233,7 @@ The renderer always falls back to a lower-level tile while the correct tile is l
 classDiagram
     class SKDeepZoomController {
         +SKDeepZoomViewport Viewport
-        +SKDeepZoomTileCache Cache
+        +SKDeepZoomMemoryTileCache Cache
         +SKDeepZoomTileScheduler Scheduler
         +SKDeepZoomRenderer Renderer
         +SKDeepZoomImageSource? TileSource
@@ -341,7 +341,7 @@ public partial class MyPage : ContentPage
 | `SKDeepZoomViewportState.cs` | Snapshot struct for viewport state |
 | `SKDeepZoomRenderer.cs` | Draws tiles onto `SKCanvas` with fallback support |
 | `SKDeepZoomTileScheduler.cs` | Selects visible tiles at the optimal pyramid level |
-| `SKDeepZoomTileCache.cs` | Thread-safe LRU cache of decoded tile bitmaps |
+| `SKDeepZoomMemoryTileCache.cs` | Thread-safe LRU cache of decoded tile bitmaps |
 | `SKDeepZoomTileId.cs` | Identifies a tile: `(level, col, row)` |
 | `SKDeepZoomTileRequest.cs` | Tile + optional fallback parent tile |
 | `SKDeepZoomTileFailedEventArgs.cs` | Event args for tile load failures |