Live image cache (flutter#50318)

Track images available on screen

Author: dnfield

dev/tracing_tests/test/image_cache_tracing_test.dart

@@ -20,7 +20,7 @@ void main() {
     final developer.ServiceProtocolInfo info = await developer.Service.getInfo();
 
     if (info.serverUri == null) {
-      throw TestFailure('This test _must_ be run with --enable-vmservice.');
+      fail('This test _must_ be run with --enable-vmservice.');
     }
     await timelineObtainer.connect(info.serverUri);
     await timelineObtainer.setDartFlags();
@@ -58,7 +58,8 @@ void main() {
           'name': 'ImageCache.clear',
           'args': <String, dynamic>{
             'pendingImages': 1,
-            'cachedImages': 0,
+            'keepAliveImages': 0,
+            'liveImages': 1,
             'currentSizeInBytes': 0,
             'isolateId': isolateId,
           }
@@ -153,7 +154,7 @@ class TimelineObtainer {
 
   Future<void> close() async {
     expect(_completers, isEmpty);
-    await _observatorySocket.close();
+    await _observatorySocket?.close();
   }
 }
 

packages/flutter/lib/src/painting/binding.dart

@@ -96,6 +96,7 @@ mixin PaintingBinding on BindingBase, ServicesBinding {
   void evict(String asset) {
     super.evict(asset);
     imageCache.clear();
+    imageCache.clearLiveImages();
   }
 
   /// Listenable that notifies when the available fonts on the system have

packages/flutter/lib/src/painting/image_cache.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:developer';
+import 'dart:ui' show hashValues;
 
 import 'package:flutter/foundation.dart';
 
@@ -15,18 +16,24 @@ const int _kDefaultSizeBytes = 100 << 20; // 100 MiB
 ///
 /// Implements a least-recently-used cache of up to 1000 images, and up to 100
 /// MB. The maximum size can be adjusted using [maximumSize] and
-/// [maximumSizeBytes]. Images that are actively in use (i.e. to which the
-/// application is holding references, either via [ImageStream] objects,
-/// [ImageStreamCompleter] objects, [ImageInfo] objects, or raw [dart:ui.Image]
-/// objects) may get evicted from the cache (and thus need to be refetched from
-/// the network if they are referenced in the [putIfAbsent] method), but the raw
-/// bits are kept in memory for as long as the application is using them.
+/// [maximumSizeBytes].
+///
+/// The cache also holds a list of "live" references. An image is considered
+/// live if its [ImageStreamCompleter]'s listener count has never dropped to
+/// zero after adding at least one listener. The cache uses
+/// [ImageStreamCompleter.addOnLastListenerRemovedCallback] to determine when
+/// this has happened.
 ///
 /// The [putIfAbsent] method is the main entry-point to the cache API. It
 /// returns the previously cached [ImageStreamCompleter] for the given key, if
 /// available; if not, it calls the given callback to obtain it first. In either
 /// case, the key is moved to the "most recently used" position.
 ///
+/// A caller can determine whether an image is already in the cache by using
+/// [containsKey], which will return true if the image is tracked by the cache
+/// in a pending or compelted state. More fine grained information is available
+/// by using the [statusForKey] method.
+///
 /// Generally this class is not used directly. The [ImageProvider] class and its
 /// subclasses automatically handle the caching of images.
 ///
@@ -71,6 +78,11 @@ const int _kDefaultSizeBytes = 100 << 20; // 100 MiB
 class ImageCache {
   final Map<Object, _PendingImage> _pendingImages = <Object, _PendingImage>{};
   final Map<Object, _CachedImage> _cache = <Object, _CachedImage>{};
+  /// ImageStreamCompleters with at least one listener. These images may or may
+  /// not fit into the _pendingImages or _cache objects.
+  ///
+  /// Unlike _cache, the [_CachedImage] for this may have a null byte size.
+  final Map<Object, _CachedImage> _liveImages = <Object, _CachedImage>{};
 
   /// Maximum number of entries to store in the cache.
   ///
@@ -163,7 +175,8 @@ class ImageCache {
         'ImageCache.clear',
         arguments: <String, dynamic>{
           'pendingImages': _pendingImages.length,
-          'cachedImages': _cache.length,
+          'keepAliveImages': _cache.length,
+          'liveImages': _liveImages.length,
           'currentSizeInBytes': _currentSizeBytes,
         },
       );
@@ -204,7 +217,7 @@ class ImageCache {
     if (image != null) {
       if (!kReleaseMode) {
         Timeline.instantSync('ImageCache.evict', arguments: <String, dynamic>{
-          'type': 'completed',
+          'type': 'keepAlive',
           'sizeiInBytes': image.sizeBytes,
         });
       }
@@ -219,6 +232,36 @@ class ImageCache {
     return false;
   }
 
+  /// Updates the least recently used image cache with this image, if it is
+  /// less than the [maximumSizeBytes] of this cache.
+  ///
+  /// Resizes the cache as appropriate to maintain the constraints of
+  /// [maximumSize] and [maximumSizeBytes].
+  void _touch(Object key, _CachedImage image, TimelineTask timelineTask) {
+    assert(timelineTask != null);
+    if (image.sizeBytes != null && image.sizeBytes <= maximumSizeBytes) {
+      _currentSizeBytes += image.sizeBytes;
+      _cache[key] = image;
+      _checkCacheSize(timelineTask);
+    }
+  }
+
+  void _trackLiveImage(Object key, _CachedImage image) {
+    // Avoid adding unnecessary callbacks to the completer.
+    if (_liveImages.containsKey(key)) {
+      assert(identical(_liveImages[key].completer, image.completer));
+      return;
+    }
+    _liveImages[key] = image;
+    // Even if no callers to ImageProvider.resolve have listened to the stream,
+    // the cache is listening to the stream and will remove itself once the
+    // image completes to move it from pending to keepAlive.
+    // Even if the cache size is 0, we still add this listener.
+    image.completer.addOnLastListenerRemovedCallback(() {
+      _liveImages.remove(key);
+    });
+  }
+
   /// Returns the previously cached [ImageStream] for the given key, if available;
   /// if not, calls the given callback to obtain it first. In either case, the
   /// key is moved to the "most recently used" position.
@@ -255,14 +298,27 @@ class ImageCache {
     final _CachedImage image = _cache.remove(key);
     if (image != null) {
       if (!kReleaseMode) {
-        timelineTask.finish(arguments: <String, dynamic>{'result': 'completed'});
+        timelineTask.finish(arguments: <String, dynamic>{'result': 'keepAlive'});
       }
+      // The image might have been keptAlive but had no listeners. We should
+      // track it as live again until it has no listeners again.
+      _trackLiveImage(key, image);
       _cache[key] = image;
       return image.completer;
     }
 
+    final _CachedImage liveImage = _liveImages[key];
+    if (liveImage != null) {
+      _touch(key, liveImage, timelineTask);
+      if (!kReleaseMode) {
+        timelineTask.finish(arguments: <String, dynamic>{'result': 'keepAlive'});
+      }
+      return liveImage.completer;
+    }
+
     try {
       result = loader();
+      _trackLiveImage(key, _CachedImage(result, null));
     } catch (error, stackTrace) {
       if (!kReleaseMode) {
         timelineTask.finish(arguments: <String, dynamic>{
@@ -282,21 +338,37 @@ class ImageCache {
     if (!kReleaseMode) {
       listenerTask = TimelineTask(parent: timelineTask)..start('listener');
     }
+    // If we're doing tracing, we need to make sure that we don't try to finish
+    // the trace entry multiple times if we get re-entrant calls from a multi-
+    // frame provider here.
     bool listenedOnce = false;
+
+    // We shouldn't use the _pendingImages map if the cache is disabled, but we
+    // will have to listen to the image at least once so we don't leak it in
+    // the live image tracking.
+    // If the cache is disabled, this variable will be set.
+    _PendingImage untrackedPendingImage;
     void listener(ImageInfo info, bool syncCall) {
       // Images that fail to load don't contribute to cache size.
       final int imageSize = info?.image == null ? 0 : info.image.height * info.image.width * 4;
+
       final _CachedImage image = _CachedImage(result, imageSize);
-      final _PendingImage pendingImage = _pendingImages.remove(key);
+      if (!_liveImages.containsKey(key)) {
+        assert(syncCall);
+        result.addOnLastListenerRemovedCallback(() {
+          _liveImages.remove(key);
+        });
+      }
+      _liveImages[key] = image;
+      final _PendingImage pendingImage = untrackedPendingImage ?? _pendingImages.remove(key);
       if (pendingImage != null) {
         pendingImage.removeListener();
       }
-
-      if (imageSize <= maximumSizeBytes) {
-        _currentSizeBytes += imageSize;
-        _cache[key] = image;
-        _checkCacheSize(listenerTask);
+      // Only touch if the cache was enabled when resolve was initially called.
+      if (untrackedPendingImage == null) {
+        _touch(key, image, listenerTask);
       }
+
       if (!kReleaseMode && !listenedOnce) {
         listenerTask.finish(arguments: <String, dynamic>{
           'syncCall': syncCall,
@@ -309,20 +381,58 @@ class ImageCache {
       }
       listenedOnce = true;
     }
+
+    final ImageStreamListener streamListener = ImageStreamListener(listener);
     if (maximumSize > 0 && maximumSizeBytes > 0) {
-      final ImageStreamListener streamListener = ImageStreamListener(listener);
       _pendingImages[key] = _PendingImage(result, streamListener);
-      // Listener is removed in [_PendingImage.removeListener].
-      result.addListener(streamListener);
+    } else {
+      untrackedPendingImage = _PendingImage(result, streamListener);
     }
+    // Listener is removed in [_PendingImage.removeListener].
+    result.addListener(streamListener);
+
     return result;
   }
 
+  /// The [ImageCacheStatus] information for the given `key`.
+  ImageCacheStatus statusForKey(Object key) {
+    return ImageCacheStatus._(
+      pending: _pendingImages.containsKey(key),
+      keepAlive: _cache.containsKey(key),
+      live: _liveImages.containsKey(key),
+    );
+  }
+
   /// Returns whether this `key` has been previously added by [putIfAbsent].
   bool containsKey(Object key) {
     return _pendingImages[key] != null || _cache[key] != null;
   }
 
+  /// The number of live images being held by the [ImageCache].
+  ///
+  /// Compare with [ImageCache.currentSize] for keepAlive images.
+  int get liveImageCount => _liveImages.length;
+
+  /// The number of images being tracked as pending in the [ImageCache].
+  ///
+  /// Compare with [ImageCache.currentSize] for keepAlive images.
+  int get pendingImageCount => _pendingImages.length;
+
+  /// Clears any live references to images in this cache.
+  ///
+  /// An image is considered live if its [ImageStreamCompleter] has never hit
+  /// zero listeners after adding at least one listener. The
+  /// [ImageStreamCompleter.addOnLastListenerRemovedCallback] is used to
+  /// determine when this has happened.
+  ///
+  /// This is called after a hot reload to evict any stale references to image
+  /// data for assets that have changed. Calling this method does not relieve
+  /// memory pressure, since the live image caching only tracks image instances
+  /// that are also being held by at least one other object.
+  void clearLiveImages() {
+    _liveImages.clear();
+  }
+
   // Remove images from the cache until both the length and bytes are below
   // maximum, or the cache is empty.
   void _checkCacheSize(TimelineTask timelineTask) {
@@ -354,6 +464,73 @@ class ImageCache {
   }
 }
 
+/// Information about how the [ImageCache] is tracking an image.
+///
+/// A [pending] image is one that has not completed yet. It may also be tracked
+/// as [live] because something is listening to it.
+///
+/// A [keepAlive] image is being held in the cache, which uses Least Recently
+/// Used semantics to determine when to evict an image. These images are subject
+/// to eviction based on [ImageCache.maximumSizeBytes] and
+/// [ImageCache.maximumSize]. It may be [live], but not [pending].
+///
+/// A [live] image is being held until its [ImageStreamCompleter] has no more
+/// listeners. It may also be [pending] or [keepAlive].
+///
+/// An [untracked] image is not being cached.
+///
+/// To obtain an [ImageCacheStatus], use [ImageCache.statusForKey] or
+/// [ImageProvider.obtainCacheStatus].
+class ImageCacheStatus {
+  const ImageCacheStatus._({
+    this.pending = false,
+    this.keepAlive = false,
+    this.live = false,
+  }) : assert(!pending || !keepAlive);
+
+  /// An image that has been submitted to [ImageCache.putIfAbsent], but
+  /// not yet completed.
+  final bool pending;
+
+  /// An image that has been submitted to [ImageCache.putIfAbsent], has
+  /// completed, fits based on the sizing rules of the cache, and has not been
+  /// evicted.
+  ///
+  /// Such images will be kept alive even if [live] is false, as long
+  /// as they have not been evicted from the cache based on its sizing rules.
+  final bool keepAlive;
+
+  /// An image that has been submitted to [ImageCache.putIfAbsent] and has at
+  /// least one listener on its [ImageStreamCompleter].
+  ///
+  /// Such images may also be [keepAlive] if they fit in the cache based on its
+  /// sizing rules. They may also be [pending] if they have not yet resolved.
+  final bool live;
+
+  /// An image that is tracked in some way by the [ImageCache], whether
+  /// [pending], [keepAlive], or [live].
+  bool get tracked => pending || keepAlive || live;
+
+  /// An image that either has not been submitted to
+  /// [ImageCache.putIfAbsent] or has otherwise been evicted from the
+  /// [keepAlive] and [live] caches.
+  bool get untracked => !pending && !keepAlive && !live;
+
+  @override
+  bool operator ==(Object other) {
+    if (other.runtimeType != runtimeType) {
+      return false;
+    }
+    return other is ImageCacheStatus
+        && other.pending == pending
+        && other.keepAlive == keepAlive
+        && other.live == live;
+  }
+
+  @override
+  int get hashCode => hashValues(pending, keepAlive, live);
+}
+
 class _CachedImage {
   _CachedImage(this.completer, this.sizeBytes);