Flutter图片加载与缓存机制的深入探究

目录

• 前言
• 图片控件
• 图片解析
• 缓存管理

• ​新增缓存
• 缓存清理

// Image
Widget result = RawImage(
      image: _imageInfo?.image,
      debugImageLabel: _imageInfo?.debugLabel,
      width: widget.width,
      height: widget.height,
      scale: _imageInfo?.scale ?? 1.0,
      color: widget.color,
      colorBlendMode: widget.colorBlendMode,
      fit: widget.fit,
      alignment: widget.alignment,
      repeat: widget.repeat,
      centerSlice: widget.centerSlice,
      matchTextDirection: widget.matchTextDirection,
      invertColors: _invertColors,
      isAntiAlias: widget.isAntiAlias,
      filterQuality: widget.filterQuality,
    );
return result;

// image.dart
void _handleImageFrame(ImageInfo imageInfo, bool synchronousCall) {
 	setState(() {
    _imageInfo = imageInfo;
  }
}

void _resolveImage() {
	final ScrollAwareImageProvider provider = ScrollAwareImageProvider<dynamic>(
      context: _scrollAwareContext,
      imageProvider: widget.image,
    );
	final ImageStream newStream =
      provider.resolve(createLocalImageConfiguration(
        context,
        size: widget.width != null && widget.height != null ? Size(widget.width, widget.height) : null,
      ));
	_updateSourceStream(newStream);
}

//ImageProvider# resolve
ImageStream resolve(ImageConfiguration configuration) {
	_createErrorHandlerAndKey(configuration,(T key, ImageErrorListener errorHandler) {
        resolveStreamForKey(configuration, stream, key, errorHandler);
      },
		(T? key, dynamic exception, StackTrace? stack) async {
        await null; // wait an event turn in case a listener has been added to the image stream.
        final _ErrorImageCompleter imageCompleter = _ErrorImageCompleter();
        stream.setCompleter(imageCompleter);
        InformationCollector? collector;
        assert(() {
          collector = () sync* {
            yield DiagnosticsProperty<ImageProvider>('Image provider', this);
            yield DiagnosticsProperty<ImageConfiguration>('Image configuration', configuration);
            yield DiagnosticsProperty<T>('Image key', key, defaultValue: null);
          };
          return true;
        }());
        imageCompleter.setError(
          exception: exception,
          stack: stack,
          context: ErrorDescription('while resolving an image'),
          silent: true, // could be a network error or whatnot
          informationCollector: collector,
        );
      }
	);
}

//resolveStreamForKey
void resolveStreamForKey(ImageConfiguration configuration, ImageStream stream, T key, ImageErrorListener handleError) {
	if (stream.completer != null) {
		final ImageStreamCompleter? completer = PaintingBinding.instance!.imageCache!.putIfAbsent(
        key,
        () => stream.completer!,
        onError: handleError,
      );
		return;
	}
	final ImageStreamCompleter? completer = PaintingBinding.instance!.imageCache!.putIfAbsent(
		key,
      () => load(key, PaintingBinding.instance!.instantiateImageCodec),
      onError: handleError,
		);
	if (completer != null) {
      stream.setCompleter(completer);
    }
}

// 整理过核心逻辑的代码
ImageStreamCompleter? putIfAbsent(Object key, ImageStreamCompleter loader(), { ImageErrorListener? onError }) {
  // 根据key从正在加载的map里获取缓存，如果有直接返回
  	ImageStreamCompleter? result = _pendingImages[key]?.completer;
  	if (result != null) {
      return result;
    }
  // 检查内存缓存，存在的话更新存活map
  final _CachedImage? image = _cache.remove(key);
  if (image != null) {
    _trackLiveImage(key, _LiveImage(image.completer, image.sizeBytes, () => _liveImages.remove(key)));
    _cache[key] = image;
    return image.completer;
  }
  // 没有缓存，从 _live 里面取
  final _CachedImage? liveImage = _liveImages[key];
  if (liveImage != null) {
    // 更新缓存
    _touch(key, liveImage, timelineTask);
    return liveImage.completer;
  }
  // 3 个 map 都没有获取到缓存的图片
  result = loader(); // 加载
  _trackLiveImage(key, _LiveImage(result, null, () => _liveImages.remove(key)));
	_PendingImage? untrackedPendingImage;
  //定义一个listener
	void listener(ImageInfo? info, bool syncCall) {
		// 加载的监听
	}
  // 包装一个listener
	final ImageStreamListener streamListener = ImageStreamListener(listener);
	if (maximumSize > 0 && maximumSizeBytes > 0) {
		// 放入缓存
		_pendingImages[key] = _PendingImage(result, streamListener);
	} else {
		untrackedPendingImage = _PendingImage(result, streamListener);
	}
	// 添加监听
	result.addListener(streamListener);
	return result;
}

// Image
_imageStream.removeListener(_getListener());
// ImageStream
void removeListener(ImageStreamListener listener) {
  for (final VoidCallback callback in _onLastListenerRemovedCallbacks) {
    callback();
  }
  _onLastListenerRemovedCallbacks.clear();
}

_trackLiveImage(key, _LiveImage(image.completer, image.sizeBytes, () => _liveImages.remove(key)));

void _checkCacheSize(TimelineTask? timelineTask) {
  while (_currentSizeBytes > _maximumSizeBytes || _cache.length > _maximumSize) {
    final Object key = _cache.keys.first;
    final _CachedImage image = _cache[key]!;
    _currentSizeBytes -= image.sizeBytes!;
    _cache.remove(key);
  }
}

return MultiFrameImageStreamCompleter(
      codec: _loadAsync(key, decode),
      scale: key.scale,
      debugLabel: key.name,
      informationCollector: collector
    );

final StreamController<ImageChunkEvent> chunkEvents =
        StreamController<ImageChunkEvent>();
    return MultiFrameImageStreamCompleter(
        chunkEvents: chunkEvents.stream,
        codec: _loadAsync(key as NetworkImage, decode, chunkEvents),
        scale: key.scale,
        debugLabel: key.url,
        informationCollector: _imageStreamInformationCollector(key));

// AssetImage _loadAsync
try {
      data = await key.bundle.load(key.name);
    } on FlutterError {
      PaintingBinding.instance!.imageCache!.evict(key);
      rethrow;
    }
if (data == null) {
// 加载数据是null，清掉这个key的缓存
	PaintingBinding.instance!.imageCache!.evict(key);
	throw StateError('Unable to read data');
}
return await decode(data.buffer.asUint8List());
/// NetworkImage _loadAsync
Future<ui.Codec> _loadAsync(
      NetworkImage key,
      image_provider.DecoderCallback decode,
      StreamController<ImageChunkEvent> chunkEvents) {
	final Uri resolved = Uri.base.resolve(key.url);
	return ui.webOnlyInstantiateImageCodecFromUrl(resolved, // ignore: undefined_function
        chunkCallback: (int bytes, int total) {
      chunkEvents.add(ImageChunkEvent(
          cumulativeBytesLoaded: bytes, expectedTotalBytes: total));
    }) as Future<ui.Codec>;
}

if (Scrollable.recommendDeferredLoadingForContext(context.context)) {
  SchedulerBinding.instance.scheduleFrameCallback((_) {
		scheduleMicrotask(() => resolveStreamForKey(configuration, stream, key, handleError));
	 });
	return;
}

static bool recommendDeferredLoadingForContext(BuildContext context) {
	final _ScrollableScope widget =
		context.getElementForInheritedWidgetOfExactType<_ScrollableScope>()?.widget as _ScrollableScope;
	if (widget == null) {
      return false;
    }
	// 存在滑动的widget
	return widget.position.recommendDeferredLoading(context);
}