简介

最近探索了一下新增Flutter的Image widget对webp做一个stopAnimation的拓展的Api，顺便了解一下Image整个结构和对一些多帧图片的处理。我们先看看Image的一个类图结构。

其中：

ImageProvider 提供加载图片的入口，不同的图片资源加载方式不一样，只要重写其load方法即可。同样，缓存图片的key值也有其生成。
FileImage 负责读取文件图片的数据，读取到的文件数据转化成ui.Codec对象交给ImageStreamCompleter去处理解析。
ImageStreamCompleter就是逐帧解析图片的类，生成之后会加入ImageCache，下载可以从缓存中得到。
ImageStream是处理Image Resource的，ImageState通过ImageStream与ImageStreamCompleter建立联系。ImageStream里也存储着图片加载完毕的监听回调。
MultiFrameImageStreamCompleter就是多帧图片解析器。 Flutter imgae支持的图片格式为：JPEG, PNG, GIF, Animated GIF, WebP, Animated WebP, BMP, and WBMP。Flutter Image是显示图片的一个Widget。 Flutter Image的几个构造方法：

方法	释义
Image()	从ImageProvider中获取图片，从本质上看，下面的几个方法都是他的具体实现。
Image.asset(String name)	从AssetBundler中获取图片
Image.network(String src)	显示网络图片
Image.file(File file)	从文件中获取图片
Image.memory(Uint8List bytes)	从Uint8List获取数据显示图片

Image

从Image的构造体上看，ImageProvider才是图片提供方，所以我们后面会看看ImageProvider究竟是要做点什么的。其他的参数是一些图片的属性和一些builder。

ImageState

关键代码：

void didUpdateWidget(Image oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (_isListeningToStream &&
        (widget.loadingBuilder == null) != (oldWidget.loadingBuilder == null)) {
      _imageStream.removeListener(_getListener(oldWidget.loadingBuilder));
      _imageStream.addListener(_getListener());
    }
    if (widget.image != oldWidget.image)
      _resolveImage();
  }

ImageProvider

其实ImageProvider是一个抽象类，让需要定制的子类去做一些实现。

比如：FileImage、MemoryImage、ExactAssetImage等等。其中对FileImage的代码进行了一些分析。

class FileImage extends ImageProvider<FileImage> {
  /// Creates an object that decodes a [File] as an image.
  ///
  /// The arguments must not be null.
  const FileImage(this.file, { this.scale: 1.0 })
      : assert(file != null),
        assert(scale != null);

  /// The file to decode into an image.
  final File file;

  /// The scale to place in the [ImageInfo] object of the image.
  final double scale;

  @override
  Future<FileImage> obtainKey(ImageConfiguration configuration) {
    return new SynchronousFuture<FileImage>(this);
  }

  @override
  ImageStreamCompleter load(FileImage key) {
    return new MultiFrameImageStreamCompleter(
      codec: _loadAsync(key),
      scale: key.scale,
      informationCollector: (StringBuffer information) {
        information.writeln('Path: ${file?.path}');
      }
    );
  }

  Future<ui.Codec> _loadAsync(FileImage key) async {
    assert(key == this);

    final Uint8List bytes = await file.readAsBytes();
    if (bytes.lengthInBytes == 0)
      return null;

    return await ui.instantiateImageCodec(bytes);
  }

  @override
  bool operator ==(dynamic other) {
    if (other.runtimeType != runtimeType)
      return false;
    final FileImage typedOther = other;
    return file?.path == typedOther.file?.path
        && scale == typedOther.scale;
  }

  @override
  int get hashCode => hashValues(file?.path, scale);

  @override
  String toString() => '$runtimeType("${file?.path}", scale: $scale)';
}

FileImage重写了 obtainKey、load的方法。但是在什么地方会调用这两个重写的方法呢？那肯定是ImageProvider这个父类了。

@optionalTypeArgs
abstract class ImageProvider<T> {
  /// Abstract const constructor. This constructor enables subclasses to provide
  /// const constructors so that they can be used in const expressions.
  const ImageProvider();

  /// Resolves this image provider using the given `configuration`, returning
  /// an [ImageStream].
  ///
  /// This is the public entry-point of the [ImageProvider] class hierarchy.
  ///
  /// Subclasses should implement [obtainKey] and [load], which are used by this
  /// method.
  ImageStream resolve(ImageConfiguration configuration) {
    assert(configuration != null);
    final ImageStream stream = new ImageStream();
    T obtainedKey;
    obtainKey(configuration).then<void>((T key) {
      obtainedKey = key;
      stream.setCompleter(PaintingBinding.instance.imageCache.putIfAbsent(key, () => load(key)));
    }).catchError(
      (dynamic exception, StackTrace stack) async {
        FlutterError.reportError(new FlutterErrorDetails(
          exception: exception,
          stack: stack,
          library: 'services library',
          context: 'while resolving an image',
          silent: true, // could be a network error or whatnot
          informationCollector: (StringBuffer information) {
            information.writeln('Image provider: $this');
            information.writeln('Image configuration: $configuration');
            if (obtainedKey != null)
              information.writeln('Image key: $obtainedKey');
          }
        ));
        return null;
      }
    );
    return stream;
  }

  /// Converts an ImageProvider's settings plus an ImageConfiguration to a key
  /// that describes the precise image to load.
  ///
  /// The type of the key is determined by the subclass. It is a value that
  /// unambiguously identifies the image (_including its scale_) that the [load]
  /// method will fetch. Different [ImageProvider]s given the same constructor
  /// arguments and [ImageConfiguration] objects should return keys that are
  /// '==' to each other (possibly by using a class for the key that itself
  /// implements [==]).
  @protected
  Future<T> obtainKey(ImageConfiguration configuration);

  /// Converts a key into an [ImageStreamCompleter], and begins fetching the
  /// image.
  @protected
  ImageStreamCompleter load(T key);

  @override
  String toString() => '$runtimeType()';
}

该方法的作用就是创建一个ImageStream，并且ImageConfiguration作为key从ImageCache中获取ImageCompleter，设置到ImageStream上面。而ImageCompleter是为了设置一些回调和帮助ImageStream设置图片的一个类。

ImageConfiguration是对于ImageCompleter的一些配置。

ImageCache是对于ImageCompleter的缓存。 ImageStreamCompleter putIfAbsent(Object key, ImageStreamCompleter loader(), { ImageErrorListener onError }) 这个方法在resolve中是一个关键方法。

ImageStreamCompleter putIfAbsentImageStreamCompleter putIfAbsent(Object key, ImageStreamCompleter loader()) {
    assert(key != null);
    assert(loader != null);
    ImageStreamCompleter result = _cache[key];
    if (result != null) {
      // Remove the provider from the list so that we can put it back in below
      // and thus move it to the end of the list.
      _cache.remove(key);
    } else {
      if (_cache.length == maximumSize && maximumSize > 0)
        _cache.remove(_cache.keys.first);
      result = loader();
    }
    if (maximumSize > 0) {
      assert(_cache.length < maximumSize);
      _cache[key] = result;
    }
    assert(_cache.length <= maximumSize);
    return result;
 }

这个方法是在imageCache里面的，提供的是内存缓存api的入口方法，putIfAbsent会先通过key获取之前的ImageStreamCompleter对象，这个key就是NetworkImage对象，当然我们也可以重写obtainKey方法自定义key，如果存在则直接返回，如果不存在则执行load方法加载ImageStreamCompleter对象，并将其放到首位（最少最近使用算法）。也就是说ImageProvider已经实现了内存缓存：默认缓存图片的最大个数是1000，默认缓存图片的最大空间是10MiB。第一次加载图片肯定是没有缓存的，所以会调用loader方法，那就是方法外面传进去的load()方法。

FileImage的load方法

@override
ImageStreamCompleter load(FileImage key) {
  return new MultiFrameImageStreamCompleter(
    codec: _loadAsync(key),
    scale: key.scale,
    informationCollector: (StringBuffer information) {
      information.writeln('Path: ${file?.path}');
    }
  );
}

Future<ui.Codec> _loadAsync(FileImage key) async {
  assert(key == this);

  final Uint8List bytes = await file.readAsBytes();
  if (bytes.lengthInBytes == 0)
    return null;

  return await ui.instantiateImageCodec(bytes);
}

load方法中使用了一个叫MultiFrameImageStreamCompleter的类：

MultiFrameImageStreamCompleter({
  @required Future<ui.Codec> codec,
  @required double scale,
  InformationCollector informationCollector
}) : assert(codec != null),
     _informationCollector = informationCollector,
     _scale = scale,
     _framesEmitted = 0,
     _timer = null {
  codec.then<void>(_handleCodecReady, onError: (dynamic error, StackTrace stack) {
    FlutterError.reportError(new FlutterErrorDetails(
      exception: error,
      stack: stack,
      library: 'services',
      context: 'resolving an image codec',
      informationCollector: informationCollector,
      silent: true,
    ));
  });
}

MultiFrameImageStreamCompleter是ImageStreamCompleter的子类，为了处理多帧的图片加载，Flutter的Image支持加载webp，通过MultiFrameImageStreamCompleter可以对webp文件进行解析，MultiFrameImageStreamCompleter拿到外面传入的codec数据对象,通过handleCodecReady来保存Codec，之后调用decodeNextFrameAndSchedule方法，从Codec获取下一帧图片数据和把数据通知回调到Image，并且开启定时解析下一帧图片数据。

到此为止，基本dart流程就走完了，所以需要做stopAnimation和startAnimation的改造就应该这这个MultiFrameImageStreamCompleter入手了。

最后

整个在Dart层面Image解析webp的流程就这样，下篇再介绍下现在Dart的一些Codec的工作流程。

探索Flutter Image显示Webp逻辑

简介

Image

ImageState

ImageProvider

最后