Adding ImageProvider.loadBuffer

• ImageProvider now has a method called loadBuffer that functions similarly to load, except that it decodes from an ui.ImmutableBuffer.
• ui.ImmutableBuffer can now be created directly from an asset key.
• The AssetBundle classes can now load an ui.ImmutableBuffer.
• The PaintingBinding now has a method called instantiateImageCodecFromBuffer, which functions similarly to instantiateImageCodec.
• ImageProvider.load is now deprecated, it will be removed in a future release.
• PaintingBinding.instantiateImageCodec is now deprecated, it will be removed in a future release.

ImageProvider.loadBuffer is a new method that must be implemented in order to load images. This API allows asset-based image loading to be performed faster and with less memory impact on application.

When loading asset images, previously the image provider API required multiple copies of the compressed data. First, when opening the asset the data was copied into the external heap and exposed to Dart as a typed data array. Then that typed data array was eventually converted into an ui.ImmutableBuffer, which internally copies the data into a second structure for decoding.

With the addition of ui.ImmutableBuffer.fromAsset, compressed image bytes can be loaded directly into the structure used for decoding. Using this approach requires changes to the byte loading pipeline of ImageProvider. This process is also faster, because it bypasses some additional scheduling overhead of the previous method channel based loader.

ImageProvider.loadBuffer otherwise has the same contract as ImageProvider.load, except it provides a new decoding callback that expects an ui.ImmutableBuffer instead of a Uint8List. For ImageProvider classes that acquire bytes from places other than assets, the convenience method ui.ImmutableBuffer.fromUint8List can be used for compatibility.

Classes that subclass ImageProvider must implement the loadBuffer method for loading assets. Classes that delegate to or call the methods of an ImageProvider directly must use loadBuffer instead of load.

Code before migration:

class MyImageProvider extends ImageProvider<MyImageProvider> {
  @override
  ImageStreamCompleter load(MyImageProvider key, DecoderCallback decode) {
    return MultiFrameImageStreamCompleter(
        codec: _loadData(key, decode),
    );
  }

  Future<ui.Codec> _loadData(MyImageProvider key, DecoderCallback decode) async {
    final Uint8List bytes = await bytesFromSomeApi();
    return decode(bytes);
  }
}

class MyDelegatingProvider extends ImageProvider<MyDelegatingProvider> {
  MyDelegatingProvider(this.provider);

  final ImageProvder provider;

  @override
  ImageStreamCompleter load(MyDelegatingProvider key, DecoderCallback decode) {
    return provider.load(key, decode);
  }
}

Code after migration:

class MyImageProvider extends ImageProvider<MyImageProvider> {
  @override
  ImageStreamCompleter loadBuffer(MyImageProvider key, DecoderBufferCallback decode) {
    return MultiFrameImageStreamCompleter(
        codec: _loadData(key, decode),
    );
  }

  Future<ui.Codec> _loadData(MyImageProvider key, DecoderBufferCallback decode) async {
    final Uint8List bytes = await bytesFromSomeApi();
    final ui.ImmutableBuffer buffer = await ui.ImmutableBuffer.fromUint8List(bytes);
    return decode(buffer);
  }
}

class MyDelegatingProvider extends ImageProvider<MyDelegatingProvider> {
  MyDelegatingProvider(this.provider);

  final ImageProvder provider;

  @override
  ImageStreamCompleter loadBuffer(MyDelegatingProvider key, DecoderCallback decode) {
    return provider.loadBuffer(key, decode);
  }
}

In both cases you might choose to keep the previous implementation of ImageProvider.load to give users of your code time to migrate as well.

Landed in version: 3.1.0-0.0.pre.976
In stable release: 3.3.0

API documentation:

• ImmutableBuffer
• ImageProvider

Relevant PR:

• Use immutable buffer for loading asset images

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