Identifies an image without committing to the precise final asset. This allows a set of images to be identified and for the precise image to later be resolved based on the environment, e.g. the device pixel ratio.

To obtain an ImageStream from an ImageProvider, call resolve, passing it an ImageConfiguration object.

ImageProvider uses the global imageCache to cache images.

The type argument T is the type of the object used to represent a resolved configuration. This is also the type used for the key in the image cache. It should be immutable and implement the == operator and the hashCode getter. Subclasses should subclass a variant of ImageProvider with an explicit T type argument.

The type argument does not have to be specified when using the type as an argument (where any image provider is acceptable).

Sample code

The following shows the code required to write a widget that fully conforms to the ImageProvider and Widget protocols. (It is essentially a bare-bones version of the widgets.Image widget.)

class MyImage extends StatefulWidget {
  const MyImage({
    Key key,
    @required this.imageProvider,
  }) : assert(imageProvider != null),
       super(key: key);

  final ImageProvider imageProvider;

  @override
  _MyImageState createState() => new _MyImageState();
}

class _MyImageState extends State<MyImage> {
  ImageStream _imageStream;
  ImageInfo _imageInfo;

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    // We call _getImage here because createLocalImageConfiguration() needs to
    // be called again if the dependencies changed, in case the changes relate
    // to the DefaultAssetBundle, MediaQuery, etc, which that method uses.
    _getImage();
  }

  @override
  void didUpdateWidget(MyImage oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (widget.imageProvider != oldWidget.imageProvider)
      _getImage();
  }

  void _getImage() {
    final ImageStream oldImageStream = _imageStream;
    _imageStream = widget.imageProvider.resolve(createLocalImageConfiguration(context));
    if (_imageStream.key != oldImageStream?.key) {
      // If the keys are the same, then we got the same image back, and so we don't
      // need to update the listeners. If the key changed, though, we must make sure
      // to switch our listeners to the new image stream.
      oldImageStream?.removeListener(_updateImage);
      _imageStream.addListener(_updateImage);
    }
  }

  void _updateImage(ImageInfo imageInfo, bool synchronousCall) {
    setState(() {
      // Trigger a build whenever the image changes.
      _imageInfo = imageInfo;
    });
  }

  @override
  void dispose() {
    _imageStream.removeListener(_updateImage);
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return new RawImage(
      image: _imageInfo?.image, // this is a dart:ui Image object
      scale: _imageInfo?.scale ?? 1.0,
    );
  }
}
Implemented by
Annotations
  • @optionalTypeArgs

Constructors

ImageProvider()
Abstract const constructor. This constructor enables subclasses to provide const constructors so that they can be used in const expressions.
const

Properties

hashCode int
The hash code for this object. [...]
read-only, inherited
runtimeType Type
A representation of the runtime type of the object.
read-only, inherited

Methods

load(T key) ImageStreamCompleter
Converts a key into an ImageStreamCompleter, and begins fetching the image.
@protected
obtainKey(ImageConfiguration configuration) Future<T>
Converts an ImageProvider's settings plus an ImageConfiguration to a key that describes the precise image to load. [...]
@protected
resolve(ImageConfiguration configuration) ImageStream
Resolves this image provider using the given configuration, returning an ImageStream. [...]
toString() String
Returns a string representation of this object.
noSuchMethod(Invocation invocation) → dynamic
Invoked when a non-existent method or property is accessed. [...]
inherited

Operators

operator ==(other) bool
The equality operator. [...]
inherited