AssetImage class

Fetches an image from an AssetBundle, having determined the exact image to use based on the context.

Given a main asset and a set of variants, AssetImage chooses the most appropriate asset for the current context, based on the device pixel ratio and size given in the configuration passed to resolve.

To show a specific image from a bundle without any asset resolution, use an AssetBundleImageProvider.

Naming assets for matching with different pixel densities

Main assets are presumed to match a nominal pixel ratio of 1.0. To specify assets targeting different pixel ratios, place the variant assets in the application bundle under subdirectories named in the form "Nx", where N is the nominal device pixel ratio for that asset.

For example, suppose an application wants to use an icon named "heart.png". This icon has representations at 1.0 (the main icon), as well as 2.0 and 4.0 pixel ratios (variants). The asset bundle should then contain the following assets:

heart.png
2.0x/heart.png
4.0x/heart.png

On a device with a 1.0 device pixel ratio, the image chosen would be heart.png; on a device with a 2.0 device pixel ratio, the image chosen would be 2.0x/heart.png; on a device with a 4.0 device pixel ratio, the image chosen would be 4.0x/heart.png.

On a device with a device pixel ratio that does not exactly match an available asset the "best match" is chosen. Which asset is the best depends on the screen. Low-resolution screens (those with device pixel ratio strictly less than 2.0) use a different matching algorithm from the high-resolution screen. Because in low-resolution screens the physical pixels are visible to the user upscaling artifacts (e.g. blurred edges) are more pronounced. Therefore, a higher resolution asset is chosen, if available. For higher-resolution screens, where individual physical pixels are not visible to the user, the asset variant with the pixel ratio that's the closest to the screen's device pixel ratio is chosen.

For example, for a screen with device pixel ratio 1.25 the image chosen would be 2.0x/heart.png, even though heart.png (i.e. 1.0) is closer. This is because the screen is considered low-resolution. For a screen with device pixel ratio of 2.25 the image chosen would also be 2.0x/heart.png. This is because the screen is considered to be a high-resolution screen, and therefore upscaling a 2.0x image to 2.25 won't result in visible upscaling artifacts. However, for a screen with device-pixel ratio 3.25 the image chosen would be 4.0x/heart.png because it's closer to 4.0 than it is to 2.0.

Choosing a higher-resolution image than necessary may waste significantly more memory if the difference between the screen device pixel ratio and the device pixel ratio of the image is high. To reduce memory usage, consider providing more variants of the image. In the example above adding a 3.0x/heart.png variant would improve memory usage for screens with device pixel ratios between 3.0 and 3.5.

ImageConfiguration can be used to customize the selection of the image variant by setting ImageConfiguration.devicePixelRatio to value different from the default. The default value is derived from MediaQueryData.devicePixelRatio by createLocalImageConfiguration.

The directory level of the asset does not matter as long as the variants are at the equivalent level; that is, the following is also a valid bundle structure:

icons/heart.png
icons/1.5x/heart.png
icons/2.0x/heart.png

assets/icons/3.0x/heart.png would be a valid variant of assets/icons/heart.png.

Fetching assets

When fetching an image provided by the app itself, use the assetName argument to name the asset to choose. For instance, consider the structure above. First, the pubspec.yaml of the project should specify its assets in the flutter section:

flutter:
  assets:
    - icons/heart.png

Then, to fetch the image, use:

const AssetImage('icons/heart.png')

class MyImage extends StatefulWidget {
  const MyImage({
    super.key,
    required this.assetImage,
  });

  final AssetImage assetImage;

  @override
  State<MyImage> createState() => _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.assetImage != oldWidget.assetImage) {
      _getImage();
    }
  }

  void _getImage() {
    final ImageStream? oldImageStream = _imageStream;
    _imageStream = widget.assetImage.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.
      final ImageStreamListener listener = ImageStreamListener(_updateImage);
      oldImageStream?.removeListener(listener);
      _imageStream!.addListener(listener);
    }
  }

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

  @override
  void dispose() {
    _imageStream?.removeListener(ImageStreamListener(_updateImage));
    _imageInfo?.dispose();
    _imageInfo = null;
    super.dispose();
  }

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

Assets in packages

To fetch an asset from a package, the package argument must be provided. For instance, suppose the structure above is inside a package called my_icons. Then to fetch the image, use:

const AssetImage('icons/heart.png', package: 'my_icons')

Assets used by the package itself should also be fetched using the package argument as above.

If the desired asset is specified in the pubspec.yaml of the package, it is bundled automatically with the app. In particular, assets used by the package itself must be specified in its pubspec.yaml.

A package can also choose to have assets in its 'lib/' folder that are not specified in its pubspec.yaml. In this case for those images to be bundled, the app has to specify which ones to include. For instance a package named fancy_backgrounds could have:

lib/backgrounds/background1.png
lib/backgrounds/background2.png
lib/backgrounds/background3.png

To include, say the first image, the pubspec.yaml of the app should specify it in the assets section:

  assets:
    - packages/fancy_backgrounds/backgrounds/background1.png

The lib/ is implied, so it should not be included in the asset path.

See also:

• Image.asset for a shorthand of an Image widget backed by AssetImage when used without a scale.