BackdropFilter class

A widget that applies a filter to the existing painted content and then paints child.

The filter will be applied to all the area within its parent or ancestor widget's clip. If there's no clip, the filter will be applied to the full screen.

The results of the filter will be blended back into the background using the blendMode parameter. The only value for blendMode that is supported on all platforms is BlendMode.srcOver which works well for most scenes. But that value may produce surprising results when a parent of the BackdropFilter uses a temporary buffer, or save layer, as does an Opacity widget. In that situation, a value of BlendMode.src can produce more pleasing results.

Multiple backdrop filters can be combined into a single rendering operation by the Flutter engine if these backdrop filters widgets all share a common BackdropKey. The backdrop key uniquely identifies the input for a backdrop filter, and when shared, indicates the filtering can be performed once. This can significantly reduce the overhead of using multiple backdrop filters in a scene. The key can either be provided manually via the backdropKey constructor parameter or looked up from a BackdropGroup inherited widget via the .grouped constructor.

Backdrop filters that overlap with each other should not use the same backdrop key, otherwise the results may look as if only one filter is applied in the overlapping regions.

The following snippet demonstrates how to use the backdrop key to allow each list item to have an efficient blur. The engine will perform only one backdrop blur but the results will be visually identical to multiple blurs.

 Widget build(BuildContext context) {
   return BackdropGroup(
     child: ListView.builder(
       itemCount: 60,
       itemBuilder: (BuildContext context, int index) {
         return ClipRect(
           child: BackdropFilter.grouped(
             filter: ui.ImageFilter.blur(
               sigmaX: 40,
               sigmaY: 40,
             ),
             child: Container(
               color: Colors.black.withOpacity(0.2),
               height: 200,
               child: const Text('Blur item'),
             ),
           ),
         );
      }
    ),
  );
}

Stack(
  fit: StackFit.expand,
  children: <Widget>[
    Text('0' * 10000),
    Center(
      child: ClipRect(  // <-- clips to the 200x200 [Container] below
        child: BackdropFilter(
          filter: ui.ImageFilter.blur(
            sigmaX: 5.0,
            sigmaY: 5.0,
          ),
          child: Container(
            alignment: Alignment.center,
            width: 200.0,
            height: 200.0,
            child: const Text('Hello World'),
          ),
        ),
      ),
    ),
  ],
)

This effect is relatively expensive, especially if the filter is non-local, such as a blur.

If all you want to do is apply an ImageFilter to a single widget (as opposed to applying the filter to everything beneath a widget), use ImageFiltered instead. For that scenario, ImageFiltered is both easier to use and less expensive than BackdropFilter.

 Widget buildBackdrop() {
   return Stack(
     children: <Widget>[
       Positioned.fill(child: Image.asset('image.png')),
       Positioned.fill(
         child: BackdropFilter(
           filter: ui.ImageFilter.blur(sigmaX: 6, sigmaY: 6),
         ),
       ),
     ],
   );
 }

 Widget buildFilter() {
   return ImageFiltered(
     imageFilter: ui.ImageFilter.blur(sigmaX: 6, sigmaY: 6),
     child: Image.asset('image.png'),
   );
 }

See also:

• ImageFiltered, which applies an ImageFilter to its child.
• DecoratedBox, which draws a background under (or over) a widget.
• Opacity, which changes the opacity of the widget itself.
• https://flutter.cn/go/ios-platformview-backdrop-filter-blur for details and restrictions when an iOS PlatformView needs to be blurred.