Push a backdrop filter.
This function applies a filter to the existing painted content and then synchronously calls the painter to paint on top of the filtered backdrop.
A RenderObject that uses this function is very likely to require its RenderObject.alwaysNeedsCompositing property to return true. That informs ancestor render objects that this render object will include a composited layer, which causes them to use composited clips, for example.
Source
// TODO(abarth): I don't quite understand how this API is supposed to work.
void pushBackdropFilter(Offset offset, ui.ImageFilter filter, PaintingContextCallback painter) {
_stopRecordingIfNeeded();
final BackdropFilterLayer backdropFilterLayer = new BackdropFilterLayer(filter: filter);
_appendLayer(backdropFilterLayer);
final PaintingContext childContext = new PaintingContext._(backdropFilterLayer, _paintBounds);
painter(childContext, offset);
childContext._stopRecordingIfNeeded();
}