SnapshotPainter class abstract

A painter used to paint either a snapshot or the child widgets that would be a snapshot.

The painter can call notifyListeners to have the SnapshotWidget re-paint (re-using the same raster). This allows animations to be performed without re-snapshotting of children. For certain scale or perspective changing transforms, such as a rotation, this can be significantly faster than performing the same animation at the widget level.

By default, the SnapshotWidget includes a delegate that draws the child raster exactly as the child widgets would have been drawn. Nevertheless, this can also be used to efficiently transform the child raster and apply complex paint effects.

void paint(PaintingContext context, Offset offset, Size size, ui.Image image, double pixelRatio) {
  const double radians = 0.5; // Could be driven by an animation.
  final Matrix4 transform = Matrix4.rotationZ(radians);
  context.canvas.transform(transform.storage);
  final Rect src = Rect.fromLTWH(0, 0, image.width.toDouble(), image.height.toDouble());
  final Rect dst = Rect.fromLTWH(offset.dx, offset.dy, size.width, size.height);
  final Paint paint = Paint()
    ..filterQuality = FilterQuality.medium;
  context.canvas.drawImageRect(image, src, dst, paint);
}