void pushShaderMask(Offset offset, Shader shader, Rect maskRect, TransferMode transferMode, PaintingContextCallback painter)

Apply a mask derived from a shader to further painting.

  • offset is the offset from the origin of the canvas' coordinate system to the origin of the caller's coordinate system.
  • shader is the shader that will generate the mask. The shader operates in the coordinate system of the caller.
  • maskRect is the region of the canvas (in the coodinate system of the caller) in which to apply the mask.
  • transferMode is the TransferMode to use when applying the shader to the painting done by painter.
  • painter is a callback that will paint with the mask applied. This function calls the painter synchronously.

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 

void pushShaderMask(Offset offset, Shader shader, Rect maskRect, TransferMode transferMode, PaintingContextCallback painter) {
  _stopRecordingIfNeeded();
  final ShaderMaskLayer shaderLayer = new ShaderMaskLayer(
    shader: shader,
    maskRect: maskRect,
    transferMode: transferMode
  );
  _appendLayer(shaderLayer);
  final PaintingContext childContext = new PaintingContext._(shaderLayer, _paintBounds);
  painter(childContext, offset);
  childContext._stopRecordingIfNeeded();
}