void pushShaderMask(Shader shader, Rect maskRect, TransferMode transferMode)

Pushes a shader mask operation onto the operation stack.

The given shader is applied to the object's rasterization in the given rectangle using the given transfer mode.

See pop for details about the operation stack.

Source 

void pushShaderMask(Shader shader, Rect maskRect, TransferMode transferMode) {
  _pushShaderMask(shader,
                  maskRect.left,
                  maskRect.right,
                  maskRect.top,
                  maskRect.bottom,
                  transferMode.index);
}