1. override
void addToScene(SceneBuilder builder, Offset layerOffset)

Override this method to upload this layer to the engine

The layerOffset is the accumulated offset of this layer's parent from the origin of the builder's coordinate system.

Source 

@override
void addToScene(ui.SceneBuilder builder, Offset layerOffset) {
  builder.pushOpacity(alpha);
  addChildrenToScene(builder, layerOffset);
  builder.pop();
}