pushOpacity method

OpacityEngineLayer pushOpacity (
  1. int alpha,
  2. {Offset offset: Offset.zero,
  3. OpacityEngineLayer oldLayer}
)

Pushes an opacity operation onto the operation stack.

The given alpha value is blended into the alpha value of the objects' rasterization. An alpha value of 0 makes the objects entirely invisible. An alpha value of 255 has no effect (i.e., the objects retain the current opacity).

If oldLayer is not null the engine will attempt to reuse the resources allocated for the old layer when rendering the new layer. This is purely an optimization. It has no effect on the correctness of rendering.

Passing a layer to addRetained or as oldLayer argument to a push method counts as usage. A layer can be used no more than once in a scene. For example, it may not be passed simultaneously to two push methods, or to a push method and to addRetained.

When a layer is passed to addRetained all descendant layers are also considered as used in this scene. The same single-usage restriction applies to descendants.

When a layer is passed as an oldLayer argument to a push method, it may no longer be used in subsequent frames. If you would like to continue reusing the resources associated with the layer, store the layer object returned by the push method and use that in the next frame instead of the original object.

See pop for details about the operation stack.

Implementation

OpacityEngineLayer? pushOpacity(
  int alpha, {
  Offset? offset = Offset.zero,
  OpacityEngineLayer? oldLayer,
}) {
  assert(_debugCheckCanBeUsedAsOldLayer(oldLayer, 'pushOpacity'));
  final EngineLayer engineLayer = EngineLayer._();
  _pushOpacity(engineLayer, alpha, offset!.dx, offset.dy);
  final OpacityEngineLayer layer = OpacityEngineLayer._(engineLayer);
  assert(_debugPushLayer(layer));
  return layer;
}