addCompositionCallback method Null safety

VoidCallback addCompositionCallback(
  1. CompositionCallback callback

Adds a callback for when the layer tree that this layer is part of gets composited, or when it is detached and will not be rendered again.

This callback will fire even if an ancestor layer is added with retained rendering, meaning that it will fire even if this layer gets added to the scene via some call to ui.SceneBuilder.addRetained on one of its ancestor layers.

The callback receives a reference to this layer. The recipient must not mutate the layer during the scope of the callback, but may traverse the tree to find information about the current transform or clip. The layer may not be attached anymore in this state, but even if it is detached it may still have an also detached parent it can visit.

If new callbacks are added or removed within the callback, the new callbacks will fire (or stop firing) on the next compositing event.

Composition callbacks are useful in place of pushing a layer that would otherwise try to observe the layer tree without actually affecting compositing. For example, a composition callback may be used to observe the total transform and clip of the current container layer to determine whether a render object drawn into it is visible or not.

Calling the returned callback will remove callback from the composition callbacks.


VoidCallback addCompositionCallback(CompositionCallback callback) {
  final int callbackId = _nextCallbackId += 1;
  _callbacks[callbackId] = () {
    assert(() {
      _debugMutationsLocked = true;
      return true;
    assert(() {
      _debugMutationsLocked = false;
      return true;
  return () {
    assert(debugDisposed || _callbacks.containsKey(callbackId));