1. override
void initState()

Called when this object is inserted into the tree.

The framework will call this method exactly once for each State object it creates.

Override this method to perform initialization that depends on the location at which this object was inserted into the tree (i.e., context) or on the widget used to configure this object (i.e., config).

If a State's build method depends on an object that can itself change state, for example a ChangeNotifier or Stream, or some other object to which one can subscribe to receive notifications, then the State should subscribe to that object during initState, unsubscribe from the old object and subscribe to the new object when it changes in didUpdateConfig, and then unsubscribe from the object in dispose.

You cannot use BuildContext.inheritFromWidgetOfExactType from this method. However, dependenciesChanged will be called immediately following this method, and BuildContext.inheritFromWidgetOfExactType can be used there.

If you override this, make sure your method starts with a call to super.initState().

Source 

@override
void initState() {
  super.initState();
  _appBarController = new AnimationController(vsync: this);
  // Use an explicit identifier to guard against the possibility that the
  // Scaffold's key is recreated by the Widget that creates the Scaffold.
  List<double> scrollValues = PageStorage.of(context)?.readState(context,
    identifier: _kScaffoldStorageIdentifier
  );
  if (scrollValues != null) {
    assert(scrollValues.length == 2);
    _scrollOffset = scrollValues[0];
    _scrollOffsetDelta = scrollValues[1];
  }
}