Creates the mutable state for this widget at a given location in the tree.
Subclasses should override this method to return a newly created instance of their associated State subclass:
@override _MyState createState() => new _MyState();
The framework can call this method multiple times over the lifetime of a StatefulWidget. For example, if the widget is inserted into the tree in multiple locations, the framework will create a separate State object for each location. Similarly, if the widget is removed from the tree and later inserted into the tree again, the framework will call createState again to create a fresh State object, simplifying the lifecycle of State objects.
@override _MaterialAppState createState() => new _MaterialAppState();