onStart method
Called at the exact moment the widget is allocated in memory. It uses an internal "callable" type, to avoid any @overrides in subclasses. This method should be internal and is required to define the lifetime cycle of the subclass.
Implementation
// @protected
@mustCallSuper
@nonVirtual
void onStart() {
  if (_initialized) {
    return;
  }
  onInit();
  _initialized = true;
}