didUpdateWidget method Null safety

  1. @override
void didUpdateWidget(
  1. covariant Scrollable oldWidget
)
override

Called whenever the widget configuration changes.

If the parent widget rebuilds and request that this location in the tree update to display a new widget with the same runtimeType and Widget.key, the framework will update the widget property of this State object to refer to the new widget and then call this method with the previous widget as an argument.

Override this method to respond when the widget changes (e.g., to start implicit animations).

The framework always calls build after calling didUpdateWidget, which means any calls to setState in didUpdateWidget are redundant.

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 be sure to subscribe and unsubscribe properly in initState, didUpdateWidget, and dispose:

  • In initState, subscribe to the object.
  • In didUpdateWidget unsubscribe from the old object and subscribe to the new one if the updated widget configuration requires replacing the object.
  • In dispose, unsubscribe from the object.

Implementations of this method should start with a call to the inherited method, as in super.didUpdateWidget(oldWidget).

Implementation

@override
void didUpdateWidget(Scrollable oldWidget) {
  super.didUpdateWidget(oldWidget);

  if (widget.controller != oldWidget.controller) {
    if (oldWidget.controller == null) {
      // The old controller was null, meaning the fallback cannot be null.
      // Dispose of the fallback.
      assert(_fallbackScrollController !=  null);
      assert(widget.controller != null);
      _fallbackScrollController!.detach(position);
      _fallbackScrollController!.dispose();
      _fallbackScrollController = null;
    } else {
      // The old controller was not null, detach.
      oldWidget.controller?.detach(position);
      if (widget.controller == null) {
        // If the new controller is null, we need to set up the fallback
        // ScrollController.
        _fallbackScrollController = ScrollController();
      }
    }
    // Attach the updated effective scroll controller.
    _effectiveScrollController.attach(position);
  }

  if (_shouldUpdatePosition(oldWidget))
    _updatePosition();
}