animation slide via AnimatedBuilder

menu_mobile.widget.dart

class _Mobile extends StatelessWidget {
  final Widget child;

  _Mobile({this.child});

  @override
  Widget build(BuildContext context) {
    double width = MediaQuery.of(context).size.width / 3;
    int elapsed = 1;
    return Center(
      child: Container(
        height: MediaQuery.of(context).size.height,
        width: width,
        child: AnimatedSwitcher(
          key: ValueKey(elapsed),
          duration: Duration(milliseconds: 500),
          transitionBuilder: (child, animation) {
            final inAnimation = Tween<Offset>(begin: Offset(1.0, 0.0), end: Offset(0.0, 0.0)).animate(animation);
            final outAnimation = Tween<Offset>(begin: Offset(-1.0, 0.0), end: Offset(0.0, 0.0)).animate(animation);

            if (child.key == ValueKey(elapsed)) {
              return ClipRect(
                child: SlideTransition(
                  position: outAnimation,
                  child: child,
                ),
              );
            } else {
              return ClipRect(
                child: SlideTransition(
                  position: inAnimation,
                  child: child,
                ),
              );
            }
          },
          child: this.child,
        ),
      ),
    );
  }