registering animation listener to our animation controller

default_app_bar.dart

bool isInSearchMode = false;

// inside initState register our status listener
_controller.addStatusListener(animationStatusListener);

// add this method to our DefaultAppBar widget
animationStatusListener(AnimationStatus animationStatus) {
  if (animationStatus == AnimationStatus.completed) {
    setState(() {
      isInSearchMode = true; 
    });
  }
}

// depending on isInSearchMode decide whether we should render SearchBar()
// or an empty Container() in render method of DefaultAppBar
render() {
  return Stack(
    children: [
      // default app bar
      
      // animated builder which handles ripple animation
      AnimatedBuilder(
        animation: _animation,
        builder: (context, child) {
          return CustomPaint(
            painter: MyPainter(
              containerHeight: widget.preferredSize.height,
              center: Offset(rippleStartX ?? 0, rippleStartY ?? 0),
              radius: _animation.value * screenWidth,
              context: context,
            ),
          );
        },
      ),
  
      // search bar depending on whether app bar is in search mode or not
      isInSearchMode ? (
        SearchBar()
      ) : (
        Container()
      )
    ]
  );
}