Animation in Flutter: How to resize a Stack?

Question

What's the right way to animate a change in size of a Stack widget?

The first idea was to wrap the Stack with a SizeTransition:

Widget build(BuildContext context) {
    return Column(
      crossAxisAlignment: CrossAxisAlignment.stretch,
      children: [
        SizeTransition(
          sizeFactor: _height, // Just a Tween<double>(begin: 200, end: 400)
          axis: Axis.vertical,
          axisAlignment: -1,
          child: Stack(
            children: [
              Positioned(
                bottom: 10,
                left: 10,
                child: Container(width: 50, height: 50, color: Colors.blue),
              ),
              Positioned(
                top: 10,
                right: 10,
                child: Container(width: 50, height: 50, color: Colors.green),
              ),
            ],
          ),
        ),
      ],
    );
  }

But running the snippet causes the following error:

'package:flutter/src/rendering/stack.dart': Failed assertion: line 588 pos 12: 'size.isFinite': is not true.

So, apparently, that's not the right way to do it. Questions now is: How do you animate resizing of a Stack?

Wrapping the Stack with a SizedBox would fix the error message, but then, how do you get the size from SizedTransition to set the proper size of a SizedBox?

Answer 1

You can use AnimatedContainer as parent of Stack widget by providing height or width one you like to change over time.

AnimatedContainer(
  color: Colors.purple,
  duration: const Duration(milliseconds: 200),
  height: height,
  child: Stack(
    children: [
      Positioned(
        bottom: 10,
        left: 10,
        child: Container(width: 50, height: 50, color: Colors.blue),
      ),
      Positioned(
        top: 10,
        right: 10,
        child: Container(width: 50, height: 50, color: Colors.green),
      ),
    ],
  ),
),