atreeon_get_child_size 0.1.0

A widget that returns the size of the child object. The [onChange] callback will run everytime the widget's size has changed.

atreeon_get_child_size

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A widget that returns the size of the child object. The [onChange] callback will run everytime the widget's size has changed.

Waring! will result in multiple passes from the layout algorithm if you rebuild a widget from onChange; it will affect performance

This is a fundamental restriction in how widgets are built -> constraints go down & sizes go up -> if you change the constraint above, you must change the size below

Example

class _GetChildSize_DemoState extends State<GetChildSize_Demo> {
  void initState() {
    super.initState();
  }

  Size myChildSize = Size(10, 10);

  Widget build(BuildContext context) {
    return Column(
      children: [
        Text(myChildSize.height.toString()),
        Expanded(
          child: GetChildSize(
            onChange: (size) {
              setState(() {
                myChildSize = size;
              });
            },
            child: Text(myChildSize.height.toString()),
          ),
        )
      ],
    );
  }
}

Discussion on code & implementation

https://github.com/flutter/flutter/issues/14488

If you just want to know the dimensions between layouts (i.e. you're not using them to affect the layout), you can just read them right out of the rendering tree (get the RenderObject from the BuildContext, etc).

However, if you are using the dimensions to actually affect the layout, then that will lead to horrible effects like lagging one frame behind, or worse. In those cases, you want to just build your own render object, or use a layout builder, or similar, depending on exactly what your use case is.

I need the size of a child to use it in the parent's build.

Fundamentally that's not possible, because at the time of the parent's build the child hasn't laid out yet. If you want to get the child's size during the parent's layout, you can do that by creating your own RenderObject. Just look at how e.g. Align is implemented.