Expand widgets inside the Stack widget

Question

I have a stack widgets with two widgets inside. One of them draws the background, the other one draws on top of that. I want both widgets to have the same size.

I want the top widget, which is a column, to expand and fill the stack vertically to the size that is set by the background widget. I tried setting the MainAxis mainAxisSize: MainAxisSize.max but it didn't work.

How can I make it work?

Answer 1

Use Positioned.fill

    Stack(       children: [         Positioned.fill(           child: Column(             children: [               //...             ],           ),         ),         //...       ],     );  

More info about Positioned in Flutter Widget of the Week

How does it work?

This is all about constraints. Constraints are min/max width/height values that are passed from the parent to its children. In the case of Stack. The constraints it passes to its children state that they can size themselves as small as they want, which in the case of some widgets means they will be their "natural" size. After being sized, Stack will place its children in the top left corner.
Positioned.fill gets the same constraints from Stack but passes different constraints to its child, stating the it (the child) must be of exact size (which meant to fill the Stack).

Positioned.fill() is the same as:

Positioned(   top: 0,   right: 0,   left: 0,   bottom:0,   child: //... ) 

For even more examples and info: How to fill a Stack widget and why? and Understanding constraints.