RenderBox was not laid out: hasSize is false

Why This Happens

In Flutter, this assertion failure occurs when you try to get the size of a widget before Flutter has completed the layout pass. This often happens when using GlobalKey to get a widget's size during initState or build, before the first frame is rendered.

The Problem

final key = GlobalKey();

@override
void initState() {
  super.initState();
  final size = key.currentContext!.size; // Not laid out yet!
}

The Fix

final key = GlobalKey();

@override
void initState() {
  super.initState();
  WidgetsBinding.instance.addPostFrameCallback((_) {
    final size = key.currentContext?.size;
    if (size != null) {
      setState(() { _widgetSize = size; });
    }
  });
}

Step-by-Step Fix

1. 1

   Identify the error

   Look at the assertion failure mentioning hasSize. This means you accessed a widget's size before layout completed.

2. 2

   Find the cause

   Check if you are reading widget size in initState or build, before the first frame is rendered.

3. 3

   Apply the fix

   Use addPostFrameCallback to defer size reading until after the first layout pass is complete.