No MediaQuery widget ancestor found

Why This Happens

In Flutter, MediaQuery.of(context) requires a MediaQuery ancestor in the widget tree, which is automatically provided by MaterialApp or WidgetsApp. If you call it outside these widgets or in a test without proper setup, this error occurs.

The Problem

void main() {
  runApp(
    Builder(
      builder: (context) {
        final size = MediaQuery.of(context).size; // No MediaQuery!
        return Text('$size');
      },
    ),
  );
}

The Fix

void main() {
  runApp(
    MaterialApp(
      home: Builder(
        builder: (context) {
          final size = MediaQuery.of(context).size;
          return Text('$size');
        },
      ),
    ),
  );
}

Step-by-Step Fix

1. 1

   Identify the error

   Look at the error message: No MediaQuery widget ancestor found. This means MediaQuery.of was called without a proper app widget ancestor.

2. 2

   Find the cause

   Check if the widget using MediaQuery.of is placed outside of MaterialApp/WidgetsApp, or if the test is missing a MediaQuery wrapper.

3. 3

   Apply the fix

   Ensure the widget is inside MaterialApp, or wrap it in a MediaQuery widget for testing.