Overview of the "No MediaQuery Widget Found" Issue
- Widget Hierarchical Structure: The error often arises because a widget that depends on the
MediaQuery context is placed outside the boundaries of where a MediaQuery widget is accessible. In Flutter, MediaQuery is typically introduced to the widget tree by the MaterialApp, CupertinoApp, or WidgetsApp class, which wraps around widgets like Scaffold, AppBar, and others. If a widget that requires MediaQuery is initialized outside these contexts, it will not have access to the MediaQuery data, hence the error.
- Improper Widget Placement: A common cause is placing a widget that depends on device size or orientation outside the scope of widgets that provide
MediaQuery data. For example, attempting to build a widget that uses MediaQuery during the initialization of a global or top-level widget without it being wrapped in MaterialApp or similar can lead to this issue.
- Widget Initialization Order: Sometimes developers try to fetch
MediaQuery data during a widget's construction phase (in the constructor or variable initialization). At this point, the widget might not yet be in the widget tree or have access to a BuildContext with a MediaQuery ancestor.
Common Scenarios Triggering the Error
- Directly Using MediaQuery in Stateless or Stateful Widgets: If a
StatelessWidget or StatefulWidget tries to access MediaQuery.of(context) in its constructor or during state initialization, this may cause the error because, at this point, the widget is not fully part of the widget tree with MaterialApp or equivalent above it.
- Custom Widgets or External Libraries: When using custom widgets or third-party library widgets expecting a
MediaQuery context, if these widgets are not placed inside a tree with the appropriate root-level wrapper that introduces MediaQuery, the error is likely to occur.
- Dialog and Overlay Usage: Using
MediaQuery in dialogs or overlays without ensuring these elements are appropriately wrapped may also lead to errors since they might not automatically be wrapped in MediaQuery context in certain setups.
Code Example Illustrating Potential Error Scenario
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
// Incorrect usage: MediaQuery used before MaterialApp is in the widget tree.
final screenHeight = MediaQuery.of(context).size.height; // Causes exception
return MaterialApp(
home: Scaffold(
appBar: AppBar(
title: Text('MediaQuery Example'),
),
body: Center(
child: Text('Screen Height: $screenHeight'),
),
),
);
}
}
In this code snippet, the MediaQuery.of(context).size.height is called before MaterialApp is in the widget tree, resulting in the "No MediaQuery widget found" exception. This illustrates the need for correct widget placement and initialization to ensure MediaQuery is available.
