Flutter throws an error when I use Text field with auto focus

Question

When I use a TextField widget in Flutter and set the value of the propriety autoFocus to true

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: TextField(
          autofocus: true,
        ),
      ),
    );
  }
}

Flutter throws an error:

I/flutter (31721): ══╡ EXCEPTION CAUGHT BY FOUNDATION LIBRARY ╞════════════════════════════════════════════════════════
I/flutter (31721): The following assertion was thrown while dispatching notifications for FocusNode:
I/flutter (31721): RenderBox was not laid out: RenderEditable#bf516 NEEDS-LAYOUT NEEDS-PAINT
I/flutter (31721): 'package:flutter/src/rendering/box.dart':
I/flutter (31721): Failed assertion: line 1687 pos 12: 'hasSize'
I/flutter (31721):
I/flutter (31721): Either the assertion indicates an error in the framework itself, or we should provide substantially
I/flutter (31721): more information in this error message to help you determine and fix the underlying cause.
I/flutter (31721): In either case, please report this assertion by filing a bug on GitHub:
I/flutter (31721):   https://github.com/flutter/flutter/issues/new?template=BUG.md
I/flutter (31721):
I/flutter (31721): When the exception was thrown, this was the stack:
I/flutter (31721): #2      RenderBox.size
package:flutter/…/rendering/box.dart:1687
I/flutter (31721): #3      EditableTextState._updateSizeAndTransform
package:flutter/…/widgets/editable_text.dart:1729
I/flutter (31721): #4      EditableTextState._openInputConnection
package:flutter/…/widgets/editable_text.dart:1415
I/flutter (31721): #5      EditableTextState._openOrCloseInputConnectionIfNeeded
package:flutter/…/widgets/editable_text.dart:1441
I/flutter (31721): #6      EditableTextState._handleFocusChanged
package:flutter/…/widgets/editable_text.dart:1707
I/flutter (31721): #7      ChangeNotifier.notifyListeners
package:flutter/…/foundation/change_notifier.dart:206
I/flutter (31721): #8      FocusNode._notify
package:flutter/…/widgets/focus_manager.dart:808
I/flutter (31721): #9      FocusManager._applyFocusChange
package:flutter/…/widgets/focus_manager.dart:1401
I/flutter (31721): (elided 12 frames from class _AssertionError and package dart:async)
I/flutter (31721):
I/flutter (31721): The FocusNode sending notification was:
I/flutter (31721):   FocusNode#ce6fe
I/flutter (31721): ════════════════════════════════════════════════════════════════════════════════════════════════════

Why, and how can I solve this problem?

My Flutter version:

[√] Flutter (Channel stable, v1.12.13+hotfix.5, on Microsoft Windows [Version 10.0.18362.535], locale en-US) [√] Android toolchain - develop for Android devices (Android SDK version 29.0.2) [√] Android Studio (version 3.5) [√] VS Code (version 1.41.0) [√] Connected device (1 available) • No issues found!

Answer 1

This issue exists even in the official example: https://flutter.dev/docs/cookbook/forms/focus

Because with auto focus, the keyboard shows up first, but the textfield does still not exist.

A workaround is to use FocusNode and WidgetsBinding.instance.addPostFrameCallback. When the TextField shows, move the focus to this TextField.

Code snippet

@override
  void initState(){
    super.initState();
    myFocusNode = FocusNode();
    WidgetsBinding.instance.addPostFrameCallback((_){
      FocusScope.of(context).requestFocus(myFocusNode);
    });
  }

Full code

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  FocusNode myFocusNode;

  @override
  void initState(){
    super.initState();
    myFocusNode = FocusNode();
    WidgetsBinding.instance.addPostFrameCallback((_){
      FocusScope.of(context).requestFocus(myFocusNode);
    });
  }

  @override
  void dispose() {
    // Clean up the focus node when the Form is disposed.
    myFocusNode.dispose();

    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: TextField(
          //autofocus: true,
          focusNode: myFocusNode,
        ),
      ),
    );
  }
}