当我使用带有自动焦点的文本字段时，Flutter 会引发错误

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 official example https://flutter.dev/docs/cookbook/forms/focus
Because with auto focus, the keyboard show up first but textfield is still not exist
Workaround is use FocusNode and WidgetsBinding.instance.addPostFrameCallback
When TextField show and then move 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,
        ),
      ),
    );
  }
}

Answer 2

This bug has been fixed, but as yet, the fix is not in the stable channel ( https://github.com/flutter/flutter/issues/52221 ). If your project allows, you can switch to the dev channel (>1.15) and just use the autofocus.

From the command line:

flutter channel dev
flutter upgrade

Rebuild your project, and it should work.

Answer 3

in 2020 and this error still occurs even in dev channel...

my bad, it is working. My variable was null and got the error, sorry