在 Flutter 中推送新页面时，Navigator 堆栈上的页面会重建

Question

我正在开发一个 Flutter 应用程序，它会提示一个表单来询问一些个人信息。

问题是每次发生某些事情时都会重新构建页面，例如当屏幕方向发生变化或文本字段获得焦点时（键盘出现并立即消失，防止用户输入任何内容）。

很明显，有些东西触发了不必要的重建，但我不知道是什么以及在哪里。

当我将此页面作为主页插入时，一切正常。 当我在初始屏幕上显示动画后将页面插入其预期位置时会发生此问题，所以我想这与我的问题有关。

主类：

import 'package:flutter/material.dart';
import './view/SplashScreen.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new SplashScreen(),
    );
  }
}

启动画面：

import 'package:flutter/material.dart';
import 'dart:async';
import './UserLoader.dart';

class SplashScreen extends StatefulWidget {
  @override
  _SplashScreenState createState() => new _SplashScreenState();
}

class _SplashScreenState extends State<SplashScreen>
    with SingleTickerProviderStateMixin {
  AnimationController _iconAnimationController;
  CurvedAnimation _iconAnimation;

  @override
  void initState() {
    super.initState();

    _iconAnimationController = new AnimationController(
        vsync: this, duration: new Duration(milliseconds: 2000));

    _iconAnimation = new CurvedAnimation(
        parent: _iconAnimationController, curve: Curves.easeIn);
    _iconAnimation.addListener(() => this.setState(() {}));

    _iconAnimationController.forward();

    startTimeout();
  }

  @override
  Widget build(BuildContext context) {
    return new Material(
      color: Colors.white,
      child: new InkWell(
        child: new Center(
          child: new Container(
            width: 275.0,
            height: 275.0,
            decoration: new BoxDecoration(
              image: new DecorationImage(
                  colorFilter: new ColorFilter.mode(
                      Colors.white.withOpacity(_iconAnimation.value),
                      BlendMode.dstATop),
                  image: new AssetImage("images/logo.png")),
            ),
          ),
        ),
      ),
    );
  }

  void handleTimeout() {
    Navigator.of(context).pushReplacement(new MaterialPageRoute(
        builder: (BuildContext context) => new UserLoader()));
  }

  startTimeout() async {
    var duration = const Duration(seconds: 3);
    return new Timer(duration, handleTimeout);
  }
}

错误页面：

import 'package:flutter/material.dart';

class UserLoader extends StatefulWidget {
  @override
  _UserLoaderState createState() => new _UserLoaderState();
}

class _UserLoaderState extends State<UserLoader> {
  @override
  Widget build(BuildContext context) {
    final _formKey = new GlobalKey<FormState>();
    final _emailController = new TextEditingController();

    return new Scaffold(
        appBar: new AppBar(
          title: new Text("Informations"),
          actions: <Widget>[
            new IconButton(
                icon: const Icon(Icons.save),
                onPressed: () {
                  // unrelated stuff happens here
                })
          ],
        ),
        body: new Center(
          child: new SingleChildScrollView(
              child: new Form(
                  key: _formKey,
                  child: new Column(children: <Widget>[
                    new ListTile(
                      leading: const Icon(Icons.email),
                      title: new TextFormField(
                        decoration: new InputDecoration(
                          hintText: "Email",
                        ),
                        keyboardType: TextInputType.emailAddress,
                        controller: _emailController,
                        validator: _validateEmail,
                      ),
                    ),
                  ]))),
        ));
    }}

谁能帮我找出为什么页面会不断地自我重建？

Answer 1

我通过像这样简单地更改类来解决问题：

import 'package:flutter/material.dart';

class UserLoader extends StatefulWidget {
  @override
  _UserLoaderState createState() => new _UserLoaderState();
}

class _UserLoaderState extends State<UserLoader> {
  Widget _form; // Save the form

  @override
  Widget build(BuildContext context) {
    if (_form == null) { // Create the form if it does not exist
      _form = _createForm(context); // Build the form
    }
    return _form; // Show the form in the application
  }

  Widget _createForm(BuildContext context) {
    // This is the exact content of the build method in the question

    final _formKey = new GlobalKey<FormState>();
    final _emailController = new TextEditingController();

    return new Scaffold(
        appBar: new AppBar(
          title: new Text("Informations"),
          actions: <Widget>[
            new IconButton(
                icon: const Icon(Icons.save),
                onPressed: () {
                  // unrelated stuff happens here
                })
          ],
        ),
        body: new Center(
          child: new SingleChildScrollView(
              child: new Form(
                  key: _formKey,
                  child: new Column(children: <Widget>[
                    new ListTile(
                      leading: const Icon(Icons.email),
                      title: new TextFormField(
                        decoration: new InputDecoration(
                          hintText: "Email",
                        ),
                        keyboardType: TextInputType.emailAddress,
                        controller: _emailController,
                        validator: _validateEmail,
                      ),
                    ),
                  ]))),
        ));
    }
  }
}

希望有一天这可能对其他人有所帮助。

Answer 2

你需要做的就是移动这一行

final _formKey = new GlobalKey<FormState>();

从build 方法到状态类声明（即在build 之外）。键必须在创建类时创建一次。在您的情况下，每个构建操作都会重新创建密钥。