flutter - StatefulWidget 默认不调用 initState 函数

Question

感谢您的关注。我是 flutter 的初学者。我不知道为什么默认情况下不调用 initState 函数。因为 print(list[0]) 语句没有运行。

import 'package:flutter/material.dart';
import 'main_page/main_page.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _MyHomePage();
}

class _MyHomePage extends State<MyHomePage> {
  int _currentIndex = 0;
  List<Widget> list = List();

  @override
  void initState() {
    list.add(MainPage());
    print(list[0]);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: MainPage(),
      bottomNavigationBar: BottomNavigationBar(
        items: [
          BottomNavigationBarItem(
            icon: Icon(Icons.home),
            title: Text('Home')
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.person),
            title: Text('Me')
          ),
        ],
        currentIndex: _currentIndex,
        onTap: (int index) {
          setState(() {
            _currentIndex = index;
          });
        },
        type: BottomNavigationBarType.fixed,
      ),
    );
  }
}

Answer 1

我试过你的代码，它仍然打印正常。请确保您RE-RUN 代码，不要进行热重载，因为 initState() 仅被调用一次。文件说:

The framework will call this method exactly once for each [State] object it creates.

我从 initState() 的文档中选择了您应该遵循的一件事:

If you override this, make sure your method starts with a call to super.initState().

这意味着您必须将所有代码放在 super.initState() 下，如下所示:

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

    list.add(MainPage());
    print('initState() ---> ${list[0]}'); // This will print "initState() ---> MainPage"
}