如果我将 bottomNotificationBar 保留在与页面其余部分相同的类中，setState 将正常工作并且按钮将正常工作。

如果我将 bottomNotificationBar 移动到另一个类，我无法让 setState 工作，因为它需要引用回主类。我已经尝试了一些方法，但我还不能全神贯注。

错误是: 处理手势时抛出以下断言: setState() 在构造函数中调用:

不工作的部分标记在底部附近:

    import 'package:flutter/material.dart';

    void main() {
      runApp(new MyApp());
    }

    class MyApp extends StatelessWidget {
      // This widget is the root of your application.
      @override
      Widget build(BuildContext context) {
        return new MaterialApp(
          title: 'My Title',
          home: new MyHomePage(),
        );
      }
    }


    class MyHomePage extends StatefulWidget {
      @override
      _MyHomePageState createState() => new _MyHomePageState();
    }


    class _MyHomePageState extends State<MyHomePage> {
      var selectedPageIndex = 0;
      var pages = [ Page1(), Page2(), ];

      @override
      Widget build(BuildContext context) {
        return new Scaffold(
          body: pages[selectedPageIndex],
          bottomNavigationBar:
              MyClass().buildBottomNavigationBar(selectedPageIndex),
        );
      }
    }

    class MyClass {
      BottomNavigationBar buildBottomNavigationBar(selectedPageIndex) {
        return new BottomNavigationBar(
          type: BottomNavigationBarType.fixed,
          items: [
            BottomNavigationBarItem(
              title: Text("Page1"),
              icon: Icon(Icons.account_circle),
            ),
            BottomNavigationBarItem(
              title: Text("Page2"),
              icon: Icon(Icons.account_circle),
            ),
          ],
          onTap: (index) {

/////////////////////////////START OF SECTION///////////////////////////
            _MyHomePageState().setState(() {
              selectedPageIndex = index;
            });
/////////////////////////////END OF SECTION///////////////////////////

          },
          currentIndex: selectedPageIndex,
        );
      }
    }

------------编辑:----------------

好的，现在我有下面的代码，我得到以下两件事:

信息: 成员“setState”只能在“package:flutter/src/widgets/framework.dart”的子类的实例成员中使用。

异常: 处理手势时抛出以下 NoSuchMethodError: 在 null 上调用了方法“setState”。 接收者:空 尝试调用:setState(Closure: () => Null)

import 'package:flutter/material.dart';

void main() {
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'My Title',
      home: new MyHomePage(),
    );
  }
}


class MyHomePage extends StatefulWidget {

  static void setIndex(BuildContext context, int _newIndex) {
    _MyHomePageState state = context.ancestorStateOfType(TypeMatcher<_MyHomePageState>());
    state.setState(() {
      state.selectedPageIndex =_newIndex;
    });
  }

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}


class _MyHomePageState extends State<MyHomePage> {
  var selectedPageIndex = 0;

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: new Container(),
      bottomNavigationBar:
          MyClass().buildBottomNavigationBar(context,selectedPageIndex),
    );
  }
}

class MyClass {
  BottomNavigationBar buildBottomNavigationBar(context,selectedPageIndex) {
    return new BottomNavigationBar(
      type: BottomNavigationBarType.fixed,
      items: [
        BottomNavigationBarItem(
          title: Text("Page1"),
          icon: Icon(Icons.account_circle),
        ),
        BottomNavigationBarItem(
          title: Text("Page2"),
          icon: Icon(Icons.account_circle),
        ),
      ],
      onTap: (index) {
    MyHomePage.setIndex(context, index);
    },
      currentIndex: selectedPageIndex,
    );
  }
}

最佳答案

您需要的是来自其他类的CallBAck 函数。由于必须在对象 -_MyHomePageState 上调用 setState。

使用 Class Constructors，我们传递初始数据并在 SetState() 上获得回调。

import 'package:flutter/material.dart';

void main() {
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'My Title',
      home: new MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  var selectedPageIndex = 0;
  var pages = [
    Page1(),
    Page2(),
  ];

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: pages[selectedPageIndex],
      bottomNavigationBar: MyClass(
        selectedPageIndex: selectedPageIndex,
        myFunc: _myFunc,
      ),
    );
  }

  void _myFunc(int index) {
    setState(() {
      selectedPageIndex = index;
    });
  }
}

class MyClass extends StatelessWidget {
  MyClass({this.selectedPageIndex, this.myFunc});
  final int selectedPageIndex;
  final Function myFunc;
  @override
  Widget build(BuildContext context) {
    return BottomNavigationBar(
      type: BottomNavigationBarType.fixed,
      items: [
        BottomNavigationBarItem(
          title: Text("Page1"),
          icon: Icon(Icons.account_circle),
        ),
        BottomNavigationBarItem(
          title: Text("Page2"),
          icon: Icon(Icons.account_circle),
        ),
      ],
      onTap: (index) {
/////////////////////////////START OF SECTION///////////////////////////
        myFunc(index);
/////////////////////////////END OF SECTION///////////////////////////
      },
      currentIndex: selectedPageIndex,
    );
  }
}

class Page1 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
      child: Container(
        child: Text('1'),
      ),
    );
  }
}

class Page2 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(child: Container(child: Text('3'),));
  }
}

关于dart - 在单独的 BottomNotifcationBar 类中使用 setState 回到主类，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/54758898/