Flutter 是否退出页面的异常 （异步调用场景）

结论

• 异步延迟操作，判断页面是否退出， StatefulWidget 要使用 mounted 做判断。StatelessWidget 目前不知道拿啥做判断（欢迎补充）
• 页面退出时，不可以使用 Navigator.maybeOf， context == null或任何context 调用,
• 没退出页面，仍要使用 Navigator.maybeOf代替 Navigator.of

示例代码：

// StatefulWidget 类中

if (!mounted) {
	return;
}

var navigator = Navigator.maybeOf(context);
if (navigator == null) {
    return;
}

场景

• 用户在网络回调执行前 退出页面。回调内调用 Navigator，show dialog， context == null, 或调用context

原因

• context在 StatefulWidget 内部是调用的getter 方法，不是属性调用。 改方法 最后做了 ！非空校验。

release 版本会在 _element!; 处异常

  BuildContext get context {
    assert(() {
      if (_element == null) {
        throw FlutterError(
          'This widget has been unmounted, so the State no longer has a context (and should be considered defunct). \n'
          'Consider canceling any active work during "dispose" or using the "mounted" getter to determine if the State is still active.',
        );
      }
      return true;
    }());
    return _element!;
  }

bool get mounted => _element != null;

报错方法栈

StateFulWidget 调用 context 报错

I/flutter (31806): Future click
I/flutter (31806): Future running context: noNull
E/flutter (31806): [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: Null check operator used on a null value
E/flutter (31806): #0      StatefulElement.state (package:flutter/src/widgets/framework.dart:4712)
E/flutter (31806): #1      Element.findAncestorStateOfType (package:flutter/src/widgets/framework.dart:4054)
E/flutter (31806): #2      Navigator.maybeOf (package:flutter/src/widgets/navigator.dart:2598)
E/flutter (31806): #3      NetPage.futureCall.<anonymous closure> (package:flutter_app/net/view/NetPage.dart:60)
E/flutter (31806): #4      new Future.delayed.<anonymous closure> (dart:async/future.dart:393)
E/flutter (31806): #5      _rootRun (dart:async/zone.dart:1420)
E/flutter (31806): #6      _CustomZone.run (dart:async/zone.dart:1328)
E/flutter (31806): #7      _CustomZone.runGuarded (dart:async/zone.dart:1236)
E/flutter (31806): #8      _CustomZone.bindCallbackGuarded.<anonymous closure> (dart:async/zone.dart:1276)
E/flutter (31806): #9      _rootRun (dart:async/zone.dart:1428)
E/flutter (31806): #10     _CustomZone.run (dart:async/zone.dart:1328)

StatelessWidget 调用 Navigator.maybeOf 报错

I/flutter (31806): Future click
I/flutter (31806): Future running context: noNull
E/flutter (31806): [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: Null check operator used on a null value
E/flutter (31806): #0      StatefulElement.state (package:flutter/src/widgets/framework.dart:4712)
E/flutter (31806): #1      Element.findAncestorStateOfType (package:flutter/src/widgets/framework.dart:4054)
E/flutter (31806): #2      Navigator.maybeOf (package:flutter/src/widgets/navigator.dart:2598)
E/flutter (31806): #3      NetPage.futureCall.<anonymous closure> (package:flutter_app/net/view/NetPage.dart:60)
E/flutter (31806): #4      new Future.delayed.<anonymous closure> (dart:async/future.dart:393)
E/flutter (31806): #5      _rootRun (dart:async/zone.dart:1420)
E/flutter (31806): #6      _CustomZone.run (dart:async/zone.dart:1328)
E/flutter (31806): #7      _CustomZone.runGuarded (dart:async/zone.dart:1236)
E/flutter (31806): #8      _CustomZone.bindCallbackGuarded.<anonymous closure> (dart:async/zone.dart:1276)
E/flutter (31806): #9      _rootRun (dart:async/zone.dart:1428)
E/flutter (31806): #10     _CustomZone.run (dart:async/zone.dart:1328)

实验代码

StateFulWidget 实验代码

import 'dart:async';
import 'dart:convert' as convert;
import 'package:http/http.dart' as http;
import 'package:flutter/material.dart';

class NetPage extends StatefulWidget {
  NetPage({Key? key}) : super(key: key);

  @override
  _NetPageState createState() => _NetPageState();
}

class _NetPageState extends State<NetPage> {
  @override
  void initState() {
    super.initState();
  }

  @override
  void dispose() {
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('NetPage'),
      ),
      body: Container(
          child: Row(
            children: [
              ElevatedButton(
                  onPressed: () {
                    futureCall();
                  },
                  child: Text("Click Future")),
              ElevatedButton(
                  onPressed: () {
                    timerCall();
                  },
                  child: Text("Click Timer")),
              ElevatedButton(
                  onPressed: () {
                    tryException();
                  },
                  child: Text("Null Exception")),
            ],
          )),
    );
  }

  Future requestNet() async {
    var url = Uri.https('www.baidu.com', '');

    var response = await http.get(url);
    if (response.statusCode == 200) {
      print("Success response body: ");
      print('${response.body}');
    } else {
      print('Request failed with status: ${response.statusCode}.');
    }
  }

  String? test;
  get message  {
    return test!;
  }

  void futureCall() {
    print("Future click");
    Future.delayed(Duration(seconds: 4), () {
      // print("Future mount test: $mounted");
      // print("Future running context: ${context == null ? "null" : "noNull"}");
      var navigatorState = Navigator.maybeOf(context);
      print("Future NavigatorState is ${navigatorState == null ? "null": "noNull"}");
      return "hello";
    });
  }

  void timerCall() {
    print("Timer click");

    Timer(Duration(seconds: 5), () {
      print("Timer running context: ${context == null ? "null" : "noNull"}");
      var navigatorState = Navigator.maybeOf(context);
      print("Timer NavigatorState is ${navigatorState == null ? "null": "noNull"}");
    });
  }

  void tryException() {
    print("Timer click getMessage $message");
  }

}

StatelessWidget 实验代码

import 'dart:async';
import 'dart:convert' as convert;
import 'package:http/http.dart' as http;
import 'package:flutter/material.dart';

class NetPage extends StatelessWidget {
  NetPage({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {


    return Scaffold(
      appBar: AppBar(
        title: Text('NetPage'),
      ),
      body: Container(
          child: Row(
            children: [
              ElevatedButton(
                  onPressed: () {
                    futureCall(context);
                  },
                  child: Text("Click Future")),
              ElevatedButton(
                  onPressed: () {
                    timerCall(context);
                  },
                  child: Text("Click Timer")),
              ElevatedButton(
                  onPressed: () {
                    tryException(context);
                  },
                  child: Text("Null Exception")),
            ],
          )),
    );
  }

  Future requestNet() async {
    var url = Uri.https('www.baidu.com', '');

    var response = await http.get(url);
    if (response.statusCode == 200) {
      print("Success response body: ");
      print('${response.body}');
    } else {
      print('Request failed with status: ${response.statusCode}.');
    }
  }

  String? test;
  get message  {
    return test!;
  }

  void futureCall(BuildContext context) {
    print("Future click");
    Future.delayed(Duration(seconds: 4), () {
      // print("Future mount test: $mounted");
      print("Future running context: ${context == null ? "null" : "noNull"}");
      var navigatorState = Navigator.maybeOf(context, rootNavigator: true);
      print("Future NavigatorState is ${navigatorState == null ? "null": "noNull"}");
      return "hello";
    });
  }

  void timerCall(BuildContext context) {
    print("Timer click");

    Timer(Duration(seconds: 5), () {
      print("Timer running context: ${context == null ? "null" : "noNull"}");
      var navigatorState = Navigator.maybeOf(context);
      print("Timer NavigatorState is ${navigatorState == null ? "null": "noNull"}");
    });
  }

  void tryException(BuildContext context) {
    print("Timer click getMessage $message");
  }
}