flutter dialog异常Another exception was thrown: Navigator operation requested with a context that does not include a Navigator

我在使用flutter里的对话框控件的时候遇到了一个奇怪的错误

Another exception was thrown: Navigator operation requested with a context that does not include a Navigator

研究了一下才知道，flutter里的dialog不是随便就能用的。

原代码如下：

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Test',
      home: new Scaffold(
        appBar: new AppBar(title: new Text('Test')),
        body: _buildCenterButton(context),
      ),
    );
  }
}


Widget _buildCenterButton(BuildContext context) {
  return new Container(
      alignment: Alignment.center,
      child: new Container(
        child: _buildButton(context),
      ));
}

Widget _buildButton(BuildContext context) {
  return new RaisedButton(
      padding: new EdgeInsets.fromLTRB(10.0, 10.0, 10.0, 10.0),
      //padding
      child: new Text(
        'show dialog',
        style: new TextStyle(
          fontSize: 18.0, //textsize
          color: Colors.white, // textcolor
        ),
      ),
      color: Theme.of(context).accentColor,
      elevation: 4.0,
      //shadow
      splashColor: Colors.blueGrey,
      onPressed: () {
        showAlertDialog(context);
      });
}
void showAlertDialog(BuildContext context) {
  showDialog(
      context: context,
      builder: (_) => new AlertDialog(
          title: new Text("Dialog Title"),
          content: new Text("This is my content"),
          actions:<Widget>[
            new FlatButton(child:new Text("CANCEL"), onPressed: (){
              Navigator.of(context).pop();

            },),
            new FlatButton(child:new Text("OK"), onPressed: (){
              Navigator.of(context).pop();

            },)
          ]

      ));
}

点击按钮的时候没有任何反应，控制台的报错是:

Another exception was thrown: Navigator operation requested with a context that does not include a Navigator

分析下源码吧~

看showDialog方法的源码：

Future<T> showDialog<T>({
  @required BuildContext context,
  bool barrierDismissible: true,
  @Deprecated(
    'Instead of using the "child" argument, return the child from a closure '
    'provided to the "builder" argument. This will ensure that the BuildContext '
    'is appropriate for widgets built in the dialog.'
  ) Widget child,
  WidgetBuilder builder,
}) {
  assert(child == null || builder == null);
  return Navigator.of(context, rootNavigator: true/*注意这里*/).push(new _DialogRoute<T>(
    child: child ?? new Builder(builder: builder),
    theme: Theme.of(context, shadowThemeOnly: true),
    barrierDismissible: barrierDismissible,
    barrierLabel: MaterialLocalizations.of(context).modalBarrierDismissLabel,
  ));
}

Navigator.of 的源码:

static NavigatorState of(
    BuildContext context, {
      bool rootNavigator: false,
      bool nullOk: false,
    }) {
    final NavigatorState navigator = rootNavigator
        ? context.rootAncestorStateOfType(const TypeMatcher<NavigatorState>())
        : context.ancestorStateOfType(const TypeMatcher<NavigatorState>());
    assert(() {
      if (navigator == null && !nullOk) {
        throw new FlutterError(
          'Navigator operation requested with a context that does not include a Navigator.
'
          'The context used to push or pop routes from the Navigator must be that of a '
          'widget that is a descendant of a Navigator widget.'
        );
      }
      return true;
    }());
    return navigator;
  }

找到了一模一样的错误信息字符串！看来就是因为Navigator.of(context)抛出了一个FlutterError。
之所以出现这个错误，是因为满足了if (navigator == null && !nullOk) 的条件， 也就是说:
context.rootAncestorStateOfType(const TypeMatcher<NavigatorState>()) 是null。

Navigator.of函数有3个参数，第一个是BuildContext，第二个是rootNavigator，默认为false，可不传，第三个是nullOk，默认为false，可不传。rootNavigator的值决定了是调用ancestorStateOfType还是rootAncestorStateOfType，nullOk的值决定了如果最终结果为null值时该抛出异常还是直接返回一个null。

我们做个测试，传入不同的rootNavigator和nullOk的值，看有什么结果：

void showAlertDialog(BuildContext context) {
  try{
     debugPrint("Navigator.of(context, rootNavigator=true, nullOk=false)="+
        (Navigator.of(context, rootNavigator: true, nullOk: false)).toString());
  }catch(e){
    debugPrint("error1 " +e.toString());
  }
  try{
    debugPrint("Navigator.of(context, rootNavigator=false, nullOk=false)="+
       (Navigator.of(context, rootNavigator: false, nullOk: false)).toString());
  }catch(e){
    debugPrint("error2 " +e.toString());
  }
  try{
    debugPrint("Navigator.of(context, rootNavigator=false, nullOk=true)="+
       (Navigator.of(context, rootNavigator: false, nullOk: true)).toString());
  }catch(e){
    debugPrint("error3 " +e.toString());
  }
  //先注释掉showDialog部分的代码
//  showDialog(
//      context: context,
//      builder: (_) => new AlertDialog(
//          title: new Text("Dialog Title"),
//          content: new Text("This is my content"),
//          actions:<Widget>[
//            new FlatButton(child:new Text("CANCEL"), onPressed: (){
//              Navigator.of(context).pop();
//
//            },),
//            new FlatButton(child:new Text("OK"), onPressed: (){
//              Navigator.of(context).pop();
//
//            },)
//          ]
//
//      ));
}

打印结果：

error1 Navigator operation requested with a context that does not include a Navigator.
error2 Navigator operation requested with a context that does not include a Navigator.
Navigator.of(context, rootNavigator=false, nullOk=true)=null

显然，无论怎么改rootNavigator和nullOk的值，Navigator.of(context, rootNavigator, nullOk)的值都是null。

为什么呢？

rootAncestorStateOfType函数的实现位于framework.dart里，我们可以看一下ancestorStateOfType和rootAncestorStateOfType的区别:

@override
  State ancestorStateOfType(TypeMatcher matcher) {
    assert(_debugCheckStateIsActiveForAncestorLookup());
    Element ancestor = _parent;
    while (ancestor != null) {
      if (ancestor is StatefulElement && matcher.check(ancestor.state))
        break; 
      ancestor = ancestor._parent;
    }
    final StatefulElement statefulAncestor = ancestor;
    return statefulAncestor?.state;
  }

  @override
  State rootAncestorStateOfType(TypeMatcher matcher) {
    assert(_debugCheckStateIsActiveForAncestorLookup());
    Element ancestor = _parent;
    StatefulElement statefulAncestor;
    while (ancestor != null) {
      if (ancestor is StatefulElement && matcher.check(ancestor.state))
        statefulAncestor = ancestor;
      ancestor = ancestor._parent;
    }
    return statefulAncestor?.state;
  }

可以看出:
ancestorStateOfType的作用是： 如果某个父元素满足一定条件, 则返回这个父节点的state属性；
rootAncestorStateOfType的作用是： 返回最顶层的满足一定条件的父元素。
这个条件是： 这个元素必须属于StatefulElement , 而且其state属性与参数里的TypeMatcher 相符合。

查询源码可以知道：StatelessWidget 里的元素是StatelessElement，StatefulWidget里的元素是StatefulElement。

也就是说，要想让context.rootAncestorStateOfType(const TypeMatcher<NavigatorState>())的返回值不为null， 必须保证context所在的Widget的顶层Widget属于StatefulWidget（注意是顶层Widget，而不是自己所在的widget。如果context所在的Widget就是顶层Widget，也是不可以的）。

这样我们就大概知道为什么会出错了。我们的showAlertDialog方法所用的context是属于MyApp的, 而MyApp是个StatelessWidget。

那么，修改方案就比较清晰了，我们的对话框所使用的context不能是顶层Widget的context，同时顶层Widget必须是StatefulWidget。

修改后的完整代码如下：

import 'package:flutter/material.dart';

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

class MyApp extends StatefulWidget {


  @override
  State<StatefulWidget> createState() {
    return new MyState();
  }
}
class MyState extends State<MyApp>{
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Test',
      home: new Scaffold(
        appBar: new AppBar(title: new Text('Test')),
        body: new StatelessWidgetTest(),
      ),
    );
  }

}
class StatelessWidgetTest extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return _buildCenterButton(context);
  }
}
Widget _buildCenterButton(BuildContext context) {
  return new Container(
      alignment: Alignment.center,
      child: new Container(
        child: _buildButton(context),
      ));
}

Widget _buildButton(BuildContext context) {
  return new RaisedButton(
      padding: new EdgeInsets.fromLTRB(10.0, 10.0, 10.0, 10.0),
      //padding
      child: new Text(
        'show dialog',
        style: new TextStyle(
          fontSize: 18.0, //textsize
          color: Colors.white, // textcolor
        ),
      ),
      color: Theme.of(context).accentColor,
      elevation: 4.0,
      //shadow
      splashColor: Colors.blueGrey,
      onPressed: () {
        showAlertDialog(context);
      });
}
void showAlertDialog(BuildContext context) {
  NavigatorState navigator= context.rootAncestorStateOfType(const TypeMatcher<NavigatorState>());
  debugPrint("navigator is null?"+(navigator==null).toString());


  showDialog(
      context: context,
      builder: (_) => new AlertDialog(
          title: new Text("Dialog Title"),
          content: new Text("This is my content"),
          actions:<Widget>[
            new FlatButton(child:new Text("CANCEL"), onPressed: (){
              Navigator.of(context).pop();

            },),
            new FlatButton(child:new Text("OK"), onPressed: (){
              Navigator.of(context).pop();

            },)
          ]
      ));
}

至于为什么flutter里的对话框控件对BuildContext的要求这么严格，暂时还不清楚原因。

后记：

在flutter里，Widget，Element和BuildContext之间的关系是什么呢?

摘抄部分系统源码如下：

abstract class Element extends DiagnosticableTree implements BuildContext{....}

abstract class ComponentElement extends Element {}

class StatelessElement extends ComponentElement {
  @override
  Widget build() => widget.build(this);
}

class StatefulElement extends ComponentElement {
  @override
  Widget build() => state.build(this);
}

abstract class Widget extends DiagnosticableTree {
  Element createElement();
}

abstract class StatelessWidget extends Widget {
  @override
  StatelessElement createElement() => new StatelessElement(this);
  @protected
  Widget build(BuildContext context);
}

abstract class StatefulWidget extends Widget {
  @override
  StatefulElement createElement() => new StatefulElement(this);
  @protected
  State createState();
}
abstract class State<T extends StatefulWidget> extends Diagnosticable {
  @protected
  Widget build(BuildContext context);
}