我在使用flutter里的對話框控件的時候遇到了一個奇怪的錯誤：

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不是隨便就能用的。

原代碼如下：

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();},) ])); } 

點擊按鈕的時候沒有任何反應，控制臺的報錯是:
Another exception was thrown: Navigator operation requested with a context that does not include a Navigator。大致意思是，context里沒有Navigator對象，卻做了Navigator相關的操作。有點莫名其妙。

分析下源碼吧~

看showDialog方法的源碼：

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/*注意這里*/).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 的源碼:

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.\n' '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; } 

找到了一模一樣的錯誤信息字符串！看來就是因為Navigator.of(context)拋出了一個FlutterError。
之所以出現這個錯誤，是因為滿足了if (navigator == null && !nullOk) 的條件， 也就是說:
context.rootAncestorStateOfType(const TypeMatcher<NavigatorState>()) 是null。

Navigator.of函數有3個參數，第一個是BuildContext，第二個是rootNavigator，默認為false，可不傳，第三個是nullOk，默認為false，可不傳。rootNavigator的值決定了是調用ancestorStateOfType還是rootAncestorStateOfType，nullOk的值決定了如果最終結果為null值時該拋出異常還是直接返回一個null。

我們做個測試，傳入不同的rootNavigator和nullOk的值，看有什么結果：

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()); } //先注釋掉showDialog部分的代碼 //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(); // //},) //] // //)); } 

打印結果：

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 

顯然，無論怎么改rootNavigator和nullOk的值，Navigator.of(context, rootNavigator, nullOk)的值都是null。

為什么呢？

rootAncestorStateOfType函數的實現位于framework.dart里，我們可以看一下ancestorStateOfType和rootAncestorStateOfType的區別:

@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： 如果某個父元素滿足一定條件, 則返回這個父節點的state屬性；
rootAncestorStateOfType的作用是： 返回最頂層的滿足一定條件的父元素。
這個條件是： 這個元素必須屬于StatefulElement , 而且其state屬性與參數里的TypeMatcher 相符合。

查詢源碼可以知道：StatelessWidget 里的元素是StatelessElement，StatefulWidget里的元素是StatefulElement。

也就是說，要想讓context.rootAncestorStateOfType(const TypeMatcher<NavigatorState>())的返回值不為null， 必須保證context所在的Widget的頂層Widget屬于StatefulWidget（注意是頂層Widget，而不是自己所在的widget。如果context所在的Widget就是頂層Widget，也是不可以的）。

這樣我們就大概知道為什么會出錯了。我們的showAlertDialog方法所用的context是屬于MyApp的, 而MyApp是個StatelessWidget。

那么，修改方案就比較清晰了，我們的對話框所使用的context不能是頂層Widget的context，同時頂層Widget必須是StatefulWidget。

修改后的完整代碼如下：

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();},) ] )); }

實驗結果:

screen1.png

至于為什么flutter里的對話框控件對BuildContext的要求這么嚴格，暫時還不清楚原因。

---

后記：

在flutter里，Widget，Element和BuildContext之間的關系是什么呢?

摘抄部分系統源碼如下：

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); }

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