我希望在我的应用程序的根目录下有一个继承的小部件，它将包含我的数据提供程序，我会在整个应用程序中使用它。所以我有这个继承的小部件，但每次我尝试加载它时，我得到这个The getter 'data' was called on null，我无法弄清楚原因。
所以这是我的main.dart：
void main() => runApp(new MatAppRoot());
class MatAppRoot extends StatelessWidget {
@override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'MyCoolApp',
routes: <String, WidgetBuilder>{
'Login': (BuildContext context) => new LoginPage(),
'Cool': (BuildContext context) => new CoolPage(),
},
home: new CoolApp(),
);
}
}
class CoolAppextends StatefulWidget {
final Widget child;
CoolApp({this.child});
@override
CoolAppState createState() => new CoolAppState();
static CoolAppState of(BuildContext context) {
return (context.inheritFromWidgetOfExactType(CoolInherit) as CoolInherit).data;
}
}
class CoolAppState extends State<CoolApp> {
String randomString = 'AYEEAS!!!';
@override
void initState() { super.initState();
Navigator.of(context).pushNamedAndRemoveUntil('Login', (Route<dynamic> route) => false);
}
@override
Widget build(BuildContext context) {
return new CoolInherit(
data: this,
child: new LoginPage(),
);
}
}
class CoolInherit extends InheritedWidget {
final CoolAppState data;
CoolInherit({
Key key,
this.data,
Widget child,
}): super(
key: key,
child: child
);
@override
bool updateShouldNotify(CoolInherit old) {
return true;
}
}
然后我的LoginPage基本上会在登录后重定向，如下所示：
if (logInSuccessful) {
Navigator.of(context).pushNamedAndRemoveUntil('Cool', (Route<dynamic> route) => false);
}
在我的Cool页面中，当我点击这样的按钮时，我会尝试加载另一个页面：
viewCoolDetails() {
Navigator.push(
context,
new MaterialPageRoute(builder: (context) => new CoolDetailsPage()),
);
}
但在我的CoolDetailsPage中，当我执行此操作时崩溃：
@override
Widget build(BuildContext context) {
final inheritedWidget = CoolApp.of(context);
print(inheritedWidget.randomString); <-- ERROR: The getter 'data' was called on null
return new Text('Cool!');
}
错误详情：
I/flutter ( 6129): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter ( 6129): The following NoSuchMethodError was thrown building CoolDetailsPage(dirty, state:
I/flutter ( 6129): _CoolDetailsPage#ba0bb):
I/flutter ( 6129): The getter 'data' was called on null.
I/flutter ( 6129): Receiver: null
I/flutter ( 6129): Tried calling: data
I/flutter ( 6129):
I/flutter ( 6129): When the exception was thrown, this was the stack:
I/flutter ( 6129): #0 Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:46:5)
I/flutter ( 6129): #1 CoolApp.of (/lib/main.dart:56:83)
... etc etc
main.dart：56是return (context.inheritFromWidgetOfExactType(CoolInherit) as CoolInherit).data;所以如果我的侦探工作达到标准，我怀疑它是导航/上下文的一部分，这阻止了我的最终小部件访问inheritedWidget，但我＆＃ 39;我不确定。
更新：
我能说的最好，我需要在更高级别插入我的InheritedWidget;导航员之前。所以我把它插入到MaterialApp：
builder: (context, child) {
return new CoolApp(child: child);
},
但是没有看到工作......
E/flutter (32321): [ERROR:topaz/lib/tonic/logging/dart_error.cc(16)] Unhandled exception:
E/flutter (32321): Navigator operation requested with a context that does not include a Navigator.
E/flutter (32321): 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.
E/flutter (32321): #0 Navigator.of.<anonymous closure> (package:flutter/src/widgets/navigator.dart:1180:9)
E/flutter (32321): #1 Navigator.of (package:flutter/src/widgets/navigator.dart:1187:6)
答案 0 :(得分：1)
很长一段时间以来，我都遇到过同样的问题，我意识到，如果您将MaterialApp与Inherited Widget封装在一起，您的数据就可以通过整个应用程序访问。但是，在您的情况下，您需要在用户登录后传递数据，您需要创建一个新的导航器并将其与继承的小部件一起包装。。您可以看到此项目https://github.com/johnstef99/inherited-widget-demo
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'InheritedWidget Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MyNav(),
);
}
}
Route generatePage(child) {
return MaterialPageRoute(builder: (context) => child);
}
class MyNav extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MyData(
data: 'omg',
child: Navigator(
onGenerateRoute: (settings) {
switch (settings.name) {
case 'page1':
return generatePage(PageOne());
case 'page2':
return generatePage(PageTwo());
case 'page3':
return generatePage(PageThree());
}
},
initialRoute: 'page1',
),
);
}
}
class MyData extends InheritedWidget {
MyData({Key key, this.child, this.data}) : super(key: key, child: child);
final Widget child;
final String data;
static MyData of(BuildContext context) {
return (context.inheritFromWidgetOfExactType(MyData) as MyData);
}
@override
bool updateShouldNotify(MyData oldWidget) {
return true;
}
}
class PageOne extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Page 1'),
),
backgroundColor: Colors.red,
body: RaisedButton(
child: Text("Goto page 2, data=${MyData.of(context).data}"),
onPressed: () {
Navigator.of(context)
.push(MaterialPageRoute(builder: (_) => PageTwo()));
},
),
);
}
}
class PageTwo extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Page 2'),
),
backgroundColor: Colors.red,
body: RaisedButton(
child: Text("Goto page 3, data=${MyData.of(context).data}"),
onPressed: () {
Navigator.of(context)
.push(MaterialPageRoute(builder: (_) => PageThree()));
},
),
);
}
}
class PageThree extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Page 3'),
),
backgroundColor: Colors.green,
body: RaisedButton(
child: Text("Goto page 4, data=${MyData.of(context).data}"),
onPressed: null,
),
);
}
}
答案 1 :(得分：0)
这是因为您尝试从另一条路线（动态）访问路线CoolApp中的/。
但在你的动态路线中，没有CoolApp。因此CoolApp.of(context)返回null，因此访问.data崩溃。
您需要找到一种在新路线中拥有CoolApp实例的方法。