问题
I want to display a simple SnackBar inside Flutter's stateful widget. My application creates new instance of MaterialApp with a stateful widget called MyHomePage.
I try to show the SnackBar in showSnackBar() method. But it fails with 'The method 'showSnackBar' was called on null'.
What's wrong with this code?
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'Flutter',
theme: new ThemeData(
primarySwatch: Colors.blue,
),
home: new MyHomePage(),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key}) : super(key: key);
@override
_MyHomePageState createState() => new _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
@override
void initState() {
super.initState();
showInSnackBar("Some text");
}
@override
Widget build(BuildContext context) {
return new Padding(
key: _scaffoldKey,
padding: const EdgeInsets.all(16.0),
child: new Text("Simple Text")
);
}
void showInSnackBar(String value) {
_scaffoldKey.currentState.showSnackBar(new SnackBar(
content: new Text(value)
));
}
}
SOLUTION:
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'Flutter',
theme: new ThemeData(
primarySwatch: Colors.blue,
),
home: new Scaffold(body: new MyHomePage()),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key}) : super(key: key);
@override
_MyHomePageState createState() => new _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
@override
void initState() {
super.initState();
}
@override
Widget build(BuildContext context) {
showInSnackBar("Some text");
return new Padding(
padding: const EdgeInsets.all(16.0),
child: new Scaffold(
body: new Text("Simple Text")
)
);
}
void showInSnackBar(String value) {
Scaffold.of(context).showSnackBar(new SnackBar(
content: new Text(value)
));
}
}
回答1:
There's three problems. The first is that you don't have a Scaffold anywhere, and the Scaffold widget is the one that knows how to show snack bars. The second is that you have a key for getting a hold of the scaffold, but you've put it on a Padding instead (and Paddings don't have any knowledge of snack bars). The third is that you've used the key before the widget that it's associated with has had a chance to be initialised, since initState is called before build.
The simplest solution is to change the home line in your MyApp widget to:
home: new Scaffold(body: new MyHomePage()),
...and then remove all mention of _scaffoldKey and instead use Scaffold.of(context) where you currently have _scaffoldKey.currentState.
回答2:
In my case i had code like this (in class state)
final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
void showInSnackBar(String value) {
_scaffoldKey.currentState.showSnackBar(new SnackBar(content: new Text(value)));
}
but i didn't setup the key for scaffold. so when i add key: _scaffoldKey
@override
Widget build(BuildContext context) {
return new Scaffold(
key: _scaffoldKey,
body: new SafeArea(
snackbar start to work :)
回答3:
There's a better and cleaner way to display a Snackbar in flutter. I found it the hard way and sharing so that maybe it's helpful for someone else.
No need to change in your main app part
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'MyApp',
theme: new ThemeData(
primarySwatch: Colors.orange),
home: new MainPage());
}
}
Page State code is where things will change.
We know Flutter provides Scaffold.of(context).showSnackBar. However, the context should be the context of a descendant of a Scaffold, and not the context that includes a Scaffold. In order to avoid error, we need to use a BuildContext for the body of the Scaffold, and store it in a variable, as below.
class MainPageState extends State<MainPage> {
BuildContext scaffoldContext;
@override
Widget build(BuildContext context) {
return new Scaffold(
backgroundColor: Colors.grey,
appBar: new AppBar(
title: const Text(APP_TITLE),
),
body: new Builder(builder: (BuildContext context) {
scaffoldContext = context;
return new Center(
child: new Text('Hello World', style: new TextStyle(fontSize: 32.0)),
);
}));
}
void createSnackBar(String message) {
final snackBar = new SnackBar(content: new Text(message),
backgroundColor: Colors.red);
// Find the Scaffold in the Widget tree and use it to show a SnackBar!
Scaffold.of(scaffoldContext).showSnackBar(snackBar);
}
}
Now, you can call this function from anywhere and it will display the Snackbar. For example, I am using it to display Internet Connectivity messages.
回答4:
I dit it, work for me :)
@override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(
title: new Text('Demo')
),
body: new Builder(
// Create an inner BuildContext so that the onPressed methods
// can refer to the Scaffold with Scaffold.of().
builder: (BuildContext context) {
return new Center(
child: new RaisedButton(
child: new Text('SHOW A SNACKBAR'),
onPressed: () {
Scaffold.of(context).showSnackBar(new SnackBar(
content: new Text('Hello!'),
));
},
),
);
},
),
);
}
回答5:
initState been called before build I guess _scaffoldKey.currentState has not been initialized when it is call.
I don't know if you can get a ScaffoldState from initState. If you change your code you can show the snackbar from build method with:
Scaffold.of(context).showSnackBar(new SnackBar(new Text(value)));
回答6:
In case someone is looking to initialise a Snackbar on initState() (in other words when the page loads here is my solution). Also, I assume you already have the _scaffoldKey in place.
void initState() {
super.initState();
Future.delayed(Duration(seconds: 1)).then(
(_) => _displaySnackbar
);
}
// Display Snackbar
void get _displaySnackbar {
_scaffoldKey.currentState.showSnackBar(SnackBar(
duration: Duration(minutes: 1),
content: Text('Your snackbar message')
));
}
回答7:
To initialise a Snackbar on initState() you can execute a function after the layout is built.
void initState() {
super.initState();
WidgetsBinding.instance
.addPostFrameCallback((_) => _scaffoldKey.currentState.showSnackBar(SnackBar(content: Text("Your message here..")));}
回答8:
Scaffold.of(context).showSnackBar(
SnackBar(content: Text("Thanks for using snackbar",
textAlign: TextAlign.center, style: TextStyle(fontSize: 16.0, fontWeight:
FontWeight.bold),), duration: Duration(seconds: 2), backgroundColor: Colors.red,)
);
回答9:
you can use this :
final _scaffoldKey = GlobalKey<ScaffoldState>();
@override
Widget build(BuildContext context) {
return Scaffold(
key: _scaffoldKey,
appBar: AppBar(
),
}
}
and then
in onPressed() can add this code
_scaffoldKey.currentState.showSnackBar(
new SnackBar(
content: new Text('Hello this is snackbar!')
)
);
来源：https://stackoverflow.com/questions/40579879/display-snackbar-in-flutter