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I'm in need to write an extension for Flutter's State<T extends StatefulWidget> so I can use a function in all of my States, let's say showSnackBar("Hello world", 5).
I tried writing a mixin
mixin BaseState on State<ProfileScreen> {
final GlobalKey<ScaffoldState> scaffoldKey = new GlobalKey<ScaffoldState>();
void showSnackBar(String text) {
setState(() {
scaffoldKey.currentState.showSnackBar(new SnackBar(
content: new Row(
children: <Widget>[
new CircularProgressIndicator(),
new Text(text == null ? " Logging in" : " $text")
],
)));
});
}
void hideSnackBar() {
setState(() {
scaffoldKey.currentState.hideCurrentSnackBar();
});
}
}
As you can see, it is now mixed on State<ProfileScreen>. It's a problem because I only can use this mixin in class ProfileScreenState extends State<ProfileScreen>. Without the type notation I end up with an error:
error: The class 'ProfileScreenState' cannot implement both 'State<ProfileScreen>' and 'State<StatefulWidget>' because the type arguments are different. (conflicting_generic_interfaces at [mobile] lib/Screens/profile.dart:17)
error: Type parameters could not be inferred for the mixin 'BaseState' because no type parameter substitution could be found matching the mixin's supertype constraints (mixin_inference_no_possible_substitution at [mobile] lib/Screens/profile.dart:17)
I tried to Google a lot, seen questions like these but without a success.
And yes I know composition is preferred over inheritance in Flutter, but I think this is a thing I don't know I would make work with composition and I feel it will be OK with inheritance.
677
Mixins can be used in two ways, the first case is when we want to make use of class code in such a way that the class doesn't have any constructor and the object of the class is extended. In such a case, we use the with keyword.
Mixin for State classes that require knowledge of changing MaterialState values for their child widgets. This mixin does nothing by mere application to a State class, but is helpful when writing build methods that include child InkWell, GestureDetector, MouseRegion, or Focus widgets.
To implement a mixin , create a class that extends Object and declares no constructors. Unless you want your mixin to be usable as a regular class, use the mixin keyword instead of class . In this way, your Car can run , but cannot be ' handleControlled '.
It's common to have mixins that want to provide a specific constructor. These will usually be applied last to create a concrete class. This is impossible now, so you have to create a new concrete class with only a constructor to give it the correct signature.
I think you are looking for something like https://github.com/flutter/flutter/blob/3fbd140e77601686acb336cd2af2838d03ba0572/packages/flutter/lib/src/widgets/ticker_provider.dart#L155
mixin BaseState<T extends StatefulWidget> on State<T> {
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