Flutter - how to correctly create a button widget that has a spinner inside of it to isolate the setState call? - flutter

Question

I have a reuable stateful widget that returns a button layout. The button text changes to a loading spinner when the network call is in progress and back to text when network request is completed.
I can pass a parameter showSpinner from outside the widget, but that requires to call setState outside of the widget, what leads to rebuilding of other widgets.
So I need to call setState from inside the button widget.
I am also passing a callback as a parameter into the button widget. Is there any way to isolate the spinner change state setting to inside of such a widget, so that it still is reusable?

Answer 1

The simplest and most concise solution does not require an additional library. Just use a ValueNotifier and a ValueListenableBuilder. This will also allow you to make the reusable button widget stateless and only rebuild the button's child (loading indicator/text).
In the buttons' parent instantiate the isLoading ValueNotifier and pass to your button widget's constructor.
final isLoading = ValueNotifier(false);
Then in your button widget, use a ValueListenableBuilder.
// disable the button while waiting for the network request
onPressed: isLoading.value
? null
: () async {
// updating the state is super easy!!
isLoading.value = true;
// TODO: make network request here
isLoading.value = false;
},
#override
Widget build(BuildContext context) {
return ValueListenableBuilder<bool>(
valueListenable: isLoading,
builder: (context, value, child) {
if (value) {
return CircularProgressIndicator();
} else {
return Text('Load Data');
}
},
);
}

Answer 2

You can use StreamBuilder to solve this problem.
First, we need to create a stream. Create a new file to store it, we'll name it banana_stream.dart, for example ;).
class BananaStream{
final _streamController = StreamController<bool>();
Stream<bool> get stream => _streamController.stream;
void dispose(){
_streamController.close();
}
void add(bool isLoading){
_streamController.sink.add(isLoading);
}
}
To access this, you should use Provider, so add a Provider as parent of the Widget that contain your reusable button.
Provider<BananaStream>(
create: (context) => BananaStream(),
dispose: (context, bloc) => bloc.dispose(),
child: YourWidget(),
),
Then add the StreamBuilder to your button widget:
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return StreamBuilder<bool>(
stream: Provider.of<BananaStream>(context, listen:false),
initialData: false,
builder: (context, snapshot){
final isLoading = snapshot.data;
if(isLoading == false){
return YourButtonWithNoSpinner();
} else{
return YourButtonWithSpinner();
}
}
);
}
}
And to change isLoading outside, you can use this code:
final provider = Provider.of<BananaStream>(context, listen:false);
provider.add(true); //here is where you change the isLoading value
That's it!
Alternatively, you can use ValueNotifier or ChangeNotifier but i find it hard to implement.

Answer 3

I found the perfect solution for this and it is using the bloc pattern. With this package https://pub.dev/packages/flutter_bloc
The idea is that you create a BLOC or a CUBIT class. Cubit is just a simplified version of BLOC. (BLOC = business logic component).
Then you use the bloc class with BlocBuilder that streams out a Widget depending on what input you pass into it. And that leads to rebuilding only the needed button widget and not the all tree.
simplified examples in the flutter counter app:
// input is done like this
onPressed: () {
context.read<CounterCubit>().decrement();
}
// the widget that builds a widget depending on input
_counterTextBuilder() {
return BlocBuilder<CounterCubit, CounterState>(
builder: (context, state) {
if (state.counterValue < 0){
return Text("negative value!",);
} else if (state.counterValue < 5){
return Text("OK: ${state.counterValue}",
);
} else {
return ElevatedButton(onPressed: (){}, child: const Text("RESET NOW!!!"));
}
},
);
}