RiverPod: Passing Future Results to Non-Future Providers

When working with Flutter and RiverPod, you may encounter a situation where you need to pass the result of a Future to a non-Future provider. This can be a bit tricky, but it's definitely possible. In this article, we'll explore how to pass the result of a Future to a non-Future provider in RiverPod.

First, let's define what we mean by a Future and a non-Future provider. A Future is a type that represents a value that may not be available yet, but will be available at some point in the future. A non-Future provider, on the other hand, is a provider that does not return a Future. Instead, it returns a value that is immediately available.

Now, let's say you have a Future that fetches some data from an API. You want to use this data in a non-Future provider, but you can't just return the Future from the non-Future provider because it expects a non-Future value. So, how can you pass the result of the Future to the non-Future provider?

One way to do this is to use a FutureProvider in combination with a Consumer to extract the value from the Future and pass it to the non-Future provider. Here's an example:

final futureProvider = FutureProvider.autoDispose.family<String, int>((ref, id) async {
final data = await fetchData(id);
return data.name;
});

final nonFutureProvider = Provider<String>((ref) {
final asyncValue = ref.watch(futureProvider(1));

return asyncValue.when(
data: (data) => data,
loading: () => 'Loading...',
error: (err, stack) => 'Error: $err',
);
});

In this example, we have a FutureProvider that fetches some data from an API using the fetchData function. The FutureProvider returns the name of the data as a String. We then have a nonFutureProvider that watches the futureProvider using the ref.watch() method. The when() method is then used to extract the value from the Future and return it as a non-Future value.

The when() method is a powerful tool in RiverPod that allows you to handle the different states of a Future. In this example, we're using it to handle the data, loading, and error states. The data state is returned as is, the loading state returns a 'Loading...' message, and the error state returns an error message.

Another way to pass the result of a Future to a non-Future provider is to use a StateProvider in combination with a Consumer. Here's an example:

final futureProvider = FutureProvider.autoDispose.family<String, int>((ref, id) async {
final data = await fetchData(id);
return data.name;
});

final stateProvider = StateProvider<String?>((ref) => null);

final nonFutureProvider = Provider<String>((ref) {
final state = ref.watch(stateProvider);

if (state == null) {
return 'Loading...';
} else {
return state;
}
});

ref.listen(futureProvider(1), (previous, next) {
ref.read(stateProvider.state).state = next.when(
data: (data) => data,
loading: () => null,
error: (err, stack) => throw Exception('Error: $err'),
);
});

In this example, we have a FutureProvider that fetches some data from an API using the fetchData function. The FutureProvider returns the name of the data as a String. We then have a stateProvider that will hold the value of the Future. We also have a nonFutureProvider that watches the stateProvider using the ref.watch() method.

We're using the ref.listen() method to listen for changes in the futureProvider. When the Future is complete, we update the stateProvider with the result of the Future. If the Future is still loading, we set the state to null. If there's an error, we throw an exception.

In conclusion, passing the result of a Future to a non-Future provider in RiverPod can be a bit tricky, but it's definitely possible. By using a FutureProvider in combination with a Consumer, or a StateProvider in combination with a Consumer, you can extract the value from the Future and pass it to the non-Future provider. Just remember to handle the different states of the Future using the when() method or the ref.listen() method.

References