AsyncValue< T> class
abstract
A utility for safely manipulating asynchronous data.
By using AsyncValue, you are guaranteed that you cannot forget to handle the loading/error state of an asynchronous operation.
It also exposes some utilities to nicely convert an AsyncValue to
a different object.
For example, a Flutter Widget may use when
to convert an AsyncValue
into either a progress indicator, an error screen, or to show the data:
/// A provider that asynchronously exposes the current user
final userProvider = StreamProvider<User>((_) async* {
// fetch the user
});
class Example extends ConsumerWidget {
@override
Widget build(BuildContext context, WidgetRef ref) {
final AsyncValue<User> user = ref.watch(userProvider);
return user.when(
loading: () => CircularProgressIndicator(),
error: (error, stack) => Text('Oops, something unexpected happened'),
data: (user) => Text('Hello ${user.name}'),
);
}
}
If a consumer of an AsyncValue does not care about the loading/error
state, consider using value/valueOrNull
to read the state:
Widget build(BuildContext context, WidgetRef ref) {
// Reading .value will be throw during error and return null on "loading" states.
final User user = ref.watch(userProvider).value;
// Reading .value will be throw both on loading and errorstates.
final User user2 = ref.watch(userProvider).requiredValue;
...
}
See also:
- FutureProvider, StreamProvider which transforms a Future into an AsyncValue.
- AsyncValue.guard, to simplify transforming a Future into an AsyncValue.
- Implementers
- Available Extensions
- Annotations
-
- @sealed
- @immutable
Constructors
- AsyncValue.data(T value)
-
Creates an AsyncValue with a data.
constfactory
- AsyncValue.error(Object error, StackTrace stackTrace)
-
Creates an AsyncValue in the error state.
constfactory
- AsyncValue.loading()
-
Creates an AsyncValue in loading state.
constfactory
Properties
- error → Object?
-
The error.
read-only
- hashCode → int
-
The hash code for this object.
read-onlyoverride
- hasValue → bool
-
Whether value is set.
read-only
- isLoading → bool
-
Whether some new value is currently asynchronously loading.
read-only
- runtimeType → Type
-
A representation of the runtime type of the object.
read-onlyinherited
- stackTrace → StackTrace?
-
The stacktrace of error.
read-only
- value → T?
-
The value currently exposed.
read-only
Methods
-
copyWithPrevious(
AsyncValue< T> previous, {bool isRefresh = true}) → AsyncValue< T> -
Clone an AsyncValue, merging it with
previous
. -
map<
R>( {required R data(AsyncData< T> data), required R error(AsyncError< T> error), required R loading(AsyncLoading< T> loading)}) → R - Perform some action based on the current state of the AsyncValue.
-
noSuchMethod(
Invocation invocation) → dynamic -
Invoked when a nonexistent method or property is accessed.
inherited
-
toString(
) → String -
A string representation of this object.
override
-
unwrapPrevious(
) → AsyncValue< T> - The opposite of copyWithPrevious, reverting to the raw AsyncValue with no information on the previous state.
Operators
-
operator ==(
Object other) → bool -
The equality operator.
override
Static Methods
-
guard<
T>( Future< T> future()) → Future< AsyncValue< T>> - Transforms a Future that may fail into something that is safe to read.