FutureOr<T> class

A type representing values that are either Future<T> or T.

This class declaration is a public stand-in for an internal future-or-value generic type, which is not a class type. References to this class are resolved to the internal type.

It is a compile-time error for any class to extend, mix in or implement FutureOr.

Examples

// The `Future<T>.then` function takes a callback [f] that returns either
// an `S` or a `Future<S>`.
Future<S> then<S>(FutureOr<S> f(T x), ...);

// `Completer<T>.complete` takes either a `T` or `Future<T>`.
void complete(FutureOr<T> value);

Advanced

The FutureOr<int> type is actually the "type union" of the types int and Future<int>. This type union is defined in such a way that FutureOr<Object> is both a super- and sub-type of Object (sub-type because Object is one of the types of the union, super-type because Object is a super-type of both of the types of the union). Together it means that FutureOr<Object> is equivalent to Object.

As a corollary, FutureOr<Object> is equivalent to FutureOr<FutureOr<Object>>, FutureOr<Future<Object>> is equivalent to Future<Object>.