Async workflows for Obj-C

Async.h

@protocol AsyncProducer;

@protocol AsyncConsumer <NSObject>

- (void)producer:(id<AsyncProducer>)producer didCompleteWithResult:(id)result;
- (void)producer:(id<AsyncProducer>)producer didCompleteWithError:(NSError *)error;

@end

@protocol AsyncProducer

// consumer is retained until completion or cancellation.
// return value is cancellation token. when return value is deallocated,
// the producer is cancelled. if producer already completed no further action is taken on deallocation.
- (id<NSObject>)startWithConsumer:(id<AsyncConsumer>)consumer;

@end

@protocol Cancellable <NSObject>

- (void)cancel;

@end

// an utility object which can be returned from implementations of -[id<AsyncProducer> startWithConsumer:]. 
// this is nice because consumers don't have to explicitly call cancel on anything, just retain/release
// the object returned from startWithConsumer
@interface Cancellation : NSObject {
    id<Cancellable> cancellable;
}

+ (id)cancellationWithCancellable:(id<Cancellable>)cancellable;

@end

@interface NSObject (AsyncConsumer)

// receiver is target. selector takes one argument: result or error
- (id<AsyncConsumer>)consumerWithResultSelector:(SEL)resultSelector;
- (id<AsyncConsumer>)consumerWithResultSelector:(SEL)resultSelector errorSelector:(SEL)errorSelector;

@end

@interface NSArray (ParallelProducers)

// all items in the array must be id<AsyncProducer>
- (id<AsyncProducer>)parallelProducer;

@end

@interface Async : NSObject

+ (void)fireAndForgetProducer:(id<AsyncProducer>)producer;

@end