Simple block-based serial queue for asynchronous operations.

QTMAsyncOperationQueue.h

//
//  QTMAsyncOperationQueue.h
//  Quantum
//
//  Created by Jonathan Willing on 10/28/13.
//  Copyright (c) 2013 AppJon. All rights reserved.
//

#import <Foundation/Foundation.h>

typedef void (^QTMAsyncOperationQueueCompletion)();

@interface QTMAsyncOperationQueue : NSObject

/// The queue on which the operations are dispatched.
///
/// Defaults to the main queue.
@property (nonatomic, assign) dispatch_queue_t dispatchQueue;

/// Adds an asynchronous operation to the queue. The operation will only be started when any
/// previously-enqueued operations have completed (serially).
///
/// The operations themselves be dispatched on `dispatchQueue`, which defaults to the main queue.
///
/// The completion block must be fired by the caller to signify the completion of the async operation.
- (void)addAsynchronousOperation:(void (^)(QTMAsyncOperationQueueCompletion completion))operation;

@end

QTMAsyncOperationQueue.m

//
//  QTMAsyncOperationQueue.m
//  Quantum
//
//  Created by Jonathan Willing on 10/28/13.
//  Copyright (c) 2013 AppJon. All rights reserved.
//

#import "QTMAsyncOperationQueue.h"

@interface QTMAsyncOperationQueue()
@property (nonatomic, strong) dispatch_group_t internalDispatchGroup;
@property (nonatomic, strong) dispatch_queue_t internalDispatchQueue;
@end

@implementation QTMAsyncOperationQueue

- (instancetype)init {
	self = [super init];
	if (self == nil) return nil;
	self.dispatchQueue = dispatch_get_main_queue();
	
	self.internalDispatchQueue = dispatch_queue_create("com.appjon.asyncOperationQueue", DISPATCH_QUEUE_SERIAL);
	self.internalDispatchGroup = dispatch_group_create();

	return self;
}

- (void)addAsynchronousOperation:(void (^)(QTMAsyncOperationQueueCompletion))operation {
	dispatch_async(self.internalDispatchQueue, ^{
		dispatch_group_enter(self.internalDispatchGroup);
		
		void (^completion)() = ^{
			dispatch_group_leave(self.internalDispatchGroup);
		};
		
		dispatch_async(self.dispatchQueue, ^{
			operation(completion);
		});
		
		dispatch_group_wait(self.internalDispatchGroup, DISPATCH_TIME_FOREVER);
	});
}

@end