Possibly part of an answer to http://stackoverflow.com/questions/22509735.

BarrierQueue.h

#import <Foundation/Foundation.h>

@interface BarrierOperation : NSOperation

@end

@interface BarrierQueue : NSObject

- (void)addOperation:(NSOperation *)op;

@end

BarrierQueue.m

#import "BarrierQueue.h"

#pragma mark - BarrierOperation

@implementation BarrierOperation

@end

#pragma mark - BarrierQueue

@interface BarrierQueue ()

@property (nonatomic, strong) NSMutableArray *queueOfQueues;

@end


@implementation BarrierQueue

- (instancetype)init
{
    if ((self = [super init])) {
        _queueOfQueues = [[NSMutableArray alloc] init];
    }

    return self;
}

- (void)addOperation:(NSOperation *)op
{
    @synchronized (self) {
        if ([op isKindOfClass:[BarrierOperation class]]) {
            [self addBarrierOperation:(id)op];
        } else {
            [[self addingQueue] addOperation:op];
        }
    }
}

// Queue to which operations are still being added.
// Should only be accessed w/i a synch(self) block.
- (NSOperationQueue *)addingQueue
{
    if ([_queueOfQueues count] == 0) {
        [_queueOfQueues addObject:[[NSOperationQueue alloc] init]];
        // start?
    }

    return [_queueOfQueues lastObject];
}

- (void)popQueue
{
    @synchronized (self) {
        NSAssert([_queueOfQueues count], @"should always be one to pop");
        [_queueOfQueues removeObjectAtIndex:0];

        if ([_queueOfQueues count]) {
            // first queue is always running, all others suspended
            [(NSOperationQueue *)_queueOfQueues[0] setSuspended:NO];
        }
    }
}

// call only from @synchronized block in -addOperation:
- (void)addBarrierOperation:(BarrierOperation *)barrierOp
{
    [[self addingQueue] setSuspended:YES];

    for (NSOperation *op in [[self addingQueue] operations]) {
        [barrierOp addDependency:op];
    }

    [[self addingQueue] addOperation:barrierOp];

    // if you are free to set barrierOp.completionTask, you could skip popCallback and do that

    __block typeof(self) weakSelf = self;
    NSOperation *popCallback = [NSBlockOperation blockOperationWithBlock:^{
        [weakSelf popQueue];
    }];
    [popCallback addDependency:barrierOp];
    [[self addingQueue] addOperation:popCallback];
    [[self addingQueue] setSuspended:NO];

    NSOperationQueue *opQueue = [[NSOperationQueue alloc] init];
    [opQueue setSuspended:YES];

    [_queueOfQueues addObject:opQueue]; // fresh empty queue to add to
}

@end