dispatch_semaphore + NSBlockOperation

gistfile1.m

- (void)runAsyncOperationWithStartBlock:(void (^)(dispatch_block_t asyncOperationCompletionBlock))startBlock {
    NSCParameterAssert(startBlock);
    if (!startBlock) {
        return;
    }
    NSOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
        dispatch_semaphore_t completionSemaphore = dispatch_semaphore_create(0);
        startBlock(^{
            dispatch_semaphore_signal(completionSemaphore);
        });
        dispatch_semaphore_wait(completionSemaphore, DISPATCH_TIME_FOREVER);
    }];
    
    [_operationQueue addOperation:operation];
}