Serializing asynchronous methods

Question

I have a number of tasks I need to execute serially but the task includes next block in a completion block.

What is a good technique for doing these tasks one at a time, starting the next task after the current one completes its completion block?

Is there a technique other than a NSOperation subclass with a serial NSOperationQueue?

Answer 1

Standard solutions:

1. NSOperationQueue with maxConcurrentOperationCount of 1. You say you don't want to do that, but you don't say why. Serial queues are the most logical solution.

   For example:

   NSOperationQueue *queue = [[NSOperationQueue alloc] init];
   queue.maxConcurrentOperationCount = 1;
   
   [queue addOperationWithBlock:^{
       NSLog(@"Starting #1");
       sleep(3);
       NSLog(@"Finishing #1");
   }];
   
   [queue addOperationWithBlock:^{
       NSLog(@"Starting #2");
       sleep(3);
       NSLog(@"Finishing #2");
   }];
   
   [queue addOperationWithBlock:^{
       NSLog(@"Starting #3");
       sleep(3);
       NSLog(@"Finishing #3");
   }];
   

2. If you don't want a serial NSOperationQueue, you can use a standard concurrent queue, but just make each operation dependent upon the prior one. You'll achieve the serial behavior you're looking for without using a serial queue.

   For example:

   NSOperationQueue *queue = [[NSOperationQueue alloc] init];
   
   NSOperation *operation;
   NSOperation *previousOperation;
   
   operation = [NSBlockOperation blockOperationWithBlock:^{
       NSLog(@"Starting #1");
       sleep(3);
       NSLog(@"Finishing #1");
   }];
   [queue addOperation:operation];
   
   previousOperation = operation;
   operation = [NSBlockOperation blockOperationWithBlock:^{
       NSLog(@"Starting #2");
       sleep(3);
       NSLog(@"Finishing #2");
   }];
   [operation addDependency:previousOperation];
   [queue addOperation:operation];
   
   previousOperation = operation;
   operation = [NSBlockOperation blockOperationWithBlock:^{
       NSLog(@"Starting #3");
       sleep(3);
       NSLog(@"Finishing #3");
   }];
   [operation addDependency:previousOperation];
   [queue addOperation:operation];
   

3. You could also create a GCD serial queue with dispatch_queue_create. It achieves the same thing as the first option, except using GCD instead of NSOperationQueue.

   For example:

   dispatch_queue_t queue = dispatch_queue_create("com.ConnerDouglass.operationtest", 0);
   
   dispatch_async(queue, ^{
       NSLog(@"Starting #1");
       sleep(3);
       NSLog(@"Finishing #1");
   });
   
   dispatch_async(queue, ^{
       NSLog(@"Starting #2");
       sleep(3);
       NSLog(@"Finishing #2");
   });
   
   dispatch_async(queue, ^{
       NSLog(@"Starting #3");
       sleep(3);
       NSLog(@"Finishing #3");
   });
   

Answer 2

I think this is an interesting solution: https://github.com/berzniz/Sequencer