Mulithreading: executing method calls only after finished executing other method

Question

I am trying to process method asynchronously, as per requirements, once the first method has completed, only then the second method should start executing. The Problem is first method itself has code that runs on background thread.

I tried dispatch_semaphore_wait, but that didnt work either.

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);

        dispatch_group_t group = dispatch_group_create();


        dispatch_group_async(group, queue, ^{

            [self firstMethod];
            NSLog(@"firstMethod Done");

        });
        dispatch_group_notify(group, queue, ^ {

            NSLog(@"1st method completed");
            NSLog(@"2nd method starting");

            [self secondMethod];

        });

FirstMethod itself runs on another worker thread like this

-(void)firstMethod
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
   //processing here.....       

 }];

What is the best way to achieve it, I cannot change the definition of firstMethod as it provided by some 3rd party and also changing it means changing lots of existing code from where this method is being called

Answer 1

You can use a completion block. You just need to modify firstMethod this way:

- (void)firstMethodWithOnComplete:(void (^)(void))onComplete {
      dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
          //processing here.....
          onComplete();
       });
}    

And then use it this way:

[self firstMethodWithOnComplete:^{
    [self secondMethod];
}];