Waiting for multiple blocks to finish

Question

I have those methods to retrieve some object information from the internet:

- (void)downloadAppInfo:(void(^)())success
                failure:(void(^)(NSError *error))failure;
- (void)getAvailableHosts:(void(^)())success
                  failure:(void(^)(NSError *error))failure;
- (void)getAvailableServices:(void(^)())success
                     failure:(void(^)(NSError *error))failure;
- (void)getAvailableActions:(void(^)())success
                    failure:(void(^)(NSError *error))failure;

The downloaded stuff gets stored in object properties, so that is why the success functions return nothing.

Now, I want to have one method like this:

- (void)syncEverything:(void(^)())success
               failure:(void(^)(NSError *error))failure;

Which does nothing else than calling all the methods above, and returning only after every single method has performed its success or failure block.

How can I do this?

Hint: I am aware that cascading the methods calls in each others success block would work. But this is neither 'clean' nor helpful when later implementations include further methods.

Attempts:

I tried running each of the calls in an NSOperation and adding those NSOperations to an NSOperationQueue followed by a "completion operation" which depends on every one of the preceding operations.

This won't work. Since the operations are considered completed even before their respective success/failure blocks return.

I also tried using dispatch_group. But it is not clear to me wether I am doing it the right way. Unfortunately, it is not working.

Answer 1

You were almost there, the problem is most likely to be that those methods are asynchronous, so you need an extra synchronization step. Just try with the following fix:

for(Appliance *appliance in _mutAppliances) {
  dispatch_group_async(
     group,
     dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
       dispatch_semaphore_t sem = dispatch_semaphore_create( 0 );

       NSLog(@"Block START");

       [appliance downloadAppInfo:^{
          NSLog(@"Block SUCCESS");
            dispatch_semaphore_signal(sem);
       }
       failure:^(NSError *error){
         NSLog(@"Block FAILURE");
         dispatch_semaphore_signal(sem);
       }];

       dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);

       NSLog(@"Block END");
 });

 dispatch_group_notify(
   group,
   dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),^{
     NSLog(@"FINAL block");
     success();
 });
}