[fleitz]

This is called a semaphore, it's already implemented in GCD.

Also, why talk about performance and then make obj-c method calls...?

It's quite easy using NSProxy to create a throttler that will wrap any object, then you can abstract throttling from the behavior of the underlying object.

  @interface Throttler : NSProxy {
     dispatch_semaphore_t _semaphore;
     id _object;
  }
  - initWithObject:(id)obj concurrentOperations:(int)ops;

  @end
  @implementation Throttler

  - (id) initWithObject:(id)obj concurrentOperations:(int)ops {
     if(self = [super init]){
       _semaphore = dispatch_semaphore_create(ops)
       _object = obj;
     }
     return self;
  }

  - (void) forwardInvocation:(NSInvocation*)invocation {
     if(dispatch_semaphore_wait(_semaphore,0)){
        @try {
        [invocation setTarget: _object];
        [invocation invoke];

        }
        @catch (NSException* e){
          @throw e;
        }
        @finally {
        dispatch_semaphore_signal(_semaphore);
        }
        return;
     }
     @throw [NSException
          exceptionWithName:@"InsufficientResourceException"
          reason:@"Insufficient Resource"
          userInfo:nil];
  }

  @end 

https://developer.apple.com/library/mac/documentation/Genera...