How to Implement Semaphores in iOS Application?

Question

Is it possible to implement Counting Semaphore in ios application?

Answer 1

Like this:

dispatch_semaphore_t sem = dispatch_semaphore_create(0);

[self methodWithABlock:^(id result){
    //put code here
    dispatch_semaphore_signal(sem);

    [self methodWithABlock:^(id result){
        //put code here
        dispatch_semaphore_signal(sem);
    }];
}];

dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);

Credit http://www.g8production.com/post/76942348764/wait-for-blocks-execution-using-a-dispatch

Answer 2

Yes, it is possible. There are quite a few synchronization tools available:

• @synchronized
• NSLock
• NSCondition
• NSConditionLock
• GCD semaphores
• pthread locks
• ...

I'd suggest reading "Threading Programming Guide" and asking something more specific.

Answer 3

In Swift 3 you can use a DispatchSemaphore.

// initialization
let semaphore = DispatchSemaphore(value: initialValue)

// wait, decrement the semaphore count (if possible) or wait until count>0
semaphore.wait()

// release, increment the semaphore count
semaphore.signal()

Answer 4

I was unable to find a native IOS object to do this but it works just fine using the C library:

#import "dispatch/semaphore.h"
...
dispatch_semaphore_t activity;
...
activity = dispatch_semaphore_create(0);
...
dispatch_semaphore_signal(activity);
...
dispatch_semaphore_wait(activity, DISPATCH_TIME_FOREVER);

Hope that helps.