Blocks are a language-level feature, which allow us to create distinct segment of code. there are three way to recognize the semaphore. dispatch_semaphore_create => creates a semaphore with initial value dispatch_semaphore_signal => increments the semaphore (send a semaphore signal) dispatch_semaphore_wait => decrements the semaphore (wait for a semaphore signal)
So, it means we can use semaphore to wait for a blocks completion. we have access the dispatch semaphore by following way.
SINGLE BLOCK

dispatch_semaphore_t sem = dispatch_semaphore_create(0);

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

MULTIPLE BLOCKS
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);
BLOCK IN BLOCK
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);