threads.m

    int multiplier = 7;
    int (^myBlock)(int) = ^(int num) {
        return num * multiplier;
    };
    multiplier = 8;
    NSLog( @"%d", myBlock( 3 ) );


----------------------------------------------------
    @interface ViewController ()
    @property (atomic,strong) NSMutableArray *array;
    @property NSOperationQueue *queue;
    @property dispatch_semaphore_t semaphore;
    @end

    @implementation ViewController


    - (void)viewDidLoad
    {
        [super viewDidLoad];
        
        self.array = [[NSMutableArray alloc] init];
        self.queue = [[NSOperationQueue alloc] init];
        
        self.semaphore = dispatch_semaphore_create(1);
        
        NSOperation *testOperation1 = [[NSInvocationOperation alloc]
                                       initWithTarget:self
                                       selector:@selector(test1)
                                       object:nil];
        NSOperation *testOperation2 = [[NSInvocationOperation alloc]
                                       initWithTarget:self
                                       selector:@selector(test2)
                                       object:nil];
        
        [self.queue addOperation:testOperation1];
        [self.queue addOperation:testOperation2];
        
    }

    -(void)test1 {
        [self.array addObject:@"test 1"];
        dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
        [self.array addObject:@"test 4"];
        NSLog(@"1 %@",self.array);
        dispatch_semaphore_signal(self.semaphore);
        NSLog(@"2 %@",self.array);
        [self.array removeAllObjects];
    }

    -(void)test2 {
        [self.array addObject:@"test 2"];
        [self.array addObject:@"test 3"];
        dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
        [self.array addObject:@"test 5"];
        dispatch_semaphore_signal(self.semaphore);
        NSLog(@"3 %@",self.array);
    }

    @end