NSArray *array = [[NSArray alloc]initWithObjects:@(1), @(1), @(1), @(1), @(1), nil];
    //创建一个信号源
    __block dispatch_semaphore_t sem = dispatch_semaphore_create(0);
    dispatch_queue_t queue = dispatch_queue_create("StudyBlocks", NULL);
    
    dispatch_async(queue, ^(void) {
        int sum = 0;
        for(int i = 0; i < array.count; i++)
            sum += (int)array[i];
        
        NSLog(@" >> Sum: %d", sum);
        
        //增加 semaphore 计数（可理解为资源数）
        dispatch_semaphore_signal(sem);
    });
    
    //减少 semaphore 的计数，如果资源数少于 0就等待资源有资源了再去处理
    dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
    
    dispatch_release(sem);  
    dispatch_release(queue);
    

 NSArray *array = [[NSArray alloc]initWithObjects:@(1), @(1), @(1), @(1), @(1), nil];
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    __block int sum = 0;
    //dispatch_apply 的定义如下：
    //dispatch_apply(size_t iterations, dispatch_queue_t queue, void (^block)(size_t));
    //参数 iterations 表示迭代的次数，void (^block)(size_t) 是 block 循环体。这么做与 for 循环相比有什么好处呢？答案是：并行，这里的求和是并行的，并不是按照顺序依次执行求和的。
    dispatch_apply(array.count, queue, ^(size_t i) {
        sum += (int)array[i];
    });
    
    NSLog(@" >> sum: %d", sum);
    
    dispatch_release(queue);

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GCD-semaphore(资源)-apply