GCD - Dispatch Semaphore

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先看代码

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);    NSMutableArray *array = [[NSMutableArray alloc]init];    dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);//信号量为1            for (int i = 0; i != 10000; ++i)        {            dispatch_async(queue, ^{                dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);//等待返回值为1或者大于1，获得执行权利后信号量－1               [array addObject:[NSNumber numberWithInt:i]];                dispatch_semaphore_signal(semaphore);//信号量+1            });        }

?百分之百不崩溃，semaphore的值表示由多少线程可以操作，根据我们的设置，永远只有一个线程可以操作数组。