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dispatch_get_specific

Posted on Edited on In Disqus:

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dispatch_get_specific 和 dispatch_queue_set_specific 用法就是用set方法在线程里设置一个key，通过get查找当前线程，是不是包含该key来判断代码是不是在特定的线程里执行。set则是给线程设置key值。

1. Get

dispatch_get_specific设置Key值。

两种方法查询线程里面包含的key值

  • dispatch_get_specific：查询当前线程的key值。
  • dispatch_queue_get_specific: 查询特定线程的key值，需要传入线程。

2. Set

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void dispatch_queue_set_specific(dispatch_queue_t queue, const void *key, void *context, dispatch_function_t destructor);

需要注意的是set方法的最后一个参数可以传入一个函数，当做析构函数。

3.示例

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const char *queueKey1 = "queueKey1";
const char *queueKey2 = "queueKey2";

void (^executionBlock)() = ^{
    if(dispatch_get_specific(queueKey1)) {
        printf("queueKey1");
    } else if(dispatch_get_specific(queueKey2)) {
        printf("queueKey2");
    } else {
        NSLog(@"other queue");
    }
};

void queueFunction() {
    NSLog(@"__queueFunction");
}
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dispatch_queue_t queue1 = dispatch_queue_create(queueKey1, NULL);
dispatch_queue_t queue2 = dispatch_queue_create(queueKey2, NULL);

dispatch_queue_set_specific(queue1, queueKey1, &queueKey1, queueFunction);
dispatch_queue_set_specific(queue2, queueKey2, &queueKey2, NULL);
    
dispatch_sync(queue1, executionBlock);
dispatch_sync(queue2, executionBlock);

if (dispatch_queue_get_specific(queue1, queueKey1)) {
    NSLog(@"包含");
}

运行结果

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queueKey1
queueKey2
包含
__queueFunction

4. FMDB如何使用dispatch_queue_set_specific和dispatch_get_specific来防止死锁

作用类似objc_setAssociatedObject跟objc_getAssociatedObject

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static const void * const kDispatchQueueSpecificKey = &kDispatchQueueSpecificKey;
//创建串行队列，所有数据库的操作都在这个队列里
_queue = dispatch_queue_create([[NSString stringWithFormat:@"fmdb.%@", self] UTF8String], NULL);
//标记队列
dispatch_queue_set_specific(_queue, kDispatchQueueSpecificKey, (__bridge void *)self, NULL);

//检查是否是同一个队列来避免死锁的方法
- (void)inDatabase:(void (^)(FMDatabase *db))block {
    FMDatabaseQueue *currentSyncQueue = (__bridge id)dispatch_get_specific(kDispatchQueueSpecificKey);
    assert(currentSyncQueue != self && "inDatabase: was called reentrantly on the same queue, which would lead to a deadlock");
}

5. References