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问题：GCD为什么不会导致循环引用
这个问题得分开回答，同步和异步线程
1.同步线程的情况
dispatch_sync苹果官方是这么写的
Unlike with dispatch_async, no retain is performed on the target queue. Because calls to this function are synchronous, it “borrows” the reference of the caller. Moreover, no Block_copy is performed on the block.
As a performance optimization, this function executes blocks on the current thread whenever possible, with one exception: Blocks submitted to the main dispatch queue always run on the main thread.
也就是说同步线程不会增加引用计数，是借用调用者的引用计数，而且也是同步执行，而且也不会把block拷贝，所以不会导致循环引用。
2. 异步线程的情况
dispatch_async苹果官方的描述是
The block to submit to the target dispatch queue. This function performs Block_copy and Block_release on behalf of callers. This parameter cannot be NULL.
也就是说会拷贝block，但是也会帮你调用release，所以也不会存在循环引用的问题。