Prevent a circular reference to a block

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2. __weak typeof (self) weakself = self ;  
4.   dispatch_after (Dispatch_time (dispatch_time_now,  (int64_t) (2 * nsec_per_sec)),  dispatch_get_main_queue (),  ^{  
6.   
8.        __strong typeof (weakself) strongself = weakself ;  
10.          
12.        [strongself dosomething];  
16. });

Weakself is for the block to not hold self, avoid circular references, and then declare a strongself because once you enter block execution, self is not allowed to be released during this execution. After the block executes, the strongself is automatically released without a circular reference problem.

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Prevent a circular reference to a block