最后更新：2017-12-12
dispatch_sync 与 dispatch_barrier_sync
https://github.com/rs/SDWebImage/pull/818
The reason why we are doing dispatch_barrier_sync and dispatch_sync is to synchronise the access to the NSMutableDictionary that is not thread-safe, according to the Threading Programming Guide. Same doc states that NSMapTable is not thread-safe, so I don't think your code is protected against crashes.