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使用dispatch_semaphore_t 实现 dispatch_semaphore_t sema = dispatch_semaphore_create(0); //创建信号量 __block ALAssetsGroup *ret = nil; ALAssetsLibraryGroupsEnumerationResultsBlock listGroupBlock = ^(ALAssetsGroup *group, BOOL *stop) { ret = group; dispatch_semaphore_signal(sema); //关键点，在此发送信号量 }; ALAssetsLibraryAccessFailureBlock failureBlock = ^(NSError *error) { ret = nil; dispatch_semaphore_signal(sema); //关键点，失败时发送 }; NSUInteger groupTypes = ALAssetsGroupSavedPhotos; [self.assetsLibrary enumerateGroupsWithTypes:groupTypes usingBlock:listGroupBlock failureBlock:failureBlock]; dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER); //关键点，在此等待信号量 return ret; 本文由Tony翻译，原文链接：Convert async block to sync | Waterworld http://www.waterworld.com.hk/en/blog/convert-async-block-sync 转：http://itony.me/65.html标签：
原文地址：http://www.cnblogs.com/ygm900/p/4551317.html
