iOS 轉換異步block為同步方式運行

使用dispatch_semaphore_t 實現 dispatch_semaphore_t sema = dispatch_semaphore_create(0); //創建信號量 __block ALAssetsGroup *ret = nil; ALAssetsLibraryGroupsEnumerationResultsBlock listGroupBlock = ^(ALAssetsGroup *group, BOOL *stop) { ret = group; dispatch_semaphore_signal(sema); //關鍵點，在此發送信號量 }; ALAssetsLibraryAccessFailureBlock failureBlock = ^(NSError *error) { ret = nil; dispatch_semaphore_signal(sema); //關鍵點，失敗時發送 }; NSUInteger groupTypes = ALAssetsGroupSavedPhotos; [self.assetsLibrary enumerateGroupsWithTypes:groupTypes usingBlock:listGroupBlock failureBlock:failureBlock]; dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER); //關鍵點，在此等待信號量 return ret; 本文由Tony翻譯，原文鏈接：Convert async block to sync | Waterworld http://www.waterworld.com.hk/en/blog/convert-async-block-sync 轉：http://itony.me/65.html