iOS 轉換非同步block為同步方式運行

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使用dispatch_semaphore_t 實現dispatch_semaphore_t sema = dispatch_semaphore_create(0); //建立訊號量__block ALAssetsGroup *ret = nil;ALAssetsLibraryGroupsEnumerationResultsBlock listGroupBlock = ^(ALAssetsGroup *group, BOOL *stop) { ret = group; dispatch_semaphore_signal(sema); //關鍵點，在此發送訊號量};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，在此等待訊號量return ret;本文由Tony翻譯，原文連結：Convert async block to sync | Waterworld http://www.waterworld.com.hk/en/blog/convert-async-block-sync轉：http://itony.me/65.html

iOS 轉換非同步block為同步方式運行