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