在iOS5以后，许多API将只提供block版本，所以调用这些API就总是以异步方式运行的了。但有时候我们不想使用异步方式，例如我们想要调用函数后立刻获取结果值的时候。
在旧的方式中，我们无法使用NSCondition做这件事：
__block NSCondition *condition = [[NSCondition alloc] init];
__block ALAssetsGroup *ret = nil;
ALAssetsLibraryGroupsEnumerationResultsBlock listGroupBlock = ^(ALAssetsGroup *group, BOOL *stop) {
[condition lock];
ret = group;
[condition signal];
[condition unlock];
};
ALAssetsLibraryAccessFailureBlock failureBlock = ^(NSError *error) {
[condition lock];
ret = nil;
[condition signal];
[condition unlock];
};
NSUInteger groupTypes = ALAssetsGroupSavedPhotos;
[self.assetsLibrary enumerateGroupsWithTypes:groupTypes usingBlock:listGroupBlock failureBlock:failureBlock];
[condition lock];
[condition wait];
[condition unlock];
return ret;
我们在block的结尾处等待一个信号量。
更简单的办法是使用dispatch_semaphore_t：
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;
