问题描述
正在学习Leancloud，想用一个工厂方法来封装查询代码，但是不能return block里的值，直接return objectID是空值，求指教，谢谢!
+ (NSString *)getObjectIDbyUsername:(NSString *)_username{ __block NSString *objectID = NULL; NSString *username = [NSString stringWithFormat:@'^%@$',_username]; AVQuery *query = [AVQuery queryWithClassName:@'_User']; [query whereKey:@'username' containsString:username]; [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {objectID = [objects valueForKey:@'objectId']; }]; return objectID;}
问题解答
回答1：异步：
+ (void)getObjectIDbyUsername:(NSString * )_username compelete:(void(^)(NSString * objectID))compelete{NSString *username = [NSString stringWithFormat:@'^%@$',_username]; AVQuery *query = [AVQuery queryWithClassName:@'_User']; [query whereKey:@'username' containsString:username]; [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {if (compelete) { compelete(nil); compelete = nil;} }];}// use:[HTBaseContainer getObjectIDbyUsername:@'userName' compelete:^(NSString *objectID) { }];
同步:
+ (NSString *)getObjectIDbyUsername:(NSString *)_username{ dispatch_semaphore_t signal = dispatch_semaphore_create(1);__block NSString *objectID = NULL; NSString *username = [NSString stringWithFormat:@'^%@$',_username]; AVQuery *query = [AVQuery queryWithClassName:@'_User']; [query whereKey:@'username' containsString:username]; [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {objectID = [objects valueForKey:@'objectId'];dispatch_semaphore_signal(signal); }];dispatch_semaphore_wait(signal, DISPATCH_TIME_FOREVER); return objectID;}回答2：
把工厂方法的形参加一个block, block 回调objectID
dispatch_semaphore_t, 在return前一直wait.
回答3：GCD group你值得拥有，假如用swift的话一个@escaping block就搞定了。。
