findObjectsInBackgroundWithBlock:从Parse获取数据,但数据仅存在于块内

Question

我做了以下测试类来尝试从Parse中检索数据:

-(void)retrieveDataFromParse
{
    PFQuery *query = [PFQuery queryWithClassName:@"TestObject"];

    [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
        if(!error){
            for (PFObject *object in objects){
                NSString *nameFromObject = [NSString stringWithFormat:@"%@", [object objectForKey:@"Name"]];
                NSString *dateFromObject = [NSString stringWithFormat:@"%@", [object createdAt]];
                NSString *scoreFromObject = [NSString stringWithFormat:@"%@", [object objectForKey:@"Score"]];
                [self addNewScore:scoreFromObject andDate:dateFromObject forUserName:nameFromObject];
                NSLog(@"The dictionary is %@", self.scoreDictionary); //<-- here it works printing out the whole dictionary
            }
        } else {
            NSLog(@"Error: %@ %@", error, [error userInfo]);
        }
    }];
    NSLog(@"The dictionary is %@", self.scoreDictionary); //<- but after the block is called, here the dictionary is again empty...
}

根据代码中的注释部分,当我self.scoreDictionary在代码中打印时,它工作正常,我看到我的整个字典,因为它逐渐被填充.但是,在块结束后,当我再次打印字典时,它现在是空的.我仔细检查了查询API文档,但我仍然不确定我做错了什么.

Answer 1

NSLog(@"The dictionary is %@", self.scoreDictionary)块完成后,最后一条语句实际上不会执行.它在findObjectsInBackgroundWithBlock方法返回后执行.findObjectsInBackgroundWithBlock大概是在一个单独的线程中运行某些东西,并且你的块实际上可能根本不会执行,直到最后一个NSLog语句之后的一段时间.从图形上看,这样的事情可能正在发生:

Thread 1 
--------
retriveDataFromParse called
invoke findObjectsInBackgroundWithBlock
findObjectsInBackgroundWithBlock queues up work on another thread
findObjectsInBackgroundWithBlock returns immediately      |
NSLog statement - self.scoreDictionary not yet updated    |
retriveDataFromParse returns                              |
.                                                         V
.                       Thread 2, starting X milliseconds later
.                       --------
.                       findObjectsInBackgroundWithBlock does some work
.                       your block is called
.                       for-loop in your block
.                       Now self.scoreDictionary has some data
.                       NSLog statement inside your block

您可能想要考虑一下,在检索到scoreDictionary数据后,您想对该怎么做？例如,您想要更新UI,调用其他方法等吗？您将希望在块内执行此操作,此时您知道已成功检索数据.例如,如果您有要重新加载的表视图,则可以执行以下操作:

for (PFObject *object in objects){
    ....
}
dispatch_async(dispatch_get_main_queue(), ^{
    [self updateMyUserInterfaceOrSomething];
});

请注意dispatch_async- 如果更新数据后需要完成的工作涉及更改UI,则需要在主线程上运行.