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,当我在代码中打印self.scoreDictionary时,它工作正常,我看到整个字典,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.
解决方法
最后一个NSLog(@“字典是％@”,self.scoreDictionary)语句在块完成后实际上不执行.它在findObjectsInBackgroundWithBlock方法返回后执行. findObjectsInBackgroundWithBlock可能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
你可能想要考虑一下,在你检索了scoreDictionary数据之后你想对它做什么？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做的工作涉及更改UI,则需要在主线程上运行.
