为什么我不能从NSArray元素中获取格式正确的字符串？ [英] Why can I not get a properly formatted string from NSArray element?

问题描述

我正在尝试从核心数据模型中检索属性，并将获取的字符串放入导航栏的标题中。我一定做错了，因为我无法将检索到的字符串设置为标题，但是如果我手动输入字符串，则可以设置标题。

I'm trying to retrieve an attribute from my core data model and put the acquired string into the title of my navigation bar. I must be doing something wrong, as I cannot set my retrieved string as the title, yet I can set the title if I manually enter the string.

这是我从核心数据中检索的方式：

Here is how I am retrieving from Core Data:

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Area" inManagedObjectContext:context];
[fetchRequest setEntity:entity];

//set predicate
fetchRequest.predicate = [NSPredicate predicateWithFormat:@"identifier == %@", self.area_id];

NSArray *area = [context executeFetchRequest:fetchRequest error:&error];

NSString *area_name = [area valueForKey:@"name"];

NSString *test = @"test";

NSLog(@"%@",test);

NSLog(@"%@",area_name);

当我使用self.tabBarController.title = area_name设置标题时；我得到 NSArrayI stringByTrimmingCharactersInSet：]：无法识别的选择器发送到实例，并且在将其设置为测试变量时设置得很好。

When I set the title using self.tabBarController.title = area_name; I get "NSArrayI stringByTrimmingCharactersInSet:]: unrecognized selector sent to instance" and it sets just fine when I set it to the test variable.

我还注意到NSLog for变量test是 test，而area_name的NSLog输出是：（ Single Area）

I also noticed that the NSLog for the variable test is "test" whereas the NSLog output for area_name is: ( "Single Area" )

这是因为我要从数组中检索我的area_name吗？我应该改用字典吗？

Is this because of how I am retrieving my area_name from the array? Should I be using a dictionary instead? Forgive me for my ignorance, I'm fairly new to obj-c.

谢谢。

推荐答案

这里是错误的

NSString *area_name = [area valueForKey:@"name"];  

区域对象的类型为 NSArray 包含 Area 对象。

因此，您需要从该数组中获取元素，然后读取名称。

area object is type of NSArray which contains Area objects.
So you need to get element from that array, and then read name.

Area *firstArea = [area firstObject];
NSString *area_name = firstArea.name;
NSLog(@"%@",area_name);

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