What is the difference between objectForKey and valueForKey?

最新推荐文章于 2025-12-03 15:49:02 发布

最新推荐文章于 2025-12-03 15:49:02 发布 · 107 阅读

文章标签：

本文详细解释了KVC（键值编码）中的valueForKey方法与NSDictionary类中objectForKey方法的区别。前者作为通用方法，允许通过字符串访问实例变量；后者则是针对字典类的方法，用于通过键获取字典中的值。

Q: What is the difference between objectForKey and valueForKey? I looked both up in the documentation and they seemed the same to me.

--------------------------------------------------------------------------------------------------------------------------------------------------

A: "objectForKey:" is an NSDictionary method. An NSDictionary is a collection class similar to an NSArray, except instead of using indexes, it uses keys to differentiate between items. A key is an arbitrary string you provide. No 2 objects can have the same key (just as no 2 objects in an NSArray can have the same index).

"valueForKey: is KVC method. It works with ANY class. "valueForKey" allows you to access an instance variable using a string. So for instance, if I have an Account class with an instance variable accountNumber, I can do the following:

NSNumber *anAccountNumber = [NSNumber numberWithInt:12345]; Account *newAccount = [[Account alloc] init]; [newAccount setAccountNumber:anAccountNUmber]; NSNumber *anotherAccountNumber = [newAccount accountNumber];

Using KVC, I can also do it like this:

NSNumber *anAccountNumber = [NSNumber numberWithInt:12345];
Account *newAccount = [[Account alloc] init];
[newAccount setValue:anAccountNumber forKey:@"accountNumber"];
NSNumber *anotherAccountNumber = [newAccount valueForKey:@"accountNumber"];

Those are equivalent sets of statements.

I know you're thinking: wow, but sarcastically. KVC doesn't look all that useful. In fact, it looks "wordy". But when you want to change things at runtime, you can do lots of cool things that are much more difficult in other languages (but this is beyond the scope of your question).

If you want to learn more about KVC, there are many tutorials if you Google especially at Scott Stevenson's blog.

Hope that helps.