valueforkey和objectforkey

objectforkey 是NSDictionary的方法，valueforkey 是KVC的方法， 两者都是键值对应，区别是valueforkey 只允许使用NSString类型，objectforkey可以是任意类型.  

 

 

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 two objects can have the same key (just as no two objects in an 

NSArray

 can have the same index).

valueForKey:

 is a 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.

 

 

objectForKey: accepts any object as a key, not just strings. The only requirement is that the key support the NSCopying protocol.

When you do 

valueForKey:

 you need to give it an NSString, whereas 

objectForKey:

 can take any NSObject subclass as a key. This is because for Key-Value Coding, the keys are always strings.

In fact, the documentation states that even when you give 

valueForKey:

 an NSString, it will invoke

objectForKey:

 anyway unless the string starts with an 

@

, in which case it invokes 

[super valueForKey:]

,
which may call 

valueForUndefinedKey:

 which may raise an exception.此段是说C字符串无效，必须是@开头的字符串