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Key Value Coding

Once you start asking how anything works, you immediately plunge into a gray area. The documentation at least tells us this:

• The default implementation will invoke the -(id)<key> or -(void)set<key> methods, if they exist, to perform the work. There are a few other method names tried if these can't be found.
• If no method is found, any instance variable with the same name as the key will be automatically accessed (unless this feature is explicitly disabled for the class).
• If no matching instance variable is found either, the -(id)valueForUndefinedKey:(NSString *)key method is invoked which raises an NSUndefinedKeyException by default.

Distilling this information down, you can (in most cases) presume that key value coding will work for any "key" on any class that supports the methods -(id)<key> and -(void)set<key>, or has an attribute named "key". Sometimes overrides valueForUndefinedKey: will support other keys too but that is highly implementation specific. Most other key and object combinations will throw an NSUndefinedKeyException.

Source cocoawithlove