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Objective-C Pocket Reference by Andrew Duncan

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Key-Value Coding

Objective-C lets you call methods specified by variables at runtime using the selector mechanism. Key-value coding is a library facility that puts field access on the same dynamic footing: you can access an object's fields by naming them. For example, you could use the following code to retrieve the parent field of a window object:

Window*parentWind = [wind valueForKey:@"parent"];

Because you can pass a string variable to -valueForKey : as well as a literal value, this is another way your program's behavior can vary based on values that aren't known until runtime.

NSObject implements -valueForKey: method as part of the NSKeyValueCoding category, which declares methods for reading from and writing to the fields of objects. These methods store and retrieve Objective-C objects, so their primary use is in accessing objects. However, even if your fields are integers or other numeric types, you can still use key-value coding to retrieve and set them. The methods will take NSNumber objects and automatically convert them to set numeric fields, and return NSNumber objects when you read numeric fields.

Access Permissions

The key-value methods can bypass the access modifiers of the static language: you can read and write to private fields as easily as to public ones. This might seem like a violation of the object's declared interface. However, you can prevent key-value methods from bypassing your access modifiers by overriding +accessInstanceVariablesDirectly to return NO.

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