IOS4 note 9

The Secret Life of Classes

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A class method may be called by sending a message directly to the name of a class. For?example, the familyNames class method of UIFont that I mentioned a moment ago might?be called like this:

NSArray* fams = [UIFont familyNames];

Clearly, this is possible because a class is an object (Chapter 2), and the name of the?class here represents that object.

To speak of a class object, you need to obtain that object formally. One way?to ?do ?this ?is ?to ?send ?the ?class message ?to ?a ?class ?or ?instance. ?For ?example, ?[MyClass class] returns the actual class object.?

To supply this as an argument, you’d need to obtain a class object formally. Take, for example, introspection on an object to inquire what its class is. The isKindOfClass: instance method is declared like this:

- (BOOL)isKindOfClass:(Class)aClass

So that means you could call it like this:

if ([someObject isKindOfClass: [MyClass class]]) // ...

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A class object is not an instance, but it is definitely a full-fledged object. Therefore, a?class object can be used wherever an object can be used. For example, it can be assigned?to a variable of type id:

id classObject = [MyClass class];

You could then call a class method by sending a message to that object, because it is?the class object:

id classObject = [MyClass class];

[classObject someClassMethod];

All class objects are also members of the Class class, so you could say this:

Class classObject = [MyClass class];

[classObject someClassMethod];

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