Programming iOS 6, 3rd Edition

Messages and Methods

An Objective-C method is defined as part of a class. It has three aspects:

Whether it’s a class method or an instance method

If it’s a class method, you call it by sending a message to the class itself. If it’s an instance method, you call it by sending a message to an instance of the class.

Its parameters and return value

As with a C function, an Objective-C method takes some number of parameters; each parameter is of some specified type. And, as with a C function, it may return a value, which is also of some specified type; if the method returns nothing, its return type is declared as void.

Its name

An Objective-C method’s name must contain as many colons as it takes parameters. The name is split after each colon in a method call or declaration, so it is usual for the part of the name preceding each colon to describe the corresponding parameter.

Sending a Message

As you’ve doubtless gathered, the syntax for sending a message to an object involves square brackets. The first thing in the square brackets is the object to which the message is to be sent; this object is the message’s receiver. Then follows the message:

NSString* s2 = [s uppercaseString]; // send "uppercaseString" message to s ...
// ... (and assign result to s2)

If the message is a method that takes parameters, each corresponding argument value comes after a colon:

[myStack1 push: @"Hello"]; // send "push:" message to myStack1 ...
// ...with one argument, the NSString @"Hello"

To send a message ...