Objective-C Fundamentals

172 CHAPTER 8 Dynamic typing and runtime type information
performSelector:withObject:withObject:. All of these methods take a selector indicating
the message to send and map directly to the messaging function. For example:
[house performSelector:@selector(setAddress:)
withObject:@"13 Adamson Crescent"];
This line is equivalent to
[house setAddress:@"13 Adamson Crescent"];
The power of these methods comes from the fact that you can vary the message being
sent at runtime just as easily as it’s possible to vary the object that receives the message. A
variable can be used in place of the method selector, unlike in a traditional message
send, which accepts only a constant in this location. In other words, the following is valid:
id obj = get_object_from_somewhere();
SEL msg = get_selector_from_somewhere();
id argument = get_argument_from_somewhere();
[obj performSelector:msg withObject:argument];
But this isn’t valid:
id obj = get_object_from_somewhere();
SEL msg = get_selector_from_somewhere();
id argument = get_argument_from_somewhere();
[obj msg:argument];
In this example, the receiver of the message (obj) is chosen at runtime, and the message
it’s sent is also determined at runtime. This leads to a lot of flexibility in the
design of an application; with functions such as NSSelectorFromString, discussed
previously, it’s even possible to use selectors that exist only at runtime.
The Target-Action design pattern
The UIKit framework makes good use of the ability to programmatically vary both the
receiver and message sent via a message send. UIView objects such as UISlider
and UIButton interpret events from hardware such as the touch screen or Bluetoothconnected
keyboard and convert these events into application-specific actions.
For instance, when the user taps on a button, a UIButton object sends a message
instructing the application that it should do something in response to the button press.
A UIButton object is initialized with details on which message to send and where to
send it. This is a common design pattern in UIKit called the Target-Action design pattern.
As an example, you could request that a button send the buttonPressed: message
to the myObject object when the user lifts their finger off the screen with the following
initialization:
[btn addTarget:myObject
action:@selector(buttonPressed:)
forControlEvents:UIControlEventTouchUpInside];
The UIButton class sends the message using NSObject’s performSelector: with-
Object: method, just as we discussed in this section. If Objective-C didn’t allow the
message name or target to be varied programmatically, UIButton objects would all have
to send the same message, and the name would be frozen in UIButton’s source code.