Objective-C Fundamentals

Runtime type information
173
8.4.3 Adding new methods to a class at runtime
The Objective-C statement [CTRentalProperty class] returns a Class object that
represents the CTRentalProperty class. With Objective-C’s dynamic features, it’s possible
to add or remove methods, protocols, and properties from a class by interacting
with this object at runtime.
We demonstrate how to dynamically add a new method implementation to an
existing class by responding to the class’s resolveInstanceMethod: message. This is
another method invoked on an object when a message sent to it doesn’t find a matching
method implementation.
At the lowest level of the Objective-C runtime, an Objective-C method is a C function
that takes, at a minimum, two additional arguments named self and _cmd (as discussed
previously). Once a suitable function is developed, it can be registered as a method of
an Objective-C class using a function called class_addMethod:. Add the following code
to CTRentalProperty.m in the current version of the Rental Manager application:
void aSimpleDynamicMethodIMP(id self, SEL _cmd) {
NSLog(@"You called a method named %@", NSStringFromSelector(_cmd));
}
+ (BOOL)resolveInstanceMethod:(SEL)sel {
if (sel == @selector(aSimpleDynamicMethod)) {
NSLog(@"Adding a method named %@ to class %@",
NSStringFromSelector(sel),
NSStringFromClass([self class]));
class_addMethod([self class],
sel,
(IMP)aSimpleDynamicMethodIMP,
"v@:");
return YES;
}
}
return [super resolveInstanceMethod:sel];
In the current version of the Rental Manager application, you can then add the
following calls to a convenient location, such as the RootViewController’s viewDid-
Load method:
id house = [CTRentalProperty rentalPropertyOfType:TownHouse
rentingFor:420.0f
atAddress:@"13 Waverly Crescent, Sumner"];
[house aSimpleDynamicMethod];
[house aSimpleDynamicMethod];
[house aSimpleDynamicMethod];
[house aSimpleDynamicMethod];
[house aSimpleDynamicMethod];
When this code is executed, you should notice that the debug console lists the following
content:
Adding a method named aSimpleDynamicMethod to class CTRentalProperty
You called a method named aSimpleDynamicMethod