Objective-C Fundamentals

174 CHAPTER 8 Dynamic typing and runtime type information
You called a method named aSimpleDynamicMethod
You called a method named aSimpleDynamicMethod
You called a method named aSimpleDynamicMethod
You called a method named aSimpleDynamicMethod
The five messages that sent the aSimpleDynamicMethod message to the CTRental-
Property object all correctly invoked the aSimpleDynamicMethod function, as evidenced
by the results of the NSLog call it contains.
An interesting point to notice is the first NSLog message, indicating that a method
named aSimpleDynamicMethod was added to the CTRentalProperty class. This
method results because the first time the house object receives the aSimpleDynamic-
Method message, it detects that the object doesn’t respond to this selector. Therefore,
the Objective-C runtime automatically invokes the object’s resolveInstanceMethod:
method to see if it’s possible to correct this problem. In the implementation of
resolveInstanceMethod:, the Objective-C runtime function class_addMethod
dynamically adds a method to the class and returns YES, indicating that you’ve
resolved the missing method. The four future message sends, which send the
aSimpleDynamicMethod selector, don’t all produce equivalent log messages, because
when these copies of the message are received, the Objective-C runtime finds that the
object does have a method implementation for the aSimpleDynamicMethod selector,
and hence it doesn’t need to dynamically attempt to resolve the situation.
Forwarding methods and dynamic method resolution are largely orthogonal. A
class has the opportunity to dynamically resolve a method before the forwarding
mechanism kicks in. If resolveInstanceMethod: returns YES, message forwarding
isn’t actioned.
The dangerous practice of method swizzling
Because you can add new methods at runtime, you might be interested to learn that
you can also replace an existing method with a new implementation at runtime, even
without access to source code for the class in question.
The Objective-C runtime function method_exchangeImplementations accepts two
methods and swaps them, so that calling one causes the implementation of the other
one to be invoked.
This technique is commonly called method swizzling, but it’s a particularly dangerous
practice that usually indicates you’re doing something wrong or something not
intended by the original creators of the object in question. It’s dangerous because it
relies on so many undocumented or brittle aspects of a class’s implementation.
8.5 Practical uses of runtime type introspection
The iOS development platform has matured with multiple versions of the iPhone
operating system and variants for the iPad all exhibiting slightly different capabilities
and restrictions.