If objc_msg_lookup() does not find a suitable method
implementation, because the receiver does not implement the required
method, it tries to see if the class can dynamically register the
method.
To do so, the runtime checks if the class of the receiver implements the method
+ (BOOL) resolveInstanceMethod: (SEL)selector;
in the case of an instance method, or
+ (BOOL) resolveClassMethod: (SEL)selector;
in the case of a class method. If the class implements it, the
runtime invokes it, passing as argument the selector of the original
method, and if it returns YES, the runtime tries the lookup
again, which could now succeed if a matching method was added
dynamically by +resolveInstanceMethod: or
+resolveClassMethod:.
This allows classes to dynamically register methods (by adding them to
the class using class_addMethod) when they are first called.
To do so, a class should implement +resolveInstanceMethod: (or,
depending on the case, +resolveClassMethod:) and have it
recognize the selectors of methods that can be registered dynamically
at runtime, register them, and return YES. It should return
NO for methods that it does not dynamically registered at
runtime.
If +resolveInstanceMethod: (or +resolveClassMethod:) is
not implemented or returns NO, the runtime then tries the
forwarding hook.
Support for +resolveInstanceMethod: and
resolveClassMethod: was added to the GNU Objective-C runtime in
GCC version 4.6.