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Dynamic Method Resolution

There are situations where you might want to provide an implementation of a method dynamically. For example, the Objective-C declared properties feature (see “Properties”) includes the @dynamic directive:

@dynamic propertyName;

which tells the compiler that the methods associated with the property will be provided dynamically.

You can implement the methods resolveInstanceMethod: and resolveClassMethod: to dynamically provide an implementation for a given selector for an instance and class method respectively.

An Objective-C method is simply a C function that take at least two arguments—self and _cmd. You can add a function to a class as a method using the function class_addMethod. Therefore, given the following function:

void dynamicMethodIMP(id self, SEL _cmd) {

    // implementation ....

}

you can dynamically add it to a class as a method (called resolveThisMethodDynamically) using resolveInstanceMethod: like this:

@implementation MyClass

+ (BOOL)resolveInstanceMethod:(SEL)aSEL

{

    if (aSEL == @selector(resolveThisMethodDynamically)) {

          class_addMethod([self class], aSEL, (IMP) dynamicMethodIMP, "v@:");

          return YES;

    }

    return [super resolveInstanceMethod:aSEL];

}

@end

Forwarding methods (as described in “Forwarding”) and dynamic method resolution are, largely, orthogonal. A class has the opportunity to dynamically resolve a method before the forwarding mechanism kicks in. If respondsToSelector: or instancesRespondToSelector: is invoked, the dynamic method resolver is given the opportunity to provide an IMP for the selector first. If you implement resolveInstanceMethod: but want particular selectors to actually be forwarded via the forwarding mechanism, you return NO for those selectors.

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