Question

ForwardInvocation does exist, but it is slow and has the annoying problem of compiler warnings. So that got me to thinking — is there a way to use macroes to quickly implement a bunch of getter methods that get the property in question from another object?

For example, if I have a Car object, it might want to implement the following:

Car.h:

@class SparkPlug;
@class Engine;

. . .

-(int) nPistons;
-(float) horsepower;
-(SparkPlug*) sparkPlug;

Car.m:

. . .

-(int) nPistons {
    return self.engine.nPistons;
}

-(float) horsepower {
    return self.engine.horsepower;
}

-(SparkPlug*) sparkPlug {
    return self.engine.sparkPlug;
}

Question — would it be possible to set up some macroes so that by making one change somewhere, I could add another such method to both the header and implementation files?

e.g. MagicForwardingMacro (nPistons, int, engine);

Ideally, in such a way that the macroes would be reusable if I later wanted to later use a similar strategy to get the firstName, lastName, placeOfBirth, and dateOfBirth properties of a Person from his or her birthCertificate.

Answer 1

The easiest way is probably to add the methods dynamically:

• Add the properties to a category so the compiler doesn’t complain too much.
• Clone a suitable IMP in +[NSObject resolveInstanceMethod:]. You’ll need to poke the Objective-C runtime.

Elaborating on the second step:

For each type, add a method like

-(int)getEngineInt {
  return (int()(id,SEL))(objc_msgSend)(engine, _cmd);
}

Note that for structs you need objc_msgSend_stret and for floats/doubles you might need objc_msgSend_fpret (I think you only need it on i386; not sure about AMD64). The easy hack to support both the simulator and device is something like (I forget the macro name GCC uses…)

#if __i386
#define objc_msgSend_fpret objc_msgSend
#endif

Now to implement +resolveInstanceMethod:, you need to know the class you’re forwarding to ahead of time. Let’s say it’s Engine.

+(BOOL)instancesRespondToSelector:(SEL)name
{
  return [Engine instancesRespondToSelector:name];
}

+(BOOL)resolveInstanceMethod:(SEL)name
{
  // Do we want to super-call first or last? Who knows...
  if ([super resolveInstanceMethod:name]) { return YES; }
  // Find the return type, which determines the "template" IMP we call.
  const char * returntype = [Engine instanceMethodSignatureForSelector:name].methodReturnType;
  if (!returnType) { return NO; }

  // Get the selector corresponding to the "template" by comparing return types...
  SEL template = NULL;
  if (0 == strcmp(returntype,@encode(int))
  {
    sel = @selector(getEngineInt);
  }
  else if (0 == strcmp(Returntype,@encode(float))
  {
    ...
  }
  if (!sel) { return NO; }
  Method m = class_getInstanceMethod(self,template);
  return class_addMethod(self, name, method_getImplementation(m), method_getTypeEncoding(m));
}

Alternatively, there’s a slightly undocumented method -forwardingTargetForSelector: which may be fast enough for your needs.

EDIT: Alternatively, you can loop over the properties/methods dynamically. There doesn’t appear to be an obvious way to introspect categories, but you can define them in a protocol, do something like @interface Engine:NSObject<Engine> ... @interface Car(DynamicEngine)<Engine> and use objc_getProtocol("Engine") and then protocol_copyMethodDescriptionList()/protocol_copyPropertyList() to get the methods, and then add the getters. I’m not sure if properties are added to the “method description list”. Also note that the “copy” functions do not copy methods/properties from superclasses, which (in this case) is what you want.