adding methods dynamically

Question

I am trying this method found in Obj-c runtime reference

BOOL class_addMethod(Class cls, SEL name, IMP imp, const char *types)

I want to add a new method like:

- [AClass drawWithFrame:(NSRect)rect inView:(id)view]

So far I have written a C function:

void drawWithFrameInView(id this, SEL this_cmd, NSRect frame, id view){
...
}

now I am ready to do:

BOOL success = class_addMethod(NSClassFromString(@"AClass"), 
                               @selector(drawWithFrame:inView:), 
                               (IMP)drawWithFrameInView, 
                               "v@:@:@:");

but success is never YES, I have tried the same approach with methods with simpler signatures and it worked. So I think the problem is last parameter: "v@:@:@:"

What should I pass in this case to get my new method working ?

Why not name the first two arguments self and _cmd, so you can write the same code you would have written in a method? — Peter Hosey
– Peter Hosey, Commented Jul 25, 2011 at 11:20
Hummm ... just for an aesthetic reason. From the C point of view self and _cmd are supposed to be normal parameters. I didn't like the fact that a parameter was colored as keywords in Obj-C (even though they represent those keywords). That is all :) — nacho4d
– nacho4d, Commented Jul 25, 2011 at 14:37

Jacob Relkin · Accepted Answer · 2011-07-25 22:32:15Z

8

This will work:

char *types = [[NSString stringWithFormat:@"v@:%s@", @encode(NSRect)] UTF8String];

BOOL success = class_addMethod(NSClassFromString(@"MyClass"), 
                               @selector(drawWithFrame:inView:), 
                               (IMP)drawWithFrameInView, 
                               types);

The reason why your code doesn't work is because NSRect is not an object, it is a typedef to a struct.

Learn more about type encodings here.

edited Jul 25, 2011 at 22:32

answered Jul 25, 2011 at 5:24

Jacob Relkin

163k34 gold badges352 silver badges321 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

nacho4d Over a year ago

That seems to work (at least I got success = YES :) ). However the docs says: Since the function must take at least two arguments—self and _cmd, the second and third characters must be “@:” (the first character is the return type) and types does not contain a return type, is this because is void or there is some kind of inference here?

Jacob Relkin Over a year ago

@nacho4d types does contain a return type - the first element is 'v', for void.

Peter Hosey Over a year ago

I'm disappointed that @encode(…) isn't considered a literal. If it were, you could concatenate them at compile time: static const char types[] = @encode(void) @encode(id) @encode(SEL) @encode(NSRect) @encode(id);

Jacob Relkin Over a year ago

@Peter Agreed - even better would be if @encode were a variadic function: @encode(void, id, SEL, NSRect, id);

Dave DeLong Over a year ago

@Peter that sounds like a great feature request... :)

Collectives™ on Stack Overflow

adding methods dynamically

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related