Adding Objective-C method to class at runtime

Question

I'm trying to support newly added methods of NSColor in 10.9 on older systems. Pre-10.9 I had these in a category which allowed my to unify code between Mac and iOS. Now that 10.9 has these methods, I get odd drawing wherever I use them. I want to add these methods dynamically to older runtimes and I've found several references for how to do it using class_addMethod. The problem is, that even though addMethod returns success, the methods aren't called.

NSColor *
fColorWithWhite(id self, SEL _cmd, float white, float alpha) {
    return [NSColor colorWithDeviceWhite: white
                                   alpha: alpha];
}

NSColor *
fColorWithRedGreenBlue(id self, SEL _cmd, float red, float green, float blue, float alpha) {
    return [NSColor colorWithDeviceRed: red
                                 green: green
                                  blue: blue
                                 alpha: alpha];
}

+ (void)addLegacySupport {
    Class class = NSClassFromString(@"NSColor");

    BOOL success = class_addMethod(class, @selector(colorWithWhite:alpha:), (IMP)fColorWithWhite, "@@:ff");
    NSLog(@"colorWithWhite:alpha: - %i", success);

    success = class_addMethod(class, @selector(colorWithRed:green:blue:alpha:), (IMP)fColorWithRedGreenBlue, "@@:ffff");
    NSLog(@"colorWithRed:green:blue:alpha: - %i", success);
}

Any pointers would be much appreciated.

Léo Natan · Accepted Answer · 2013-12-23 01:25:34Z

6

You are trying to add class methods. You need to add them to the metaclass.

Class meta_cls = objc_getMetaClass("NSColor");

answered Dec 23, 2013 at 1:25

Léo Natan

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2 Comments

greg Over a year ago

This looked so amazing that I couldn't believe I didn't see it in the API docs but also returned false on class_addMethod.

greg Over a year ago

Solved: The above worked. The underlying problem had to do with casting of double to CGFloat to a float.

jscs · Accepted Answer · 2013-12-23 01:30:34Z

6

class_addMethod() adds an instance method to the class. You are trying to add a class method. Thus, you need to add the method to the metaclass (classes are instances of their metaclasses), which you can get by calling object_getClass() with the class as the argument:

Class metaclass = object_getClass(NSClassFromString(@"NSColor"));

BOOL success = class_addMethod(metaclass, @selector(colorWithWhite:alpha:), (IMP)fColorWithWhite, "@@:ff");

edited Dec 23, 2013 at 1:30

answered Dec 23, 2013 at 1:25

jscs

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3 Comments

greg Over a year ago

No success. class_addMethod returns false.

jscs Over a year ago

Huh. It's working here. What SDK are you testing against? It'll fail if the method already exists (i.e., on 10.9).

greg Over a year ago

I am building against 10.8 so that I actually see the errors and don't have the code backing. I don't have a pre 10.9 system to run on at the moment, so this seemed like my best bet for testing this successfully before releasing.

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