Dynamically invoke a class method in Objective C

Question

Suppose I have Objective C interface SomeClass which has a class method called someMethod:

@interface SomeClass : NSObject {
}

+ (id)someMethod;
@end

In some other interface I want to have a helper method that would dynamically invoke someMethod on a class like this:

[someOtherObject invokeSelector:@selector(someMethod) forClass:[SomeClass class];

What should be the implementation for invokeSelector? Is it possible at all?

- (void)invokeSelector:(SEL)aSelector forClass:(Class)aClass {
   // ???
}

In your example code you are declaring SomeClass as a root class. There should incredibly rarely be a need for this. Is it just a mistake in your question? — Mike Abdullah
– Mike Abdullah, Commented Nov 23, 2009 at 11:45

shim · Accepted Answer · 2015-10-09 20:35:01Z

84

Instead of:

[someOtherObject invokeSelector:@selector(someMethod) forClass:[SomeClass class];

call:

[[SomeClass class] performSelector:@selector(someMethod)];

Example (using GNUstep ...)

file A.h

#import <Foundation/Foundation.h>
@interface A : NSObject {}

- (NSString *)description;
+ (NSString *)action;
@end

file A.m

#import <Foundation/Foundation.h>
#import "A.h"

@implementation A

- (NSString *)description
{
    return [NSString stringWithString: @"A"];
}

+ (NSString *)action
{
    return [NSString stringWithString:@"A::action"];
}

@end

Somewhere else:

A *a = [[A class] performSelector:@selector(action)];
NSLog(@"%@",a);

Output:

2009-11-22 23:32:41.974 abc[3200] A::action

nice explanation from http://www.cocoabuilder.com/archive/cocoa/197631-how-do-classes-respond-to-performselector.html:

"In Objective-C, a class object gets all the instance methods of the root class for its hierarchy. This means that every class object that descends from NSObject gets all of NSObject's instance methods - including performSelector:."

edited Oct 9, 2015 at 20:35

shim

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answered Nov 23, 2009 at 4:13

stefanB

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7 Comments

GregK Over a year ago

I knew I could use performSelector on NSObject. However, I was confused because I thought the type Class didn't have performSelector method. I didn't bother to try.

GregK Over a year ago

Code Sense doesn't suggest performSelector for [SomeClass class] experssion.

stefanB Over a year ago

I'm not in front of my Mac, so I tried on Linux with GNUStep and it works ... :)

vanja. Over a year ago

His class doesn't implement NSObject.

user102008 Over a year ago

you can just do [A performSelector:@selector(action)];

|

Adrian · Accepted Answer · 2009-11-23 11:29:42Z

4

In Objective-C, classes are objects as well. The class objects are treated differently, however, as they can call the instance methods of their root class (NSObject or NSProxy in Cocoa).

So it's possible to use all the instance methods defined in NSObject on class objects as well and the right way to dynamically invoke a class method is:

[aClass performSelector:@selector(aSelector)];

The apple docs are a bit more specific.

answered Nov 23, 2009 at 11:29

Adrian

1,0547 silver badges5 bronze badges

Comments

Ben S · Accepted Answer · 2009-11-23 04:09:49Z

2

You shouldn't implement this yourself.

The NSObject Protocol has a performSelector: method that does exactly this.

answered Nov 23, 2009 at 4:09

Ben S

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10 Comments

GregK Over a year ago

Type Class is not NSObject. It doesn't have performSelector method

mipadi Over a year ago

@Ben S: He's trying to invoke a class method. Class objects don't descend from NSObject.

Mike Abdullah Over a year ago

In Objective-C classes are objects, direct subclasses of NSObject. Every instance method defined on NSObject will work on a class.

mipadi Over a year ago

@Mike: Yes, but a Class object doesn't descend from NSObject.

user102008 Over a year ago

@mipadi: Class is not a class, but a generic type for pointers to class objects. Each class object is an instance of its meta-class, which follows an inheritance chain that ultimately ends in the root class of the class in question, which is in this case NSObject. So yet, it descends from NSObject.

|

Cameron Lowell Palmer · Accepted Answer · 2018-06-04 13:56:51Z

2

Is this built-in method what you want?

id objc_msgSend(id theReceiver, SEL theSelector, ...)

(See the runtime reference docs for this function.)

edited Jun 4, 2018 at 13:56

Cameron Lowell Palmer

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answered Nov 23, 2009 at 4:10

vanja.

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4 Comments

Ben S Over a year ago

From your link: "This reference is useful primarily for developing bridge layers between Objective-C and other languages, or for low-level debugging. You typically do not need to use the Objective-C runtime library directly when programming in Objective-C."

vanja. Over a year ago

But isn't avoiding built-in checks and balances the whole point of dynamically sending messages?

Chris Long Over a year ago

Never mind; didn't notice that it wasn't a subclass of NSObject.

Chris Long Over a year ago

I think this is the best option then.

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