Find the Selector of a Class method - objective-c

Question

I'm quite a newbie in Objective C, though I have some background in Java reflection.
Here, I have a classic class method findAll that find all the domain objects from the database. The class Univers directly inherits from DomainObject
#interface DomainObject : NSObject
- (NSString *) execute : (NSString*) method withJson:(NSString*)json;
+ (NSString*)findAll: (NSString*)json;
#end
#implementation DomainObject
- (NSString *) execute: (NSString*) method withJson:(NSString*)json{
method = [NSString stringWithFormat:#"%#%#", method, #":"];
//method is 'findAll:'
NSString* result = [ self performSelector:
NSSelectorFromString(method) withObject:json];// Error here
return result;
}
#end
The code was working when findAll was NOT a class method (ie -findAll declaration), but now I have the error : NSInvalidArgumentException -[Univers findAll:]
It clearly seems that the runtime is looking for an instance method.
Any idea to find my class method ?

Answer 1

Instead of calling
NSString* result = [self performSelector:NSSelectorFromString(method) withObject:json];
you need to call
NSString* result = [[self class] performSelector:NSSelectorFromString(method) withObject:json];
for class methods.
After all it's the object instance's class that supposed to be calling the method, not the instance itself.
Short explanation: NSObject implements - (Class)class; (not to be mistaken with + (Class)class of similar effect, which NSObject implements, too!) which returns the Class object of your instance object. Keep in mind that in Objective-C in addition to plain instance objects, Classes are actual objects, too: objects of type Class, that is (vs. id, NSObject, …).
See the documentation for the -class method here.
Btw, you should probably wrap your method call into an conditional block to prevent exceptions caused by calls to missing methods.
SEL selector = NSSelectorFromString(method);
if ([[self class] respondsToSelector:selector]) {
NSString* result = [[self class] performSelector:selector withObject:json];
}
In general it's a common pattern in Objective-C to call an object's class method by receiving the class object via [object class].
Consider this case of a class called Foo implementing a convenience method for returning an autporeleased instance of itself (to be called via: Foo *newFoo = [Foo foo];):
While it would certainly be possible to implement said method like this (after all we know the object's class name, right?):
+ (id)foo {
return [[[Foo alloc] init] autorelease];
}
the correct way is this:
+ (id)foo {
return [[[self alloc] init] autorelease];
}
As the first one would cause problems with polymorphism in subclasses (Such as a subclass called FooBar, for which it should clearly be [FooBar alloc] …, not [Foo alloc] …. Luckily [[self class] alloc] solves this dynamically).
While this is clearly not the right place for a thorough explanation of this (rather offtopic one might say) it's certainly worth noting/warning about, imho.