如何为Class方法构建NSInvocation？

Question

I would like to build the invocation using NSClassFromString and NSSelectorFromString for the class method and the selector. I tried to build invocation in the following way:

NSMethodSignature *signature;
NSInvocation *inv;

Class targetClass = NSClassFromString(@"NSString");
SEL selector   = NSSelectorFromString(@"stringWithString:");
id arg = @"argument";

Method method = class_getInstanceMethod(targetClass, selector); 

// why is the method nil when I run the code?

struct objc_method_description* desc = method_getDescription(method);

if (desc == NULL || desc->name == NULL){
    return nil;
}

signature = [NSMethodSignature signatureWithObjCTypes:desc->types];
inv = [NSInvocation invocationWithMethodSignature:signature];
[inv setSelector:selector];
[inv setArgument:&arg atIndex:2];
[inv invoke];

__autoreleasing id returnObj;    
[inv getReturnValue:&returnObj];    // get created object

Since 'method' is always 'nil' this approach does not work. Why?
What is the proper way to execute class method via invocation for the above code?

Answer 1

Your code is too complicated. You can get an NSMethodSignature* object without messing around with the runtime.

signature = [NSString methodSignatureForSelector:@selector(stringWithString:)];

Answer 2

stringWithString: is a class method, your code is using class_getInstanceMethod() .

Try changing class_getInstanceMethod() to class_getClassMethod() .