Use an NSString to programatically access or create a method in Objective-C

Question

I am trying to use an array of strings dynamically access methods at runtime within my class. For now the methods are already there, eventually I want to create them. Is this possible?

For example:

bool nextLevel=NO;
for(NSString * match in gameLevels)
{

    if([match isEqualToString:self.level])
    {
        nextLevel=YES;
    }
    else if(nextLevel==YES)
    {
        self.level=match;
        nextLevel=NO;
    }
}
//access method named self.level

Thank you in advance!

@KhawarAli I think he wants to have it so he can do [self @"methodThatDoesSomething"] where the string comes from possible functuon names in an array — Oscar Apeland
– Oscar Apeland, Commented May 13, 2014 at 18:11
Are you saying the NSString contains the names of methods to call? Yes that's possible. Look at NSSelectorFromString — Piepants
– Piepants, Commented May 13, 2014 at 18:11
There are several ways to call a method by name. See, eg, the Objective-C Runtime Reference. Creating a method is a bit (actually, a whole lot) more difficult. — Hot Licks
– Hot Licks, Commented May 13, 2014 at 18:13

atreat · Accepted Answer · 2014-05-13 22:04:08Z

1

I use:

NSSelectorFromString(selectorString)

In your case, the selectorString would be:

NSString * selectorString = @"setLevel:";

This is 'setLevel' instead of 'level' because the Objective-C runtime will automatically expand dot properties to these selector names when assignment occurs.

edited May 13, 2014 at 22:04

answered May 13, 2014 at 18:19

atreat

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2 Comments

Bryan Chen Over a year ago

you mean setLevel:?

atreat Over a year ago

Why yes I do. Good catch. The colon is necessary because we are passing a parameter.

J2theC · Accepted Answer · 2014-05-13 21:00:57Z

To access a method based on a string, check the other answer.

To add a method in the runtime you need to create a IMP function or block.

If using a function, could be something like:

void myMethodIMP(id self, SEL _cmd)
{
    // implementation ....
}

You could also use a block like this:

IMP blockImplementation=imp_implementationWithBlock(^(id _self, ...){
    //Your Code here
}

Then you need to add the method, like this:

class_addMethod(yourClass, @selector(selectorName), (IMP) blockImplementation, encoding);

The encoding part is a special runtime encoding to describe the type of parameters your method receives. You can find that on the Objective-C runtime reference.

If you receive dynamic arguments on your generated methods, you need to use the va_list to read the values.

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