How can I dynamically create a selector at runtime with Objective-C?

Question

I know how to create a SEL at compile time using @selector(MyMethodName:) but what I want to do is create a selector dynamically from an NSString. Is this even possible?

What I can do:

SEL selector = @selector(doWork:);
[myobj respondsToSelector:selector];

What I want to do: (pseudo code, this obviously doesn't work)

SEL selector = selectorFromString(@"doWork");
[myobj respondsToSelector:selector];

I've been searching the Apple API docs, but haven't found a way that doesn't rely on the compile-time @selector(myTarget:) syntax.

PLG · Accepted Answer · 2018-02-13 13:51:06Z

180

I'm not an Objective-C programmer, merely a sympathizer, but maybe NSSelectorFromString is what you need. It's mentioned explicity in the Runtime Reference that you can use it to convert a string to a selector.

edited Feb 13, 2018 at 13:51

PLG

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answered Sep 22, 2008 at 0:34

Torsten Marek

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2 Comments

craigb Over a year ago

I need to brush up on my Google-fu. that's exactly what I was (or wasn't as it may be) looking for.

Torsten Marek Over a year ago

Well, I still had the links flying around in my bookmarks since I've read through the Objective-C 2.0 docs a couple of days ago.

Josh Gagnon · Accepted Answer · 2008-09-22 00:42:32Z

40

According to the XCode documentation, your psuedocode basically gets it right.

It’s most efficient to assign values to SEL variables at compile time with the @selector() directive. However, in some cases, a program may need to convert a character string to a selector at runtime. This can be done with the NSSelectorFromString function:

setWidthHeight = NSSelectorFromString(aBuffer);

Edit: Bummer, too slow. :P

answered Sep 22, 2008 at 0:42

Josh Gagnon

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3 Comments

bendytree Over a year ago

NSStringFromSelector(@"doWork") converts it the other way (just fyi)

jpswain Over a year ago

I think you mean, NSStringFromSelector(@selector(doWork))

user4951 Over a year ago

And what does that selector supposedly do? Shouldn't we specify a block or something?

Alex Gray · Accepted Answer · 2014-02-01 05:59:18Z

I'd have to say that it's a little more complicated than the previous respondents' answers might suggest... if you indeed really want to create a selector... not just "call one" that you "have laying around"...

You need to create a function pointer that will be called by your "new" method.. so for a method like [self theMethod:(id)methodArg];, you'd write...

void (^impBlock)(id,id) = ^(id _self, id methodArg) { 
     [_self doSomethingWith:methodArg]; 
};

and then you need to generate the IMP block dynamically, this time, passing, "self", the SEL, and any arguments...

void(*impFunct)(id, SEL, id) = (void*) imp_implementationWithBlock(impBlock);

and add it to your class, along with an accurate method signature for the whole sucker (in this case "v@:@", void return, object caller, object argument)

 class_addMethod(self.class, @selector(theMethod:), (IMP)impFunct, "v@:@");

You can see some good examples of this kind of runtime shenanigans, in one of my repos, here.

Krypton · Accepted Answer · 2014-02-26 05:11:16Z

5

I know this has been answered for long ago, but still I wanna share. This can be done using sel_registerName too.

The example code in the question can be rewritten like this:

SEL selector = sel_registerName("doWork:");
[myobj respondsToSelector:selector];

answered Feb 26, 2014 at 5:11

Krypton

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1 Comment

PLG Over a year ago

Actually, NSSelectorFromString mentioned by @torsten-marek uses sel_registerName under the hood. appledev: "NSSelectorFromString passes a UTF-8 encoded character representation of aSelectorName to sel_registerName and returns the value returned by that function"

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