Objective-C Passing Method Reference As Parameter

Question

I've seen a lot of discussions NEAR this subject, but none that actually work in Xcode 5.x, especially using ARC. I have a simple problem:

I need to pass a method reference to a CreateButton method so that when the button is called it calls my custom function, and not some generic one.

I've tried using (SEL) type, but that doesn't work with ARC. I've tried using the &func method, but that claims I haven't declared the function yet.

So my need is:

Class A calls Class B and sends over the info to make a UIButton. Within that call, I want to send over the action:method in a reference. I'm sure this is done routinely, but I can't seem to find an iOS 7 / Xcode 5.x method of doing it. I've also reviewed the O'Reilly iOS 7 fundamentals and cookbook code and couldn't find this discussed anywhere.

Thanks for you help.

Marc Liyanage · Accepted Answer · 2019-09-26 05:43:50Z

4

When I have to pass selectors around, I convert them to strings with NSStringFromSelector() and back to selectors with NSSelectorFromString().

Passing the strings around is a lot easier. You can store them in collections (arrays, dictionaries), serialize and unserialize them, and they will work naturally with ARC.

Example:

In your class A where you gather the information to create a button:

NSString *selectorString = NSStringFromSelector(@selector(yourActionMethodNameHere:));
// Gather more information needed by Class B here, then package
// it all up into a dictionary, for example
NSDictionary *buttonInfo = @{@"selectorString": selectorString, /* more stuff here */};

At this point, you can call your button-constructing method in Class B, passing along buttonInfo, which contains all the information that that helper method needs, including the selector. The method can convert the string back to a selector and use it like this:

SEL actionSelector = NSSelectorFromString(buttonInfo[@"selectorString"]);
// configure your button to use actionSelector here

edited Sep 26, 2019 at 5:43

answered Dec 25, 2013 at 2:03

Marc Liyanage

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6 Comments

Mark Löwe Over a year ago

How does one call the NSStringFromSelector()? I tried NSStringFromSelector(func), self.func, [self func], (SEL)func. Nothing seemed to work. The back half is done and ready! I continually get an error claiming my method "func" isn't declared in my class when it's right about the method making the reference.

Marc Liyanage Over a year ago

I added a small example. Obviously I don't know enough about your use case but that's how you could do it.

Mark Löwe Over a year ago

TY. So for some reason I keep getting an ^ error on the @selector for this line telling me my method isn't declared. Is this something to do with the instance of my class? The class is merely referencing itself. Its a UIViewController that's trying to handle it's own buttons. So your first line when entered into my code fails wth "Undeclared selector 'test:'". :(

Marc Liyanage Over a year ago

Does one of your classes implement a method named "test:"?

Mark Löwe Over a year ago

I finally got it to work! Thank you very much. I had a typo that was killing me. ARG. You're the best.

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PaReeOhNos · Accepted Answer · 2013-12-25 01:58:25Z

1

You should be able to use SEL parameters? I know I have done before. ARC might complain and give you a warning, but it won't fail to compile. It's simply a warning because it can't quite figure out what to do memory wise.

If you really can't get that to work though, another alternative would be to use a block, so you might call your method like

[objectA performMethodWithParam:paramA paramb:paramB completion:^{ ... do somethhing ... }];

Then in that method you can just call

completion();

Instead of actually calling a method.

Another alternative would be to use the delegate pattern. Create a @protocol defining a method such as classADidFinish then make class B implement that method. Then set the instance of classB as the delegate for your classA instance, and have it call that method when it's done.

Both of these approaches will stop ARC moaning at you.

But as I said, using SEL params should work fine. There is a way you can even get the compiler to stop showing you the warnings but it's a little ugly.

answered Dec 25, 2013 at 1:58

PaReeOhNos

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1 Comment

Mark Löwe Over a year ago

I looked into using blocks, but it's so friggin messy. If a block can't simply reference a cleanly written method in my class, .i.e, ^methodName, I can't see myself using it until absolutely necessary.

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