Has anyone written an `apply` method on iOS that’s...

# multiplatform

[vanniktech]

Has anyone written an apply method on iOS that’s behaving similar to Kotlin’s apply?

[Sam]

As in a Swift extension?

[vanniktech]

Yeah

[Sam]

I’ve never been able to figure out a good way to do it. You can’t add extensions to Any or AnyObject. You can do something like this:

protocol Applier {
    associatedtype Applying
    func apply(_ fn: (Applying) -> Void) -> Applying
}

extension Applier {
    func apply(_ fn: (Applying) -> Void) -> Applying{
        fn(self as! Self.Applying)
        return self as! Self.Applying
    }
}

class Foo {
    var bar = "baz"
}

extension Foo: Applier {
    typealias Applying = Foo
}


let foo = Foo().apply { `self` in
    self.bar = "baz2"
}

print(foo.bar)

class Foo2: Foo {
    var baz = "foo"
}

let foo2 = Foo2().apply { `self` in
    self.bar = "baz2"
    //self.baz = "Does not work.  Self is of type Foo"
}

It isn’t very useful though as you have to declare the conformance of the type to the Applier protocol for each class you want to use it on. The other caveat is the subclass issue. If you make UIView conform to the protocol and try to use it on a subclass such as UILabel , the type of the argument passed into your apply function is UIView which is only minimally useful. Also there should be a way to do the above without the force casts. I’m just too tired to parse through all of the protocol associated type nuances right now.

[Sam]

I do have a decent version of let for Swift. It can only be used on optionals which is about 99% of when I use a let function anyway.

extension Optional {
    /// Function similar to Kotlin let that executes a block of code on an optional if the optional has a value.
    ///
    /// - Parameter execute: The block to be executed.
    func `let`(_ execute: (Wrapped) -> Void) {
        if let wrapped = self {
            execute(wrapped)
        }
    }
}

[vanniktech]

Argh. That apply looks really distasteful. That let function should come handy though, thanks

[Sam]

Yeah, I don’t use it nor recommend it. I did ask on Stackoverflow how to improve it. Maybe someone there has an idea. https://stackoverflow.com/questions/61259143/implementing-kotlin-apply-function-in-swift

[Sam]

So my mistake was in specifying the function in the protocol. Have a look at this gist that is linked in one of the SO articles from my question. https://gist.github.com/kakajika/0bb3fd14f4afd5e5c2ec You still have to add the protocol conformance for every swift object you would want to add apply to. However adding it to NSObject will make it apply to every object in UIKit and any other Swift object that has the @objc attribute.

[vanniktech]

That’s neat