Swift: Configuring a Constant Using Shorthand Argument Names

Posted on May 25th, 2016

There is a tweet going around on initializing constants in Swift using positional references:

Just learned from .@ericasadun's Github that I can initialize a constant using positional references! 🙌 #swiftlang pic.twitter.com/KzhXhPliWV

— Nicholas Sakaimbo (@nick_skmbo) May 24, 2016

The original code is here.

Since it is an ephemeral tweet, and I was confused when it didn’t work as I expected on first glance, I wanted to write a more permanent blog post on the topic here.

The Problem

It’s a common pattern in Swift (and a really nice one!) to configure constants right when they are initialized in a closure vs later on in a viewDidLoad or another such method:

let purpleView: UIView = {
    // have to initialize a view here
    // is "view" the correct name for it?
    let view = UIView()
    view.backgroundColor = .purpleColor()
    return view
}()

let purpleView: UIView = {

// have to initialize a view here

// is "view" the correct name for it?

let view = UIView()

view.backgroundColor = .purpleColor()

return view

}()

I’ve always found it kind of awkward to name another UIView in the closure. Now there is a “purpleView” and a “view”. Should “view” actually be named “purpleView” also? I haven’t figured out a good solution for the naming problem here.

The Solution

So I was super excited to see the tweet that uses $0 instead of bothering to name the variable! I immediately tried it like this:

// spoiler: this does not work...
let yellowView: UIView = {
    $0.backgroundColor = .yellowColor()
    return $0
}()

// spoiler: this does not work...

let yellowView: UIView = {

$0.backgroundColor = .yellowColor()

return $0

}()

But it didn’t work… Upon closer inspection of @ericasadun’s code, I learned that I had to pass in the initialized UIView when the closure is executed:

let yellowView: UIView = {
    $0.backgroundColor = .yellowColor()
    return $0
 // make sure to pass in UIView() here!
}(UIView())

let yellowView: UIView = {

$0.backgroundColor = .yellowColor()

return $0

// make sure to pass in UIView() here!

}(UIView())

This of course makes a lot of sense!

Conclusion

I do really like the idea of using $0 here instead of explicitly naming the thing, but I’m upset that it didn’t come naturally to me. However, now that I know I have to pass in an initialized UIView() in there, it does logically make sense. I think I’ll use this in the future. Getting to use $0 here is just too beautiful not to!

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Update

As @khanlou pointed out, there is a super cool library called Then to make this even more readable and better:

let label = UILabel().then {
    $0.textAlignment = .Center
    $0.textColor = .blackColor()
    $0.text = "Hello, World!"
}

let label = UILabel().then {

$0.textAlignment = .Center

$0.textColor = .blackColor()

$0.text = "Hello, World!"

}

While I’m not a fan of adding this library as a dependency to my project, @khanlou pointed out once again that it’s only 15 lines of code. So I’ll definitely consider copying this over manually into my project!

Make sure to also check out @ericasadun’s nice solution!

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ioc

You can also check out my solution here: https://github.com/illusionofchaos/swift-package-bullet
Kersten Broich

I like the approach a lot. However if I refer to self within the closure outside of viewDidLoad in the object’s declaration scope I get a weird compiler error “Cannot convert value of type ‘NSObject -> () -> MyViewController’ to expected argument type ‘AnyObject?'”

Also see this gist: https://gist.github.com/Broich/702d3d04d30de420e7c09fef9e664975

Looks like self is not completely initialized within the closure? I’m not sure…
- Roman Roibu
  
  You are right, self is not fully initialised at the time you call the closure. You need to use lazy var instead of let for that to work.
  
  I use an approach similar to the one in the mentioned Then library, but with a slight modification to accommodate lazy instantiation.
  
  See: https://gist.github.com/romanroibu/8325edc8d3887136842aa0825b5c7ee8
  
  This lets you start with a let property, and then change it to a lazy var if, for example, at some point you need to set the delegate.
  - Kersten Broich
    
    Thanks Roman, that looks fancy 🙂
Yariv

Uber cool!
elio d’antoni

Apparently “Then” stopped working on Xcode 7.3.1 (only on projects / Playgrounds are still ok). Is anyone experiencing code completion issue when using this syntax?
Can Sürmeli

Really nice thinking!

Although I’m having trouble creating a CLBeaconRegion:

private let beaconRegion: CLBeaconRegion = {
let uuidString = “”
let uuid = NSUUID(UUIDString: uuidString)
return CLBeaconRegion(proximityUUID: uuid!, identifier: “Estimotes”)
}(CLBeaconRegion())

Gives the following error “argument passed to call takes no arguments” for the last line. How exactly should I configure it? Thanks.
- Derrick Ho
  
  Basically you aren’t using your $0 value. And it is complaining as a result. To fix you issue just remove the first argument you have passed to your closure.
  
  private let beaconRegion: CLBeaconRegion = {
  let uuidString = “”
  let uuid = NSUUID(UUIDString: uuidString)
  return CLBeaconRegion(proximityUUID: uuid!, identifier: “Estimotes”)
  }()

Cap’n Slipp

I’ve been using single-letter variable names for let-initialization-closures, E.G.:


let mainNode:SCNNode = {
    let n = SCNNode()
    n.pivot = SCNMatrix4MakeTranslation(0.0, 0.0, 1.5)
    n.position = SCNVector3(0, 0, 0)
    return n
}()

let mainNode:SCNNode = {

let n = SCNNode()

n.pivot = SCNMatrix4MakeTranslation(0.0, 0.0, 1.5)

n.position = SCNVector3(0, 0, 0)

return n

}()

I’m normally vehemently opposed to single-letter identifiers, but that’s because I often see them used in public APIs as shorthands for mathematical/technical terms that would make sense to someone who knows that math/tech, but aren’t so helpful for someone who just wants to use the function/method without having to know how it works.

However, here it’s used in a short, private scope, and all usages of the single-letter identifier are within a few lines of each other, so I give this a pass. That said, the $0 trick is much cooler, so I think I’ll use that from now on!

Still, there are situations when single-letter variable names come in handy, such as this usage of nested let-initialization-closures:


let scnCameraNode:SCNNode = {
    $0.camera = {(c) in
        c.automaticallyAdjustsZRange = true
        c.yFov = 35
        return c
    }(SCNCamera())
    $0.position = SCNVector3(-40, -40, 20)
    return $0
}(SCNNode())

let scnCameraNode:SCNNode = {

$0.camera = {(c) in

c.automaticallyAdjustsZRange = true

c.yFov = 35

return c

}(SCNCamera())

$0.position = SCNVector3(-40, -40, 20)

return $0

}(SCNNode())

muescha

i was looking a longtime for a swift equivalent of JAVA static/instance initializer – here i found the solution – thanks 🙂

muescha

if someone don’t like the $0 then this with this is also possible:


        let label: UILabel = { this in
            this.textAlignment = .center
            this.textColor = .black
            this.text = "Hello, My World!"
            return this
        }(UILabel())

let label: UILabel = { this in

this.textAlignment = .center

this.textColor = .black

this.text = "Hello, My World!"

return this

}(UILabel())

instead of


        let label: UILabel = {
            $0.textAlignment = .center
            $0.textColor = .black
            $0.text = "Hello, World!"
            return $0
        }(UILabel())

let label: UILabel = {

$0.textAlignment = .center

$0.textColor = .black

$0.text = "Hello, World!"

return $0

}(UILabel())