We've looked at protocols in Swift in Bite #232, but we haven't really seen a ton of "real world" examples. Today we'll check out a library called CostumeKit. It's a set of base protocols that can assist us in visually styling or "themeing" our apps. Along the way we'll use some protocols "for real". Let's jump in.

Author's Note: Full disclosure, CostumeKit is written by me. I'm a huge fan of conventions, also it's a nice basic example of Protocols. I use it in all my apps.

To try this out, we'll be working on an imaginary Little Bites of Cocoa app.

We'll start by adding CostumeKit to our project with Carthage:

github "jakemarsh/CostumeKit"

Next, we'll start implementing our types. First up is the colors we'd like to use.

We'll make an enum and paste in all the colors we want to use, giving each a friendly name:

public enum LittleBitesColors : Color, ColorPalette {
  case beige = "FEFAF1"
  case lightBrown = "AB9372"
  case brown = "8C5637"
  case lightGray = "ECECEC"
  case darkGray = "6A6A6A"
  case sponsorRowGray = "F5F5F5"
}

Sharped-eyed-readers will actually recognize this technique from Bite #255 on Creating a ColorConvertible Protocol.

Moving along, LBOC uses the Source Sans Pro font. Let's implement CostumeKit's Font protocol for this. We'll add the font files to our app then implement the protocol:

public struct LittleBitesFont : Font {
  public init(size: FontSize = .textStyle(.body)) {
    self.size = size
  }

  // Font

  public var size: FontSize

  // FontConvertible

  public var FontValue: UIFont {
    return UIFont(name: "SourceSansPro", size: pointSize)!
  }
}

We're almost there, next we need to create the actual Costume our app will wear. For this we'll implement one more protocol. This protocol has no requirements. It serves as more of a convention really.

open class LittleBitesCostume : Costume {
  let spacing = CGFloat(8)

  public func wearRootBackground(_ view: UIView) {
    view.backgroundColor = Color.white.colorValue
  }

  public func wearHeadline(_ label: UILabel) {
    label.font = LittleBitesFont(size: .textStyle(.title1)).fontValue
    label.textColor = contentTextColor().colorValue
  }

  public func contentTextColor() -> Color {
    return LittleBitesColors.darkGray
  }

  public var name: String { return "Default" }
  public var description: String { return "The default costume." }

  public init() { }
}

For good measure, we'll define a night-mode costume as well:

open class LittleBitesNightCostume : LittleBitesCostume {
  public override func contentTextColor() -> Color {
    return Color.white
  }

  override public var name: String { return "Night Mode" }
  override public var description: String { return "For reading in the dark." }
}

That's where CostumeKit stops. We get a super-tiny bit of functionality "for free" (fonts, color parsing), but the idea is that we're merely following a set of conventions to help guide us through writing this code.

Let's finish up by looking at how we might actually use costumes in our app. Everything from this point on would live inside our app's code (and isn't part of CostumeKit).

We'll make a Wardrobe type and static instance to manage all our costumes:

import CostumeKit

public class Wardrobe {
  private(set) var current: LittleBitesCostume

  init(initialCostume: LittleBitesCostume) {
    current = initialCostume
  }

  public func change(costume: LittleBitesCostume, animated: Bool = true) {
    // TODO: In the future, we'll animate this change using UIView transitions and RxSwift.

    current = costume
  }
}

public let wardrobe = Wardrobe(initialCostume: LittleBitesCostume())
public var costume: LittleBitesCostume { return wardrobe.current }

Now in our regular UIKit code, we can use our current costume:

let headlineLabel = UILabel()
headlineLabel.text = "Hello World".
costume.wearHeadline(headlineLabel)

Learn more about CostumeKit at git.io/costumekit