Snippets of code to prepare your app for dark mode (https://developer.apple.com/videos/play/wwdc2019/214)

DynamicCGColor.swift

// Only contents of UIKit can use DynamicColors. So if you need the dynamic color behavior 
// on CALayer, CGColor etc you need to use one of these 3 options:

// Option 1
let resolvedColor = UIColor.label.resolvedColor(with: traitCollection)
layer.borderColor = resolvedColor.cgColor

// Option 2
traitCollection.perfomAsCurrent {
  layer.borderColor = UIColor.label.cgColor
}

// Option 3
let savedTraitCollection = UITraitCollection.current

UITraitCollection.current = traitCollection
layer.borderColor = UIColor.label.cgColor

UITraitCollection.current = savedTraitCollection

DynamicColor.swift

// Defines a dynamic color that responds to traitCollection.userInterfaceStyle

let dynamicColor = UIColor { (traitCollection: UITraitCollection) -> UIColor in 
    if traitCollection.userInterfaceStyle == .dark {
        return .black
    } else {
        return .white
    }
}

DynamicImages.swift

// Setting dynamic images manually

let image = UIImage(named: "aImage")
let asset = image?.imageAsset
let resolvedImage = asset?.image(with: traitCollection)

OverrideUserInterfaceStyle.swift

// on iOS 13, UIKit now provides a new property that can be used to change the user interface style
// this way, even if the user is using iOS 13 on dark mode we can have screens on light mode or the opposite 

class UIViewController {
  var overrideUserInterfaceStyle: UIUserInterfaceStyle
}

class UIView {
  var overrideUserInterfaceStyle: UIUserInterfaceStyle
}