Mysterious UIColor behaviour with UIView and in General in Swift

Question

Recently I came across an interesting(may be naive) problem regarding UIColor in Swift..

import UIKit

let view = UIView(frame: CGRect(x: 0,
                                y: 0,
                                width: 50,
                                height: 50))
view.backgroundColor = .systemBlue

let a = UIColor.systemBlue
switch a {
case .red:
    print("red")
case .systemBlue:
    print("blue")
default:
    print("unknown")
}

Following code prints "blue" on playground perfectly fine, but changing

let a = UIColor.systemBlue
to
let a = view.backgroundColor ?? .red

prints "unknown" in playground, May someone help what is happening here? I could not resolve it.. Is it something related to value type or reference type at some point?? Please help!!

Answer 1

Printing the two values gives you the explanation:

print(UIColor.systemBlue)

<UIDynamicSystemColor: 0x600000b47880;
  name = systemBlueColor
>

print(view.backgroundColor!)

<UIDynamicModifiedColor: 0x60000058bed0;
  contrast = normal,
  baseColor = <UIDynamicSystemColor: 0x600000b47880;
    name = systemBlueColor
  >
>

When setting the backgroundColor property, UIKit wraps the color in a private class UIDynamicModifiedColor.

If you compare the resolved colors using the view's traits, you'll get true:

UIColor.systemBlue.resolvedColor(with: view.traitCollection) ==
  view.backgroundColor!.resolvedColor(with: view.traitCollection)

The resolved color is an absolute color:

print(UIColor.systemBlue.resolvedColor(with: view.traitCollection))

UIExtendedSRGBColorSpace 0 0.478431 1 1

A color such as UIColor.systemBlue is a dynamic color that might result in different colors, depending on the view's traits, which include factors such as high-contrast mode and dark/light mode.

From the docs of UIColor.systemBlue:

A blue color that automatically adapts to the current trait environment.