There are a few gotchas in Swift's variable didSet logic. At a high level the didSet() function is called whenever a variable value is set.

private var colorPreference: UIColor = UIColor.red {
  didSet() {
    // logic
  }
}

The main catch is that this does not apply to initializing values. The first case is when the variable is initialized with a value.

private var colorPreference: UIColor = UIColor.red

The second is when the variable is set in the initialized func.

init() {
  colorPreference = UIColor.purple
}

This caught be up a few too many times so I needed to write it out