如何使用共享的init方法初始化let变量？

Question

I want to setup a variable using let so that its not optional, won't change over time and doesn't require messy unwrapping to access each time. I know I can legally set it, but only inside one of the init methods. The problem is that Swift requires you to override the init(coder aDecoder: NSCoder) initializer. However I've never had an app that ever has that initializer called. Its always the init(frame: CGRect) initializer that is called. So if I didn't have to override init(coder aDecoder: NSCoder) then I'd just do the self.imageView = UIImageView.new() in init(frame: CGRect) and I'd be done, but the compiler complains that its not set in the other init as well. I tried making a sharedInit() method that is called from both inits, but the compiler won't let me set the imageView from there since it's read only outside of the init methods. How are you supposed to accomplish this without doing all of your init twice directly in the init methods?

class SuperCoolView : UIView {

    let imageView: UIImageView

    required init(coder aDecoder: NSCoder) {
        sharedInit()
        super.init(coder: aDecoder)
    }

    override init(frame: CGRect) {
        sharedInit()
        super.init(frame: frame)
    }

    func sharedInit() {
        self.imageView = UIImageView.new()
        // Other similar shared init
    }

}

Answer 1

Just use a combination of lazy initialization and private 'set' to get:

class SuperCoolView : UIView {
  public private(set) lazy var imageView: UIImageView = {
    // In practice, do more here.  Access state in this instance.
    // Or, include more computation.
    UIImageView.new()
  }()

  // inits w/o setting imageView
}

The 'private set' makes this externally identical to a let ; the lazy lets you initialize the value based on the other state in the SuperCoolView instance.

Note that if the initialization of imageView, and others, doesn't depend on state in the instance, then you can avoid the lazy and just do the initialization from the closure.

Answer 2

I found this answer and liked it's approach. It fixed the problem, and is much cleaner IMHO. By implementing init(coder aDecoder: NSCoder) normally we would be keeping up the appearance that this view can be initialized from a nib/storyboard when it can't.

class SuperCoolView : UIView {

let imageView: UIImageView

required init(coder aDecoder: NSCoder) {
    fatalError("NSCoding not supported")
}

override init(frame: CGRect) {
    self.imageView = UIImageView.new()
    // Other stuff
    super.init(frame: frame)
}

}