readableContentGuide is a mess.

ReadableContentGuideHelperView.swift

class ReadableContentGuideHelperView: UIView {
    var onChange: ((CGRect)->())?
    
    static func addToView(view: UIView) -> ReadableContentGuideHelperView {
        let helperView = ReadableContentGuideHelperView()
        view.addSubview(helperView)
        
        if #available(iOS 9.0, *) {
            helperView.setupAnchors(view)
        } else {
            helperView.setupFrame(view)
        }
        
        return helperView
    }
    
    @available(iOS 9.0, *)
    private func setupAnchors(view: UIView) {
        translatesAutoresizingMaskIntoConstraints = false
        leadingAnchor.constraintEqualToAnchor(view.readableContentGuide.leadingAnchor).active = true
        trailingAnchor.constraintEqualToAnchor(view.readableContentGuide.trailingAnchor).active = true
        topAnchor.constraintEqualToAnchor(view.readableContentGuide.topAnchor).active = true
        bottomAnchor.constraintEqualToAnchor(view.readableContentGuide.bottomAnchor).active = true
    }
    
    private func setupFrame(view: UIView) {
        translatesAutoresizingMaskIntoConstraints = true
        autoresizingMask = [.FlexibleWidth, .FlexibleHeight]
        frame = view.bounds
    }
    
    init() {
        readableContentFrame = CGRect.zero
        super.init(frame: CGRect.zero)
        hidden = true
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError()
    }
    
    private (set) var readableContentFrame: CGRect
    
    override func layoutSubviews() {
        super.layoutSubviews()
        guard frame != readableContentFrame else { return }
        readableContentFrame = frame
        log.debug("ReadableContentGuideHelperView new frame: \(frame)")
        onChange?(frame)
    }
}

Maybe this fugly thing works with KVO?