可扩展视图错误，因为它未注册为观察者

Question

我想要一个可扩展的表视图，但是当我尝试展开表格视图时，我有这个错误：

  

2015-10-04 19：42：25.030 SideBarMenu [7692：304078] ***由于未捕获的异常'NSRangeException'而终止应用程序，原因是：'无法删除关键路径“frame”的观察者，因为它是没有注册为观察员。'

这是完整的源代码：https://github.com/MyHappyHog/ProjectPrototype/tree/master/src/03.iOS/UI

这是源代码的一部分：

SettingViewController.swift

import UIKit

let cellID = "cell"

class SettingViewController: UITableViewController {
    var selectedIndexPath : NSIndexPath?

    override func numberOfSectionsInTableView(tableView: UITableView) -> Int {
        return 1
    }

    override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return 2
    }

    override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCellWithIdentifier(cellID, forIndexPath: indexPath)
        cell.textLabel!.text = "Test Title"
        return cell
    }

    override func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {
        let previousIndexPath = selectedIndexPath
        if indexPath == selectedIndexPath {
            selectedIndexPath = nil
        }
        else {
            selectedIndexPath = indexPath
        }

        var indexPaths : Array<NSIndexPath> = []
        if let previous = previousIndexPath {
            indexPaths += [previous]
        }
        if let current = selectedIndexPath {
            indexPaths += [current]
        }
        if indexPaths.count > 0 {
            tableView.reloadRowsAtIndexPaths(indexPaths, withRowAnimation: UITableViewRowAnimation.Automatic )
        }
    }

    override func tableView(tableView: UITableView, willDisplayCell cell: UITableViewCell, forRowAtIndexPath indexPath: NSIndexPath) {
        (cell as! SettingViewCell).watchFrameChanges()
    }

    override func tableView(tableView: UITableView, didEndDisplayingCell cell: UITableViewCell, forRowAtIndexPath indexPath: NSIndexPath) {
        (cell as! SettingViewCell).ignoreFrameChanges()
    }

    override func tableView(tableView: UITableView, heightForRowAtIndexPath indexPath: NSIndexPath) -> CGFloat {
        if indexPath == selectedIndexPath {
            return SettingViewCell.expandedHeight
        }
        else {
            return SettingViewCell.defaultHeight
        }
    }
}

SettingViewCell.swift

import UIKit

class SettingViewCell : UITableViewCell {

    @IBOutlet weak var titleLabel: UILabel!
    @IBOutlet weak var datePicker: UIDatePicker!
    class var expandedHeight: CGFloat { get {return 200 } }
    class var defaultHeight: CGFloat { get {return 44 } }

    func checkHeight() {
        datePicker.hidden = (frame.size.height < SettingViewCell.expandedHeight)
    }

    func watchFrameChanges() {
        addObserver(self, forKeyPath: "frame", options: .New, context: nil)
        checkHeight()
    }

    func ignoreFrameChanges() {

        removeObserver(self, forKeyPath: "frame")
    }

    override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
        if keyPath == "frame" {
            checkHeight()
        }
    }
}