func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {     // 得到当前应用的版本号     let infoDictionary = NSBundle.mainBundle().infoDictionary     let currentAppVersion = infoDictionary!["CFBundleShortVersionString"] as! String     // 取出之前保存的版本号     let userDefaults = NSUserDefaults.standardUserDefaults()     let appVersion = userDefaults.stringForKey("appVersion")     let storyboard = UIStoryboard(name: "Main", bundle: nil)     // 如果 appVersion 为 nil 说明是第一次启动；如果 appVersion 不等于 currentAppVersion 说明是更新了     if appVersion == nil || appVersion != currentAppVersion {         // 保存最新的版本号         userDefaults.setValue(currentAppVersion, forKey: "appVersion")         let guideViewController = storyboard.instantiateViewControllerWithIdentifier("GuideViewController") as! GuideViewController         self.window?.rootViewController = guideViewController     }     return true }

class GuideViewController: UIViewController {     @IBOutlet weak var pageControl: UIPageControl!     @IBOutlet weak var startButton: UIButton!     private var scrollView: UIScrollView!     private let numOfPages = 3     override func viewDidLoad() {         super.viewDidLoad()         let frame = self.view.bounds         scrollView = UIScrollView(frame: frame)         scrollView.pagingEnabled = true         scrollView.showsHorizontalScrollIndicator = false         scrollView.showsVerticalScrollIndicator = false         scrollView.scrollsToTop = false         scrollView.bounces = false         scrollView.contentOffset = CGPointZero         // 将 scrollView 的 contentSize 设为屏幕宽度的3倍(根据实际情况改变)         scrollView.contentSize = CGSize(width: frame.size.width * CGFloat(numOfPages), height: frame.size.height)         scrollView.delegate = self         for index  in 0..<numOfPages {             // 这里注意图片的命名             let imageView = UIImageView(image: UIImage(named: "GuideImage\(index + 1)"))             imageView.frame = CGRect(x: frame.size.width * CGFloat(index), y: 0, width: frame.size.width, height: frame.size.height)             scrollView.addSubview(imageView)         }         self.view.insertSubview(scrollView, atIndex: 0)         // 给开始按钮设置圆角         startButton.layer.cornerRadius = 15.0         // 隐藏开始按钮         startButton.alpha = 0.0     }     // 隐藏状态栏     override func prefersStatusBarHidden() -> Bool {         return true     } }

// MARK: - UIScrollViewDelegate extension GuideViewController: UIScrollViewDelegate {     func scrollViewDidScroll(scrollView: UIScrollView) {         let offset = scrollView.contentOffset         // 随着滑动改变pageControl的状态         pageControl.currentPage = Int(offset.x / view.bounds.width)         // 因为currentPage是从0开始，所以numOfPages减1         if pageControl.currentPage == numOfPages - 1 {             UIView.animateWithDuration(0.5) {                 self.startButton.alpha = 1.0             }         } else {             UIView.animateWithDuration(0.2) {                 self.startButton.alpha = 0.0             }         }     } }