为什么这个 didSet 属性在 Swift 中不打印任何输出

Question

我已经正确设置了一个UIPageViewController。

我想检查每个视图的 itemIndex 并相应地显示内容。代码如下所示：

var itemIndex: Int = 0 {
    didSet {
        if itemIndex == 1 {
            println("Character is Lela!")
        }
        if itemIndex == 0 {
            println("Character is John!")
    }  
  } 
}

我尝试了什么

1. 我运行应用程序
2. 控制台输出："角色是约翰！
3. 我向前滑动（向右）
4. 控制台输出："角色是莱拉！
5. 我向后滑动（向左）
6. 没有输出！
7. 我向前滑动
8. 没有输出！再！

这是由于什么原因？以下是完整代码：

import UIKit
var currentIndex: Int = 0
var nextIndex: Int = 0
class ProView: UIViewController, UIPageViewControllerDataSource, UIPageViewControllerDelegate {
var pageViewController: UIPageViewController?
let characterImages = ["character1", "character2", "character1", "character2", "character1", "character2", "character1", "character2"]
override func viewDidLoad() {
    super.viewDidLoad()
    createPageViewController()
    setupPageControl()
    character = 1
}
override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
}
// Forward, check if this IS NOT the last controller
func pageViewController(pageViewController: UIPageViewController,
    viewControllerAfterViewController ProView: UIViewController) -> UIViewController? {
        let itemController = ProView as PageItemController
        // Check if there is another view
        if itemController.itemIndex+1 < characterImages.count {
            return getItemController(itemController.itemIndex+1)
        }
        return nil
}

// Check if this IS NOT the first controller
func pageViewController(pageViewController: UIPageViewController,
    viewControllerBeforeViewController ProView: UIViewController) -> UIViewController? {
    let itemController = ProView as PageItemController
        if itemController.itemIndex < 0 {
            return getItemController(itemController.itemIndex-1)
        }
        return nil
}
private func getItemController(itemIndex: Int) -> PageItemController? {
    if itemIndex < characterImages.count {
        let pageItemController = self.storyboard!.instantiateViewControllerWithIdentifier("ItemController") as PageItemController
        pageItemController.itemIndex = itemIndex
        pageItemController.imageName = characterImages[itemIndex]
        return pageItemController
}
    return nil
}
func createPageViewController() {
    let pageController = self.storyboard!.instantiateViewControllerWithIdentifier("PageController") as UIPageViewController
    pageController.dataSource = self
    pageController.delegate = self
    if characterImages.count > 0 {
        let firstController = getItemController(0)!
        let startingViewControllers: NSArray = [firstController]
        pageController.setViewControllers(startingViewControllers, direction: UIPageViewControllerNavigationDirection.Forward, animated: false, completion: nil)
    }
    pageViewController = pageController
    addChildViewController(pageViewController!)
    self.view.addSubview(pageViewController!.view)
    pageViewController?.didMoveToParentViewController(self)
}
func setupPageControl() {
    let appearance = UIPageControl.appearance()
    appearance.pageIndicatorTintColor = UIColor.grayColor()
    appearance.currentPageIndicatorTintColor = UIColor.whiteColor()
}
func presentationCountForPageViewController(pageViewController: UIPageViewController) -> Int {
    return characterImages.count
}
func presentationIndexForPageViewController(pageViewController: UIPageViewController) -> Int {
return 0
}
// BETA
func pageViewController(PageItemController: UIPageViewController,
    didFinishAnimating finished: Bool,
    previousViewControllers pageViewController: [AnyObject],
    transitionCompleted completed: Bool)
{
    if (!completed)
    {
        // You do nothing because whatever page you thought
        // the book was on before the gesture started is still the correct page
        return;
    }
    // This is where you would know the page number changed and handle it appropriately
    }
}
class PageItemController: UIViewController {
@IBOutlet weak var imageCharacterChoose: UIImageView!
var itemIndex: Int = 0 {
    didSet {
        if itemIndex == 1 {
            println("Character is Lela!")
        }
        if itemIndex == 0 {
            println("Character is John!")
    }
    }}
var imageName: String = "" {
    didSet {
        if let imageView = imageCharacterChoose {imageCharacterChoose.image = UIImage(named: imageName)
        }
    }
}
override func viewDidLoad() {
    super.viewDidLoad()
    imageCharacterChoose!.image = UIImage(named: imageName)
}
override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
}
}

Answer 1

didSet 和 willSet 只有在您更改相应变量的值时才会被调用，因此在您的情况下，您应该在代码中的某个地方执行 itemIndex = 1 ，或其他一些值来触发函数。