如何获得屏幕上 UIview 的百分比

Question

我正在构建一个 IOS 应用程序，当您第一次开始使用该应用程序时，它会以 UIPageViewController 开始。我一直在寻找一种方法来淡入 imageView，具体取决于用户滑动到下一个屏幕的距离。即：当他们刷过一半屏幕时alpha = 0.5，当屏幕完全刷完时alpha = 1.0？

我尝试过使用“animateWithDuration”，但这似乎要等到视图完全显示在屏幕上，所以它显示出延迟淡入。我考虑过使用gestureRecognizers，但pageViewController 已经有了它们，因此我必须覆盖它们。

Answer 1

您还可以简单地利用 UIScrollView API 来帮助您实现 UIView 查看/滚动的百分比。这里有一个想法。

随着 UIScrollView 对象被拖动并正确连接到 Storyboard 中的相应场景上，或者，在代码中正确实例化，继续符合您当前相应的 ViewController 类，您想要计算屏幕查看到 UIScrollViewDelegate 的百分比 协议。

例如class YourCustomViewController: UIViewController, UIScrollViewDelegate {}

然后，在YourCustomViewController 类的某个地方，您可以实现以下代码。这是实现屏幕垂直滚动百分比的代码：

func computePercentage(in scrollView: UIScrollView) {
    let height = scrollView.contentSize.height
    let viewed = scrollView.contentOffset.y + scrollView.bounds.size.height
    let pagePercentScrolled = viewed / height * 100.0
   // print("Percent Scrolled: \(pagePercentScrolled)")

    if pagePercentScrolled >= 50.0 && pagePercentScrolled < 100.0 {
        //Do something.
        //Example, set imageView alpha to half
        yourCustomImageView.alpha = 0.5
    } else if pagePercentScrolled == 100 {
        //Do something.
        //Example, fade in/show full imageView
        yourCustomImageView.alpha = 1.0
    }
}

同样，您可以稍微改变上面的内容来计算屏幕水平滚动的百分比：

    func computePercentage(in scrollView: UIScrollView) {
    let width = scrollView.contentSize.width
    let viewed = scrollView.bounds.size.width - scrollView.contentOffset.x
    let pagePercentScrolled = viewed / width * 100.0
    //print("Percent Scrolled: \(pagePercentScrolled)")

    if pagePercentScrolled >= 50.0 && pagePercentScrolled < 100.0 {
        //Do something.
        //Example, set imageView alpha to half
        yourCustomImageView.alpha = 0.5
    } else if pagePercentScrolled == 100 {
        //Do something.
        //Example, fade in/show full imageView
        yourCustomImageView.alpha = 1.0
    }
}

当然，您可以简单地将上述两种方法合二为一。但为了简单和可读性，我将它们分开。 然后在YourCustomViewController 上实现 UIScrollViewDelegate 方法，如下所示：

    // MARK: Scroll View Delegate
func scrollViewDidScroll(_ scrollView: UIScrollView) {
    computePercentage(in: scrollView)
}

func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
    computePercentage(in: scrollView)
}

func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView) {
    computePercentage(in: scrollView)
}

func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
    computePercentage(in: scrollView)
}

Answer 2

在视图中添加平移手势以了解在屏幕上滑动了多少。定义一个变量说CGFloat xpostion;

- (void)panRecognized:(UIPanGestureRecognizer *)gesture
{
  CGPoint translate = [gesture translationInView:self.view];

  if (gesture.state == UIGestureRecognizerStateBegan)
  {
     xpostion = translate.x;
  }
  else if (gesture.state == UIGestureRecognizerStateEnded)
  {
     CGFloat totalXSwipe = translate.x - xpostion;

    CGFloat width = [UIScreen mainScreen].bounds.size.width;

    CGFloat percentage = fabs(totalXSwipe)/width;
    NSLog(@"Swipe percentage %f",percentage);

    //Can use in your requirement to set as alpha value as it will be always less or equal to zero
  }
}

Answer 3

这是 Paresh 的 Objective-C 代码转换成 Swift：

var xpostion: CGFloat = 0.0
func panRecognized(_ gesture: UIPanGestureRecognizer?) {
    guard let translate: CGPoint? = gesture?.translation(in: view) else { return }

    if gesture?.state == .began {
        xpostion = (translate?.x)!
    } else if gesture?.state == .ended {
        let totalXSwipe: CGFloat = (translate?.x ?? 0.0) - xpostion

        let width: CGFloat = UIScreen.main.bounds.size.width

        let percentage = CGFloat(fabs(Float(totalXSwipe))) / width
        print("Swipe percentage \(percentage)")

        //Can use in your requirement to set as alpha value as it will be always less or equal to zero
    }
}

编码愉快！