视差视图滚动（雅虎天气之类）

Question

这不是严格意义上的编程问题，而是更多“如何完成这个”问题。

我很好奇（并且正在开发可能需要此功能的应用程序）左右视差滚动是如何实现的。要确切了解我的意思，请查看 Yahoo Weather 应用程序（它是免费的 - 不用担心）。

他们是只使用一个视图控制器，还是为那里显示的每个视图使用一个单独的控制器？
实现这一点的最简单方法是什么？我找到了this topic here，这有点解释了，但是他们什么时候从他们的服务器获取信息？是在视图更改时还是在应用启动时？

任何关于如何实现这种滚动的信息将不胜感激。

Answer 1

其实很简单：

• 子类 UIScrollView
• 添加一个 UIView *parallaxView;作为@property
• 添加一个 CGFloat parallaxFactor 作为@property
• 覆盖 layoutSubviews
• 调用super，然后使用self.scrollOffset*parallaxFactor定位视差视图

就是这样！

我自己制作了一个多功能 UIScrollView 子类，使用起来非常简单，非常适合这种情况！在 GitHub 上获取：https://github.com/LeonardPauli/LPParallaxScrollView

祝你好运！

Answer 2

我实现了一个基于标准 UIPageViewController 子类的小型原型，以及用于视差效果的自动布局约束。 如果您想看一下，它的文档非常完整：TestParallax

简而言之，视差的核心是在scrollViewDidScroll委托方法中完成的：

extension PageViewController: UIScrollViewDelegate {

    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        let screenWidth = scrollView.bounds.width
        /*
         In a UIPageViewController, the initial contentOffset.x is not 0, but equal to the screen width
         (so that the offset goes between (1 * screenWidth) and 0 when going to the previous view controller, 
         and from (1 * screenWidth) to (2 * screenWidth) when going to the next view controller).
         Also, it's reset to the screenWidth when the scroll to a previous or next view controller is complete.
         Therefore, we calculate a new 'horizontalOffset' starting at 0, and going:
         - negative from 0 to (-screenWidth/2) when scrolling to the next view controller,
         - and from 0 to (screenWidth/2) when scrolling to the previous view controller.
         */
        let horizontalOffset = (scrollView.contentOffset.x - screenWidth)/2

        // Special case: initial situation, or when the horizontalOffset is reset to 0 by the UIPageViewController.
        guard horizontalOffset != 0 else {
            previousPageController?.offsetBackgroundImage(by: screenWidth/2)
            currentPageController?.offsetBackgroundImage(by: 0)
            nextPageController?.offsetBackgroundImage(by: -screenWidth/2)
            return
        }

        // The background image of the current page controller should always be offset by the horizontalOffset (which may be positive or negative)
        guard let currentPageController = currentPageController else { return }
        currentPageController.offsetBackgroundImage(by: horizontalOffset)

        if horizontalOffset > 0 { // swiping left, to the next page controller

            // The background image of the next page controller starts with an initial offset of (-screenWidth/2), then we apply the (positive) horizontalOffset
            if let nextPageController = nextPageController {
                let nextOffset = -screenWidth/2 + horizontalOffset
                nextPageController.offsetBackgroundImage(by: nextOffset)
            }
        } else { // swiping right, to the previous page controller

            // The background image of the previous page controller starts with an initial offset of (+screenWidth/2), then we apply the (negative) horizontalOffset
            if let previousPageController = previousPageController {
                let previousOffset = screenWidth/2 + horizontalOffset
                previousPageController.offsetBackgroundImage(by: previousOffset)
            }
        }
    }
}

Answer 3

您不应该更改页面视图控制器的滚动视图的委托。它可能会破坏其正常行为。

相反，您可以：

1. 向页面视图控制器的视图添加平移手势：

   let panGesture = UIPanGestureRecognizer(target: self, action: #selector(panRecognized(gesture:)))
   view.addGestureRecognizer(panGesture)
   panGesture.delegate = self
   

2. 添加新函数以了解视图是如何滚动的。

   @objc func panRecognized(gesture: UIPanGestureRecognizer) {
       // Do whatever you need with the gesture.translation(in: view)
   }
   

3. 将您的 ViewController 声明为 UIGestureRecognizerDelegate。

4. 实现这个功能：

   func gestureRecognizer(_ gestureRecognizer: UIGestureRecognizer, shouldRecognizeSimultaneouslyWith otherGestureRecognizer: UIGestureRecognizer) -> Bool {
       return true
   }