Swift Set Scrollview Subviews Frame Location based off content offset

Question

问题：

我正在重新创建 iPhone App Switcher 页面，其中应用视图在屏幕左侧相互重叠。看起来每个应用视图框架的 x 位置是根据它在可见屏幕上的索引和位置设置的。

我在滚动视图中有一组视图。您将如何设置视图框架以复制 iPhone 应用切换页面？

尝试：

`func scrollViewDidScroll(_ scrollView: UIScrollView) {
    items.enumerated().forEach { (index, tabView) in
        let screenWidth = UIScreen.main.bounds.width
        
        // This returns a value between 0 and 1 depending on the location of the tab view within the visible screen
        let xOffset = scrollView.convert(CGPoint(x: tabView.frame.minX, y: 0), to: view).x
        let percentViewMovedOnVisibleScreen: CGFloat = xOffset / screenWidth
        
        // Spacing - NOT CORRECT
        let someSpacingAmount: CGFloat = 80
        tabView.frame.origin.x = CGFloat(index) * percentViewMovedOnVisibleScreen * someSpacingAmount
    }
}`

我认为这很接近，但它并不能完全让您了解 Apple 所拥有的。也许需要 didScroll 以外的其他东西才能让它感觉流畅？

GitHub 示例项目：https://github.com/Alexander-Frost/ViewContentOffset

Answer 1

这很可能（很可能，我会说）这不是在滚动视图中开始的。

考虑这个例子：

class TestSwitcherViewController: UIViewController {
    
    var leadConstraint: NSLayoutConstraint!
    
    override func viewDidLoad() {
        super.viewDidLoad()
    
        let colors: [UIColor] = [
            // light red
            UIColor(red: 1.0, green: 0.5, blue: 0.5, alpha: 1.0),
            // light green
            UIColor(red: 0.5, green: 1.0, blue: 0.5, alpha: 1.0),
            // light blue
            UIColor(red: 0.0, green: 0.5, blue: 1.0, alpha: 1.0),
            // light orange
            UIColor(red: 0.9, green: 0.7, blue: 0.5, alpha: 1.0),
            // yellow
            UIColor(red: 1.0, green: 1.0, blue: 0.0, alpha: 1.0),
        ]
        
        var prevView: UIView!
        
        // create a view for each color (a label with the view number as its text)
        // for each of the views
        //  if it's the first one,
        //      constrain leading to view leading
        //  else
        //      constrain leading to previous view leading, constant 1.0, multiplier 2.0

        var i = 1
        colors.forEach { c in
            let v = UILabel()
            v.backgroundColor = c
            v.text = "\(i)"
            v.textAlignment = .center
            v.layer.cornerRadius = 20
            v.layer.masksToBounds = true
            v.translatesAutoresizingMaskIntoConstraints = false
            view.addSubview(v)
            v.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
            v.widthAnchor.constraint(equalToConstant: 200.0).isActive = true
            v.heightAnchor.constraint(equalToConstant: 400.0).isActive = true
            if i == 1 {
                leadConstraint = v.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 0.0)
                leadConstraint.isActive = true
            } else {
                NSLayoutConstraint(item: v, attribute: .leading, relatedBy: .equal, toItem: prevView, attribute: .leading, multiplier: 2.0, constant: 1.0).isActive = true
            }
            prevView = v
            i += 1
        }
        
        // add a pan gesture recognizer to the view
        let pan = UIPanGestureRecognizer(target: self, action: #selector(self.didPan(_:)))
        view.addGestureRecognizer(pan)
        
    }
    
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        updateScales()
    }
    
    @objc func didPan(_ gesture: UIPanGestureRecognizer) -> Void {
        
        let translation = gesture.translation(in: view)
        
        // increment or decrement the leading anchor constant
        let tmpX = leadConstraint.constant + (translation.x * 0.25)

        // don't let it go past either side
        leadConstraint.constant = min(max(tmpX, 0.0), view.frame.width - 200.0)
        
        gesture.setTranslation(.zero, in: view)

        updateScales()
        
    }
    
    func updateScales() -> Void {
        view.subviews.forEach { v in
            // percentage of distance from leading edge of label to 1/5th width of view
            let pct = min(v.frame.origin.x / (view.frame.width * 0.2), 1.0)
            let scale = 0.8 + 0.2 * pct
            v.transform = .identity
            v.transform = CGAffineTransform(scaleX: scale, y: scale)
        }
    }
    
}

它创建 5 个彩色视图（编号标签），并使用具有常量和乘数的前导锚将它们相互约束。它向视图添加了平移手势，因此当您向左或向右平移时，它会修改“底部”视图的前导约束的常量以将其向左/向右移动。这反过来又会移动其他视图...并且由于我们使用了一个乘数作为约束，所以当我们向右滑动时，移动会“增长”。

这是它在发布时的样子：

这是向右拖动一点后的样子：

显然，要复制 App Switcher 的所有功能需要做更多的工作，但它可能会让您顺利上路。