我有一个 ScrollView ，我用它来控制其他元素的 alpha，具体取决于用户向下 ScrollView 的距离。

首先，我设置了模糊 View 。 这里的Alpha好像没用，首先

var effect: UIBlurEffectStyle = .light
if #available(iOS 10.0, *) {
    effect = .prominent
}
let blurView = UIVisualEffectView(effect: UIBlurEffect(style: effect))
blurView.alpha = 0
if let navigationBar = navigationController?.navigationBar {
    blurView.frame = navigationBar.bounds
}
self.blurView = blurView

然后在我的 UIScrollViewDelegate 中:

func scrollViewDidScroll(_ scrollView: UIScrollView) {
        var alpha = (scrollView.contentOffset.y / 200)
        alpha = alpha > 1.0 ? 1.0 : alpha

        blurView?.alpha = alpha
}

这似乎也不起作用。

关于如何实现这种效果有什么建议吗？谢谢!

最佳答案

当您阅读 Apple documentation你可以找到:

When using the UIVisualEffectView class, avoid alpha values that are less than 1.

事实上你也应该得到一个警告:

[Warning] <UIVisualEffectView 0x7af7a3b0> is being asked to animate its opacity. This will cause the effect to appear broken until opacity returns to 1.

附注: 请注意，我的意思是您可以更改视觉效果的 alpha，但 View 可能只是部分透明而不是模糊。

如果您不相信这个文档，您可以在一个空白项目中测试我下面的小示例，您可以看到，如果您更改 alpha 值，您的效果将停止正常工作:

(您可以在我的 GitHUB 存储库 here 下找到所有项目)

在一个空白项目中准备一个连接到 viewController 的 navigationController，如下所示:

将带有 UIImageView 的通用图像放到您的 viewController 的 View 中，如下图:

class ViewController: UIViewController {
    var blurView: UIVisualEffectView!
    var effect: UIBlurEffectStyle = .light
    var blurEffect : UIBlurEffect!
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        // This code make more transparent our navigation
        if let navigationBar = navigationController?.navigationBar {
            navigationBar.setBackgroundImage(UIImage(), for: UIBarMetrics.default)
            navigationBar.tintColor = .white
            let textAttributes = [NSForegroundColorAttributeName:UIColor.white]
            navigationBar.titleTextAttributes = textAttributes
            navigationBar.shadowImage = UIImage()
            navigationBar.backgroundColor = UIColor(red: 0.0, green: 0.3, blue: 0.5, alpha: 0.3)
            navigationBar.isTranslucent = true
        }
    }
    override func viewDidLoad() {
        super.viewDidLoad()
        if #available(iOS 10.0, *) {
            effect = .prominent
        } else {
            // Fallback on earlier versions
        }
        if let navigationBar = navigationController?.navigationBar {
            let noEffectView = UIVisualEffectView.init(frame: navigationBar.bounds)
            self.blurEffect = UIBlurEffect(style: effect)
            self.blurView = noEffectView
            navigationBar.addSubview(self.blurView)
            // This line below to don't blur buttons and title
            navigationBar.sendSubview(toBack: self.blurView)
            // Apply the effect:
            Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(self.applyBlur), userInfo: nil, repeats: false)
        }
    }
    func applyBlur() {
        print("Apply the blur effect..")
        UIView.animate(withDuration: 10.2, animations: {
            self.blurView.effect = self.blurEffect
        }, completion:{ (finished: Bool) in
            // Make test with a simple timer:
            Timer.scheduledTimer(timeInterval: 0.3, target: self, selector: #selector(self.changeAlpha), userInfo: nil, repeats: true)
        })
    }
    func changeAlpha() {
        let startNum:CGFloat = 0.0; let stopNum:CGFloat = 200.0
        let randomNum = CGFloat(arc4random()) / CGFloat(UINT32_MAX) * abs(startNum - stopNum) + min(startNum, stopNum)
        var alpha = (randomNum / 200)
        alpha = alpha > 1.0 ? 1.0 : alpha
        print("we change alpha to : \(alpha)")
        self.blurView.alpha = alpha
    }
}

输出模糊效果:

alpha 更改期间的输出:

解决方案:

可能您不想更改 alpha 值，而是更改滚动期间的模糊效果量，因此解决方案可能是使用 的新 fractionComplete >UIViewPropertyAnimator 属性仅适用于 iOS 10.0(如 this 其他 SO 答案中所建议)

import UIKit
@available(iOS 10.0, *)
class ViewController: UIViewController {
    var blurView: UIVisualEffectView!
    var effect: UIBlurEffectStyle = .light
    var blurEffect : UIBlurEffect!
    var animator: UIViewPropertyAnimator!
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        // This code make more transparent our navigation
        if let navigationBar = navigationController?.navigationBar {
            navigationBar.setBackgroundImage(UIImage(), for: UIBarMetrics.default)
            navigationBar.tintColor = .white
            let textAttributes = [NSForegroundColorAttributeName:UIColor.white]
            navigationBar.titleTextAttributes = textAttributes
            navigationBar.shadowImage = UIImage()
            navigationBar.backgroundColor = UIColor(red: 0.0, green: 0.3, blue: 0.5, alpha: 0.3)
            navigationBar.isTranslucent = true
        }
    }
    override func viewDidLoad() {
        super.viewDidLoad()
            effect = .prominent
            if let navigationBar = navigationController?.navigationBar {
                let noEffectView = UIVisualEffectView.init(frame: navigationBar.bounds)
                self.blurEffect = UIBlurEffect(style: effect)
                self.blurView = noEffectView
                navigationBar.addSubview(self.blurView)
                // This line below to don't blur buttons and title
                navigationBar.sendSubview(toBack: self.blurView)
                // Apply the effect:
                Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(self.applyBlur), userInfo: nil, repeats: false)
            }
    }
    func applyBlur() {
        print("Apply the blur effect..")
        animator = UIViewPropertyAnimator(duration: 0.5, curve: .linear)
        self.blurView.effect = self.blurEffect
        animator.addAnimations {
            self.blurView.effect = nil
        }
        Timer.scheduledTimer(timeInterval: 0.3, target: self, selector: #selector(self.changeBlurFraction), userInfo: nil, repeats: true)

    }
    func changeBlurFraction() {
        let startNum:CGFloat = 0.0; let stopNum:CGFloat = 200.0
        let randomNum = CGFloat(arc4random()) / CGFloat(UINT32_MAX) * abs(startNum - stopNum) + min(startNum, stopNum)
        var blurFraction = (randomNum / 200)
        blurFraction = blurFraction > 1.0 ? 1.0 : blurFraction
        print("we change the blur fraction to : \(blurFraction)")
        animator.fractionComplete = blurFraction
    }
}

如您所见，这改变了模糊效果而不是 alpha，并且每次触发计时器时，您都可以看到导航栏的不同模糊值。

诀窍是应用模糊效果并添加一个新动画以将效果报告为零，但每次都使用一个新的“分数”将动画更改为模糊的确切值。

输出:

关于swift - 如何以编程方式更改 NavigationBar 中 UIVisualEffectView 的 alpha？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/43099786/