https://github.com/pgorzelany/ScratchCardView
这有一个实现刮奖效果的第三方,一个灰色遮罩,手指划过会显现图片内容.
如图:

效果图

看到效果第一时间,认为是在imageView 上方遮盖一层灰色 View ,在手指划过时,路径变成透明,以达到效果.当然,现实中是这样实现的没错.
但是,在运行 demo 时,看到层级结构
当相关如下时:

效果

层级结构

????

顶着崩坏的世界观继续看源码
直到

        canvasMaskView.backgroundColor = UIColor.clear
        canvasMaskView.strokeColor = UIColor.orange
        canvasMaskView.lineWidht = scratchWidth
        contentViewContainer.mask = canvasMaskView

canvasMaskView 即 我写了 Hi 的图层

mask 是什么属性? 点击去还是系统的属性?

于是去百了个度 看到文章
http://www.jianshu.com/p/e4ac8266d9f0
恍然大悟! 我的世界观得以修正,谢谢!

说:

UIView有一个maskView属性（遮罩），这个属性是通过改变alpha通道来改变透明度的，可以理解为maskview将自己“投影”到view上，实现改变alpha一样的效果。

1.设置了遮罩属性后view只显示和遮罩重叠的区域。

2.通过改变遮罩的alpha和颜色实现透明、半透明的效果。

至于划线部分.代码非常简单(以下为框架:ScratchCardView 中摘抄)

import UIKit

class CanvasView: UIView {
    
    // MARK: Properties
    
    @IBInspectable var lineWidht: CGFloat = 1
    @IBInspectable var strokeColor = UIColor.red
    
    fileprivate var paths: [CGMutablePath] = []
    fileprivate var currentPath: CGMutablePath?
    
    // MARK: Lifecycle
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        
        self.configureView()
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        
        self.configureView()
    }
    
    init(strokePaths paths: [CGMutablePath]) {
        self.init()
        
        self.paths = paths
    }
    
    override func draw(_ rect: CGRect) {
        let context = UIGraphicsGetCurrentContext()
        for path in self.paths + [self.currentPath].flatMap({$0}) {
            context?.addPath(path)
            context?.setLineWidth(lineWidht)
            context?.strokePath()
        }
    }
    
    // MARK: Actions
    
    func panGestureRecognized(_ recognizer: UIPanGestureRecognizer) {
        let location = recognizer.location(in: self)
        
        switch recognizer.state {
        case .began:
            self.beginPath(at: location)
        case .changed:
            self.addLine(to: location)
        default:
            self.closePath()
        }
    }
    
    // MARK: Public methods
    
    func beginPath(at point: CGPoint) {
        self.currentPath = CGMutablePath()
        self.currentPath?.move(to: point)
    }
    
    func addLine(to point: CGPoint) {
        self.currentPath?.addLine(to: point)
        self.setNeedsDisplay()
    }
    
    func closePath() {
        if let currentPath = self.currentPath {
            self.paths.append(currentPath)
        }
        self.currentPath = nil
        self.setNeedsDisplay()
    }
    
    func clear() {
        self.paths = []
        self.setNeedsDisplay()
    }
    
    // MARK: Private Methods
    
    fileprivate func configureView() {
        self.addGestureRecognizers()
    }
    
    fileprivate func addGestureRecognizers() {
        let panRecognizer = UIPanGestureRecognizer(target: self, action: #selector(panGestureRecognized))
        self.addGestureRecognizer(panRecognizer)
    }
}