Swift中如何清除Core Graphics Context绘制的内容？

要实现清除绘制内容的功能，核心是清空存储线条数据的数组并触发视图重绘，具体步骤如下：

1. 给CanvasView添加清除方法 #

在CanvasView类中新增一个公共的清除方法，用于清空线条数组并通知视图重绘：

struct Line {
    var points: [CGPoint]
    var strokeColor: UIColor
    var strokeWidth: CGFloat
}

class CanvasView: UIView {
    private var lines: [Line] = []
    private var strokeWidth: CGFloat = 8.0
    private var strokeColor: UIColor = .white
    
    // 新增清除方法
    func clear() {
        lines.removeAll()
        setNeedsDisplay()
    }
    
    override func draw(_ rect: CGRect) {
        super.draw(rect)
        
        guard let context = UIGraphicsGetCurrentContext() else { return }
        
        lines.forEach { line in
            context.setStrokeColor(line.strokeColor.cgColor)
            context.setLineWidth(line.strokeWidth)
            context.setLineCap(.round)
            
            for (index, point) in line.points.enumerated() {
                if index == 0 {
                    context.move(to: point)
                } else {
                    context.addLine(to: point)
                }
            }
            context.strokePath()
        }
    }
    
    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        guard let point = touches.first?.location(in: self) else { return }
        let newLine = Line(points: [point], strokeColor: strokeColor, strokeWidth: strokeWidth)
        lines.append(newLine)
        setNeedsDisplay()
    }
    
    override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
        guard let point = touches.first?.location(in: self) else { return }
        guard var lastLine = lines.popLast() else { return }
        
        lastLine.points.append(point)
        lines.append(lastLine)
        setNeedsDisplay()
    }
}

2. 绑定清除按钮的点击事件 #

在包含CanvasView的视图控制器中，给清除按钮添加点击事件，调用CanvasView的clear()方法：

class ViewController: UIViewController {
    // 通过IBOutlet关联你的CanvasView实例
    @IBOutlet weak var canvasView: CanvasView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    
    // 清除按钮的点击响应方法
    @IBAction func clearButtonTapped(_ sender: UIButton) {
        canvasView.clear()
    }
}

原理说明 #

你的绘制逻辑完全依赖lines数组中的数据——draw(_:)方法会遍历数组内的每一条线完成绘制。清空数组后调用setNeedsDisplay()，会触发视图重新执行draw(_:)方法，此时数组为空，自然不会绘制任何线条，视图就恢复到初始的清空状态。

内容的提问来源于stack exchange，提问作者Deltrin Sam