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Core Graphics

问题​

Core Graphics 怎么用？draw(_:) 和 drawRect 的工作原理？

答案​

自定义绘制​

class ChartView: UIView {
    override func draw(_ rect: CGRect) {
        guard let context = UIGraphicsGetCurrentContext() else { return }

        // 绘制背景
        context.setFillColor(UIColor.white.cgColor)
        context.fill(rect)

        // 绘制折线
        context.setStrokeColor(UIColor.blue.cgColor)
        context.setLineWidth(2.0)
        context.move(to: CGPoint(x: 20, y: 100))
        context.addLine(to: CGPoint(x: 80, y: 50))
        context.addLine(to: CGPoint(x: 140, y: 80))
        context.strokePath()

        // 绘制圆
        let circleRect = CGRect(x: 50, y: 50, width: 60, height: 60)
        context.setFillColor(UIColor.red.withAlphaComponent(0.3).cgColor)
        context.fillEllipse(in: circleRect)
    }
}

UIBezierPath​

// 更高层 API，封装 CGPath
let path = UIBezierPath()
path.move(to: CGPoint(x: 0, y: 100))
path.addCurve(
    to: CGPoint(x: 200, y: 100),
    controlPoint1: CGPoint(x: 50, y: 0),
    controlPoint2: CGPoint(x: 150, y: 200)
)

let shapeLayer = CAShapeLayer()
shapeLayer.path = path.cgPath
shapeLayer.strokeColor = UIColor.blue.cgColor
shapeLayer.fillColor = UIColor.clear.cgColor
shapeLayer.lineWidth = 2.0
view.layer.addSublayer(shapeLayer)

异步绘制​

// 主线程绘制可能卡顿，使用异步绘制
DispatchQueue.global().async {
    UIGraphicsBeginImageContextWithOptions(size, false, 0)
    // ... 复杂绘制
    let image = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    DispatchQueue.main.async {
        imageView.image = image
    }
}

常见面试问题​

Q1: setNeedsDisplay 和 setNeedsLayout 的区别？​

答案：

  • setNeedsDisplay：标记需要重绘，下次 RunLoop 调用 draw(_:)
  • setNeedsLayout：标记需要重新布局，下次 RunLoop 调用 layoutSubviews()

相关链接​