iOS：使用 UIView 的“drawRect：”与其图层的委托“drawLayer：inContext：”

使用UIView的“drawRect：”与其图层的委托“drawLayer：inContext：”是iOS开发中常用的两种绘制界面的方法。这两种方法可以帮助开发者实现自定义的界面效果，并且可以提高界面渲染的效率。

swift
class CustomView: UIView {
    override func drawRect(rect: CGRect) {
        super.drawRect(rect)
        
        let context = UIGraphicsGetCurrentContext()
        
        CGContextSetFillColorWithColor(context, UIColor.redColor().CGColor)
        CGContextFillRect(context, rect)
    }
}

swift
class CustomLayerDelegate: NSObject, CALayerDelegate {
    func drawLayer(layer: CALayer, inContext ctx: CGContext) {
        let colors = [UIColor.redColor().CGColor, UIColor.blueColor().CGColor]
        let locations: [CGFloat] = [0.0, 1.0]
        
        let gradient = CGGradientCreateWithColors(CGColorSpaceCreateDeviceRGB(), colors, locations)
        let startPoint = CGPoint.zero
        let endPoint = CGPoint(x: 0, y: layer.bounds.height)
        
        CGContextDrawLinearGradient(ctx, gradient, startPoint, endPoint, [])
    }
}
class CustomView: UIView {
    override class func layerClass() -> AnyClass {
        return CAGradientLayer.self
    }
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        
        let gradientLayer = self.layer as! CAGradientLayer
        gradientLayer.colors = [UIColor.redColor().CGColor, UIColor.blueColor().CGColor]
        gradientLayer.locations = [0.0, 1.0]
        gradientLayer.startPoint = CGPoint.zero
        gradientLayer.endPoint = CGPoint(x: 0, y: 1)
        gradientLayer.delegate = CustomLayerDelegate()
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }
}