使用Swift 3.0实时绘制线条

您必须开始图像上下文：

UIGraphicsBeginImageContextWithOptions(view.bounds.size, false, 0)

您还必须摸索路径：

context?.strokePath()

您也没有绘制上一张图像：

imageView.image?.draw(in: view.bounds)

从而：

func drawLine(from fromPoint: CGPoint, to toPoint: CGPoint) {    UIGraphicsBeginImageContextWithOptions(view.bounds.size, false, 0)    imageView.image?.draw(in: view.bounds)    let context = UIGraphicsGetCurrentContext()    context?.move(to: fromPoint)    context?.addLine(to: toPoint)    context?.setLineCap(CGLineCap.round)    context?.setLineWidth(brushWidth)    context?.setStrokeColor(red: red, green: green, blue: blue, alpha: 1.0)    context?.setBlendMode(CGBlendMode.normal)    context?.strokePath()    imageView.image = UIGraphicsGetImageFromCurrentImageContext()    imageView.alpha = opacity    UIGraphicsEndImageContext()}