Drawing simple lines on iPhone with CoreGraphics

Question

I would like to draw a straight line between where a user touches the screen, and where the touch ends. i need multiple lines, for if the user repeats the touch-drag-release act

Answer 1

Swift 3

Here is a full example for drawing lines (a grid) in a separate image and then added this image (as an overlay) to an existing image, in this case called 'boardImage' .

// ----------------------------------------------------------------------------------------
// DRAW LINES ON THE BOARD IMAGE
// ----------------------------------------------------------------------------------------
    private func drawLinesOnBoard() {

        // 1. DEFINE AN OFFSET AND THE SIZE OF ONE GRIDFIELD

        let offSet      : CGFloat = 20
        let fieldWidth  : CGFloat = 60

        // 2. CREATE A IMAGE GRAPHICS CONTEXT AND DRAW LINES ON IT

        UIGraphicsBeginImageContext(boardImage.boundsSize)

        if let currentContext = UIGraphicsGetCurrentContext() {

            currentContext.setLineWidth(1)  // set strokeWidth
            currentContext.setStrokeColor(UIColor.init(colorLiteralRed: 0.85, green: 1, blue: 0.85, alpha: 0.85).cgColor)
            currentContext.setLineJoin(.round)
            currentContext.setLineDash(phase: 1, lengths: [offSet / 4, offSet / 5])

            // VERTICAL LINES
            for multiplyer in (1...5) {

                let startPoint  : CGPoint = CGPoint(x: offSet + CGFloat(multiplyer) * fieldWidth, y: offSet)
                let endPoint    : CGPoint = CGPoint(x: startPoint.x, y: boardImage.frame.height - offSet)

                /* Start the line at this point */
                currentContext.move(to: startPoint)

                /* And end it at this point */
                currentContext.addLine(to: endPoint)
            }

            // HORIZONTAL LINES
            for multiplyer in (1...5) {

                let startPoint  : CGPoint = CGPoint(x: offSet, y: offSet + CGFloat(multiplyer) * fieldWidth)
                let endPoint    : CGPoint = CGPoint(x:boardImage.frame.width - offSet, y: startPoint.y)

                /* Start the line at this point */
                currentContext.move(to: startPoint)

                /* And end it at this point */
                currentContext.addLine(to: endPoint)
            }

            currentContext.strokePath()

            // 3. CREATE AN IMAGE OF THE DRAWN LINES AND ADD TO THE BOARD

            if let linesImage : UIImage = UIGraphicsGetImageFromCurrentImageContext() {
                let linesImageView : UIImageView = UIImageView(image: linesImage)
                let theCenter : CGPoint = CGPoint(x: boardImage.bounds.width / 2, y: boardImage.bounds.height / 2)
                boardImage.addSubview(linesImageView)
                linesImageView.center = theCenter
                isBoardLinesDrawn = true
            }
        }

        // 4. END THE GRAPHICSCONTEXT
        UIGraphicsEndImageContext()