How to draw a line in the simplest way in swift

Swift 3.1, inside a UIView:

public override func draw(_ rect: CGRect) {
    let context = UIGraphicsGetCurrentContext()
    context!.setLineWidth(2.0)
    context!.setStrokeColor(UIColor.red.cgColor)
    context?.move(to: CGPoint(x: 0, y: self.frame.size.height))
    context?.addLine(to: CGPoint(x: self.frame.size.width, y: 0))
    context!.strokePath()
}

If you want to add a line to a UIViewController, just add a UIView with this function as a subview to the UIViewController, ei:

let line = LineView(CGRect(x: 0, y: 0, width: self.view.frame.width, height: self.view.frame.height))
self.view.addSubview(line)

Try looking into UIBezierPath, it will help you a lot for drawing lines. Here is documentation. Here is an example:

override func drawRect(rect: CGRect) {
    let aPath = UIBezierPath()

    aPath.move(to: CGPoint(x:<#start x#>, y:<#start y#>))
    aPath.addLine(to: CGPoint(x: <#end x#>, y: <#end y#>))

    // Keep using the method addLine until you get to the one where about to close the path
    aPath.close()

    // If you want to stroke it with a red color
    UIColor.red.set()
    aPath.lineWidth = <#line width#>
    aPath.stroke()
}

Make sure you put this code in the drawRect, like in the example above.

If you need to update the drawing just call setNeedsDisplay() to update.

The questioner isn't really asking what code to use, he is asking where to put the code. It's actually a good question because if you put any of the graphics code in the 'wrong' place, nothing at all will be drawn. The 'wrong' place to put the code is in a class that is not a subclass of UIView. If you try doing that, the code will compile but when it runs, the call to get the graphics context will fail and the calls to the following graphics functions, such as context?.move(), will not run because the context is nil.

As an example, let's create a storyboard with a View, a Button and a Label by dragging the respective objects from the Object Library onto the storyboard. My example looks like this, where the white square is the View (actually a sub-view because the main screen is also a view):

It's possible to draw graphics on Button and Label controls (and the other types of UI controls) because UIButton and UILabel are subclasses of UIView.

I also created a new file in the project, of type Swift, and called it ExampleDraw.swift and put the following code in it. The code contains three classes to write graphics on the view, button and label. Each class overrides the draw() function in the base UIView class:

import UIKit

class DrawOnView: UIView {
    override func draw(_ rect: CGRect) {
        let context = UIGraphicsGetCurrentContext()
        context?.setLineWidth(2.0)
        context?.setStrokeColor(UIColor.blue.cgColor)
        context?.move(to: CGPoint(x:0, y: 0))
        context?.addLine(to: CGPoint(x: 20, y: 30))
        context?.strokePath()

        print("in DrawOnView")
    }
}

class DrawOnButton: UIButton {
    override func draw(_ rect: CGRect) {
        let context = UIGraphicsGetCurrentContext()
        context?.setLineWidth(4.0)
        context?.setStrokeColor(UIColor.green.cgColor)
        context?.move (to: CGPoint(x: 0, y: 0))
        context?.addLine (to: CGPoint(x: 40, y: 45))
        context?.strokePath()

        print("in DrawOnButton")
    }
}

class DrawOnLabel: UILabel {
    override func draw(_ rect: CGRect) {
        let context = UIGraphicsGetCurrentContext()
        context?.setLineWidth(1.0)
        context?.setStrokeColor(UIColor.red.cgColor)
        context?.move (to: CGPoint(x: 0, y: 0))
        context?.addLine (to: CGPoint(x: 35, y: 45))
        context?.strokePath()

        print("in DrawOnLabel")
    }
}

Then in the main storyboard, select the view, like this:

Then open the Identity Inspector and associate this view with the custom class (i.e. the first class in ExampleDraw.swift) called DrawOnView() like this:

Do the same for the Button and the Label, like this:

Then run the project in the simulator and hopefully the graphics will write on the view, button and label like this:

Update for Swift 3.x using Epic Defeater's example

override func draw(_ rect: CGRect) {
        let aPath = UIBezierPath()

        aPath.move(to: CGPoint(x:20, y:50))

        aPath.addLine(to: CGPoint(x:300, y:50))

        //Keep using the method addLineToPoint until you get to the one where about to close the path

        aPath.close()

        //If you want to stroke it with a red color
        UIColor.red.set()
        aPath.stroke()
        //If you want to fill it as well
        aPath.fill()
    }