sutartmelson sutartmelson - 2 months ago 7x
iOS Question

How can I draw a shape in a view based on input in the view controller?

A very basic version of what I want to do is have a slider and a view within a view controller. The slider signifies the number of sides on a polygon, as you move the slider, a shape is drawn in the subview.

A very simple push in the right direction is all I need. I just am unsure of how the interaction works between view controllers and views.


Here you go. You create a custom view to draw a polygon. When its sides property is set, it calls setNeedsDisplay on itself which tells iOS to call drawRect.

class PolygonView: UIView {
    var sides: Int = 0 {
        didSet {

    override func drawRect(rect: CGRect) {
        // Draw polygon using sides

In your ViewController, update sides on the polygonView when the slider value changes:

class ViewController: UIViewController {
    @IBOutlet weak var polygonView: PolygonView!

    @IBAction func sliderChanged(sender: UISlider) {
        let sides = Int(sender.value)
        polygonView.sides = sides

So, when the slider is moved, sliderChanged will be called. The value of the slider is read and saved to the sides property of the polygonView. didSet gets called when sides is set, and the polygonView calls setNeedsDisplay on itself. This tells iOS that the view needs to be drawn, so iOS calls drawRect on the polygonView.