Programmatically add a dynamic number of subviews to a UIScrollView using Auto Layout

Auto Layout and UIScrollView seem to be a popular, but sometimes problematic couple. So here is another post about this topic. This time we will take a couple of subviews, add them to a UIScrollView and make everything work by programmatically adding layout constraints.

Because I am a lazy guy, we will use SnapKit (a.k.a. TheFrameworkFormerlyKnownAsMasonry) for the Auto Layout stuff. And because I get bored easily and I have already written a post about a vertical UIScrollView with Auto Layout, we’ll do a horizontal UIScrollView this time. Yay!

So, let’s take 4 empty UIViews and give them each a different background color, so that we can see the scrolling. To make things simple we make each subview the same size as the scrollview (so each subview covers the whole visible area).

Here is the code, you’ll find some explanations below it:

import UIKit
import SnapKit

class ViewController: UIViewController {
    
    let scrollView = UIScrollView()
    let subViews = [UIView(), UIView(), UIView(), UIView()]
    let colors = [UIColor.greenColor(), UIColor.blueColor(), UIColor.redColor(), UIColor.orangeColor()]

    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 1
        view.addSubview(scrollView)
        scrollView.snp_makeConstraints { (make) in
            make.edges.equalTo(view)
        }
        
        // 2
        subViews.enumerate().forEach { index, subview in
            subview.backgroundColor = colors[index]
            // 3
            scrollView.addSubview(subview)
            subview.snp_makeConstraints(closure: { (make) in
                // 4
                make.top.equalTo(0)
                make.size.equalTo(scrollView)
                switch index {
                // 5
                case 0:
                    make.left.equalTo(0)
                // 6
                case subViews.count - 1:
                    make.left.equalTo(subViews[index - 1].snp_right)
                    make.right.equalTo(0)
                // 7
                default:
                    make.left.equalTo(subViews[index - 1].snp_right)
                }
            })
        }
    }
}

First we add the UIScrollView to the UIViewController and set its size to the size of the UIViewController’s view. // 1
We iterate over the subviews // 2, add them to the UIScrollView // 3 and add the Auto Layout constraints following this 4 simple rules:

1. All subviews have a top constraint of 0 and a size constraint that is equal to the scroll view. // 4
2. The first subview (in other words the leftmost subview) has a left constraint of 0 // 5
3. The last subview (the rightmost subview) has a right constraint of 0 // 5
4. All other subviews have a left constraint to the right of the previous subview // 6

And that’s all. When you run this code you can rotate the device to see that the subview sizes and the UIScrollView’s contentSize are adjusted automatically. If you don’t want the UIViews to be resized you just have to give them a static size.

This example also works for a vertical UIScrollView. Just connect the top and bottom constraints of the subviews instead of the left and right constraints. See the blog post I mentioned above for more details.