Xcode – How to make UIView which is inside scrollview adapt to screen orientation when user changes screen from portrait to landscape in swift

You really want to be using auto-layout instead of trying to calculate frame sizes. Let it do all the work for you.

Based on your code, it looks like you want each "contentView" to be the width of the scrollView’s frame, minus 20 (so you have 10-pts of space on each side).

You can quite easily do this by embedding your contentViews in a UIStackView.

Here’s a simple example:

class ViewController: UIViewController {
    
    var scrollView: UIScrollView = {
        var scroll = UIScrollView()
        scroll.translatesAutoresizingMaskIntoConstraints = false
        
        return scroll
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()

        view.addSubview(scrollView)
        
        // use a stack view to hold and arrange the scrollView's subviews
        let stackView = UIStackView()
        stackView.translatesAutoresizingMaskIntoConstraints = false
        stackView.spacing = 20

        // add the stackView to the scrollView
        scrollView.addSubview(stackView)

        // respect safe area
        let safeG = view.safeAreaLayoutGuide
        
        // use scrollView's Content Layout Guide to define scrollable content
        let layoutG = scrollView.contentLayoutGuide

        // use scrollView's Frame Layout Guide to define content height (since you want horizontal scrolling)
        let frameG = scrollView.frameLayoutGuide

        NSLayoutConstraint.activate([
            
            scrollView.topAnchor.constraint(equalTo: safeG.topAnchor, constant: 100),
            
            // you're setting leading and trailing, so no need for centerX
            //scrollView.centerXAnchor.constraint(equalTo: view.centerXAnchor, constant: 0),
            
            scrollView.leadingAnchor.constraint(equalTo: safeG.leadingAnchor, constant: 0),
            scrollView.trailingAnchor.constraint(equalTo: safeG.trailingAnchor, constant: 0),
            
            // let's constrain the scrollView bottom to the view (safe area) bottom
            //scrollView.heightAnchor.constraint(equalToConstant: 500),
            scrollView.bottomAnchor.constraint(equalTo: safeG.bottomAnchor, constant: -10.0),
            
            // constrain Top and Bottom of the stackView to scrollView's Content Layout Guide
            stackView.topAnchor.constraint(equalTo: layoutG.topAnchor),
            stackView.bottomAnchor.constraint(equalTo: layoutG.bottomAnchor),
            
            // 10-pts space on leading and trailing
            stackView.leadingAnchor.constraint(equalTo: layoutG.leadingAnchor, constant: 10.0),
            stackView.trailingAnchor.constraint(equalTo: layoutG.trailingAnchor, constant: -10.0),
            
            // constrain stackView's height to scrollView's Frame Layout Guide height
            stackView.heightAnchor.constraint(equalTo: frameG.heightAnchor),
            
        ])
        
        // add some views to the stack view
        let arr: [UIColor] = [
            .red, .green, .blue, .yellow, .purple,
        ]
        
        for i in 0..<arr.count {
            let contentView = UIView()
            contentView.backgroundColor = arr[i]
            stackView.addArrangedSubview(contentView)
            
            // constrain each "contentView" width to scrollView's Frame Layout Guide width minus 20
            contentView.widthAnchor.constraint(equalTo: frameG.widthAnchor, constant: -20).isActive = true
            
            // don't do this
            //contentView.frame = CGRect(x: i * Int(view.bounds.size.width) + 10, y: 0, width: Int(view.bounds.size.width) - 20 , height: Int(view.frame.height))
            
            // don't do this
            //scrollView.contentSize = CGSize(width: (view.frame.size.width * CGFloat((Double(i)+1))) ,height: scrollView.frame.size.height)
            
        }

    }

}

Run that and see if that’s what you’re going for.