Swift - self.bounds.size.width doesn't return the correct width - ios

Question

In another thread I found a solution for an underline for a segmented control.
The important line for my problem seems to be this one:
let underlineWidth: CGFloat = self.bounds.size.width / CGFloat(self.numberOfSegments)
It turns out that the calculation of the width is not entirely correct. Since I have always 3 segments I expected this value to be a third of the screen width (regardless of the device).
On this screenshot you can see the application running on two different devices:
Screenshot
As you can see, on the iPhone 6S, the width of the underline is slightly too big, where as on the iPhone 8Plus it's too small.
That can only mean that self.bounds.size.width doesn't return the correct width.
The whole class for the segmented control:
import UIKit
import Foundation
extension UISegmentedControl{
func removeBorder(){
let backgroundImage = UIImage.getColoredRectImageWith(color: UIColor.clear.cgColor, andSize: self.bounds.size)
let backgroundImageTest = UIImage.getColoredRectImageWith(color: UIColor.red.cgColor, andSize: self.bounds.size)
self.setBackgroundImage(backgroundImage, for: .normal, barMetrics: .default)
self.setBackgroundImage(backgroundImageTest, for: .selected, barMetrics: .default)
self.setBackgroundImage(backgroundImage, for: .highlighted, barMetrics: .default)
let deviderImage = UIImage.getColoredRectImageWith(color: UIColor.clear.cgColor, andSize: CGSize(width: 1.0, height: self.bounds.size.height))
self.setDividerImage(deviderImage, forLeftSegmentState: .selected, rightSegmentState: .normal, barMetrics: .default)
self.setTitleTextAttributes([NSAttributedStringKey.foregroundColor: UIColor.gray], for: .normal)
self.setTitleTextAttributes([NSAttributedStringKey.foregroundColor: UIColor(red: 67/255, green: 129/255, blue: 244/255, alpha: 1.0)], for: .selected)
}
func addUnderlineForSelectedSegment(){
removeBorder()
let underlineWidth: CGFloat = self.bounds.size.width / CGFloat(self.numberOfSegments)
let underlineHeight: CGFloat = 2.0
let underlineXPosition = CGFloat(selectedSegmentIndex * Int(underlineWidth))
let underLineYPosition = self.bounds.size.height - 1.0
let underlineFrame = CGRect(x: underlineXPosition, y: underLineYPosition, width: underlineWidth, height: underlineHeight)
let underline = UIView(frame: underlineFrame)
underline.backgroundColor = UIColor(red: 67/255, green: 129/255, blue: 244/255, alpha: 1.0)
underline.tag = 1
self.addSubview(underline)
}
func changeUnderlinePosition(){
guard let underline = self.viewWithTag(1) else {return}
let underlineFinalXPosition = (self.bounds.width / CGFloat(self.numberOfSegments)) * CGFloat(selectedSegmentIndex)
UIView.animate(withDuration: 0.1, animations: {
underline.frame.origin.x = underlineFinalXPosition
})
}
}
extension UIImage{
class func getColoredRectImageWith(color: CGColor, andSize size: CGSize) -> UIImage{
UIGraphicsBeginImageContextWithOptions(size, false, 0.0)
let graphicsContext = UIGraphicsGetCurrentContext()
graphicsContext?.setFillColor(color)
let rectangle = CGRect(x: 0.0, y: 0.0, width: size.width, height: size.height)
graphicsContext?.fill(rectangle)
let rectangleImage = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
return rectangleImage!
}
}

Answer 1

It is possible that you are trying to obtain dimensions in the method func viewDidLoad().
Try to take the exact dimensions in the method func viewDidLayoutSubviews().
See also this answer

Answer 2

The exact size of the view is only correct after the view has rendered. Before this has been done the view has a default size (e.g. as in the xib or storyboard).
In viewDidLayoutSubviews() or viewDidAppear()/viewWillAppear() the size will definitely be calculated right.