A UIView that fills its bounds with a black rectangle in light mode, or a white rectangle in dark mode.

ResizableLineView.swift

//
//  ResizableLineView.swift
//  YourProjectName
//
//  Created by conath on 09/12/20. Should work on iOS 12 and later.
//  
//  This is free and unencumbered software released into the public domain.
//
//  Anyone is free to copy, modify, publish, use, compile, sell, or
//  distribute this software, either in source code form or as a compiled
//  binary, for any purpose, commercial or non-commercial, and by any
//  means.
//
//  In jurisdictions that recognize copyright laws, the author or authors
//  of this software dedicate any and all copyright interest in the
//  software to the public domain. We make this dedication for the benefit
//  of the public at large and to the detriment of our heirs and
//  successors. We intend this dedication to be an overt act of
//  relinquishment in perpetuity of all present and future rights to this
//  software under copyright law.
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
//  EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
//  MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
//  IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
//  OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
//  ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
//  OTHER DEALINGS IN THE SOFTWARE.
//
//  For more information, please refer to <https://unlicense.org>
//

import UIKit

@IBDesignable
class ResizableLineView: UIView {

    var oldBounds: CGRect = .zero
    
    private func regenerateLine() {
        layer.sublayers?.removeAll()
        // add separator path to view
        let line = CAShapeLayer()
        line.path = CGPath(rect: bounds, transform: nil)
        line.fillColor = UIColor.getLabelColor().cgColor
        layer.addSublayer(line)
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        
        if bounds != oldBounds {
            regenerateLine()
        }
    }
    
    override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
        super.traitCollectionDidChange(previousTraitCollection)
        
        if let prev = previousTraitCollection,
            traitCollection.userInterfaceStyle != prev.userInterfaceStyle {
            // dark mode changed
            regenerateLine()
        }
    }
}

extension UIColor {
    /**
     Returns the label color to use according to system appearance (iOS 13) or interface style (iOS 12).
     */
    static func getLabelColor() -> UIColor {
        if #available(iOS 13.0, *) {
            return UIColor.label
        } else {
            return UITraitCollection().userInterfaceStyle == UIUserInterfaceStyle.dark ? .white : .black
        }
    }
}