UISearchBar with UIActivityIndicator source: https://stackoverflow.com/a/45905342

UISearchBar.swift

import UIKit

extension UISearchBar {
    public var textField: UITextField? {
        let subViews = subviews.flatMap { $0.subviews }
        guard let textField = subViews.first(where: { $0 is UITextField }) as? UITextField else {
            return nil
        }
        return textField
    }

    public var activityIndicator: UIActivityIndicatorView? {
        return textField?.leftView?.subviews.compactMap { $0 as? UIActivityIndicatorView }.first
    }

    var isLoading: Bool {

        get {
            return activityIndicator != nil
        }

        set {
            if newValue {
                if activityIndicator == nil {
                    let newActivityIndicator = UIActivityIndicatorView(activityIndicatorStyle: .gray)
                    newActivityIndicator.transform = CGAffineTransform(scaleX: 0.7, y: 0.7)
                    newActivityIndicator.startAnimating()
                    newActivityIndicator.backgroundColor = UIColor.white
                    textField?.leftView?.addSubview(newActivityIndicator)
                    let leftViewSize = textField?.leftView?.frame.size ?? CGSize.zero
                    newActivityIndicator.center = CGPoint(x: leftViewSize.width / 2, y: leftViewSize.height / 2)
                }
            } else {
                activityIndicator?.removeFromSuperview()
            }
        }
    }
}