How to add system icons for a UIButton programmatically?

import UIKit

extension UIImage {

    public convenience init?(_ systemItem: UIBarButtonItem.SystemItem) {

        guard let sysImage = UIImage.imageFrom(systemItem: systemItem)?.cgImage else {
            return nil
        }

        self.init(cgImage: sysImage)
    }

    private class func imageFrom(systemItem: UIBarButtonItem.SystemItem) -> UIImage? {

        let sysBarButtonItem = UIBarButtonItem(barButtonSystemItem: systemItem, target: nil, action: nil)

        //MARK:- Adding barButton into tool bar and rendering it.
        let toolBar = UIToolbar()
        toolBar.setItems([sysBarButtonItem], animated: false)
        toolBar.snapshotView(afterScreenUpdates: true)

        if  let buttonView = sysBarButtonItem.value(forKey: "view") as? UIView{
            for subView in buttonView.subviews {
                if subView is UIButton {
                    let button = subView as! UIButton
                    let image = button.imageView!.image!
                    return image
                }
            }
        }
        return nil
    }
}

This is an example of how do we use it:

 let button = UIButton() ;
 let systemImage = UIImage(systemItem: .trash) ;
 button.setImage(systemImage, for: .normal)

For Swift 5 the syntax is:

button.setImage(UIImage(systemName: "search"), for: .normal)

You can also set the weight of the icon by adding SymbolConfiguration:

let boldConfig = UIImage.SymbolConfiguration(weight: .bold)
let boldSearch = UIImage(systemName: "search", withConfiguration: boldConfig)

button.setImage(boldSearch, for: .normal)

See: Apple documentation for available names (API column) or go to Interface Builder, select UIButton and in the Attributes Inspector select Image which will give you list of all available icons.

I found 2521 system icons, as numerated values.

@available(iOS 13.0, *)
@objc public extension UIImage{

    var assetName: String? {
        guard let imageAsset = imageAsset else { return nil }
        return imageAsset.value(forKey:"assetName") as? String
    }

    static var square_and_arrow_up: UIImage? {
        return UIImage(systemName: "square.and.arrow.up")
    }
    static var square_and_arrow_up_fill: UIImage? {
        return UIImage(systemName: "square.and.arrow.up.fill")
    }
    static var square_and_arrow_down: UIImage? {
        return UIImage(systemName: "square.and.arrow.down")
    }
    ...

screenshot: github: link

Now you can just do

button.setImage(UIImage(.search), for: .normal)