Create a button programatically in differrent class swift

After doing this

let renderobj = render()

And after calling this function, a new button is initialized and added to the view of render view controller.

renderobj.sign()

But renver view controller is only initialized and its view is not added to current view controller which is ViewController

If you want to show render controller, then present it like usual. If you want to add that button to your ViewController then either declare a button variable and access it or return it in sign() function

I think there is a lack of understanding around the concepts of a UIViewController here. A UIViewController is used to manage the views displayed to a user, think about it as a single screen of your app. You can embed view controllers in other view controllers, but typically you won't need to do this for basic stuff.

A UIViewController has a lifecycle which you can find more about here: https://developer.apple.com/reference/uikit/uiviewcontroller#1652793

Now what it appears like you want to do is to wrap up the functionality of creating a button, you can do this by either creating a factory method in you ViewController class (or other class somewhere else), or by subclassing UIButton and creating a convenience initialiser method to create the button with the properties you want (you can also use Swift extensions but I won't go into that here).

Lets look at the simplest method and create a factory method inside your ViewController class:

// The button method takes a title and a selector (the selector is the method that should be called when the button is tapped)
func makeButton(title: String, action: Selector) -> UIButton {

    // Create a basic button with a default size
    let button = UIButton(frame: CGRect(x: 0, y: 0, width: 100, height: 50))

    // Customise the button with your settings
    button.backgroundColor = .green
    button.setTitle(title, for: .normal)
    button.addTarget(self, action: action, for: .touchUpInside)

    // Return the newly created button
    return button
}

Now we have a method that creates a button of a default size and position, we want to call that method to create a button and then add it as a subview of the ViewController's view. To do that we can do something in the viewDidLoad() method that we override:

override func viewDidLoad() {

    // Always remember to call the super method to ensure all super classes have a chance to do any work thats required by them
    super.viewDidLoad()

    // Create the button by calling our new method
    let button = self.makeButton(title: "Test Button", action: #selector(buttonTapped))

    // Set the origin of the button to move it to the right position
    button.frame.origin = CGPoint(x: 100, y: 100)

    // Finally add the new button as a subclass of the view controllers view
    self.view.addSubview(button)
}

Now we understand whats going on in the methods we can put it all together to have a class that looks like this:

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {

        super.viewDidLoad()

        let button = self.makeButton(title: "Test Button", action: #selector(buttonTapped))
        button.frame.origin = CGPoint(x: 100, y: 100)
        self.view.addSubview(button)
    }

    func makeButton(title: String, action: Selector) -> UIButton {

        let button = UIButton(frame: CGRect(x: 0, y: 0, width: 100, height: 50))

        button.backgroundColor = .green
        button.setTitle(title, for: .normal)
        button.addTarget(self, action: action, for: .touchUpInside)

        return button
    }

    func buttonTapped(sender: UIButton) {

        print("Button tapped")
    }
}

You create new UIViewController here:

let renderobj = render()
renderobj.sign()

but ViewController instance is showing in this moment.