Unable to call method of ViewController() - ios

Question

I am simply trying to make a method in the ViewController class and be able to call it.
Here is my code (I note the 2 ways I tried calling it, and the errors I got):
import UIKit
class ViewController: UIViewController{
func sayHi(name: String){
print("Hi \(name)")
}
}
/*
let viewcontroller = ViewController()
viewcontroller.sayHi(name: "Bob")
*/
//Error: Expressions are not allowed at the top level
/*
ViewController.sayHi(name: "Bob")
*/
//Error: Expressions are not allowed at the top level
//Error: Instance member 'sayHi' cannot be used on type 'ViewController'; did you mean to use a value of this type instead?
So as you can see in the commenting, I tried to call sayHi as a type method and as an instance method. Neither worked. I will ultimately create a function that can take input from a text input, and manipulate it. Is ViewController.swift even the right file to be doing this? If so, how do I call a method that I have defined?

Answer 1

There will be this delegate in appDelegate which will be called when you app is launched. Create your viewController there and add it to the window.
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
window = UIWindow(frame: UIScreen.main.bounds)
window?.makeKeyAndVisible()
let viewController = ViewController()
window?.rootViewController = viewController
viewController.sayHi()
return true
}

Answer 2

This will work on Playground, but not on Xcode.
Xcode's code is compiled and then you have an app. The first point where a call happens is AppDelegate and from there your first controller and its methods are initialised. Nothing outside a class will be executed.
Use playground for tests or any other online swift playground.
If you want to run sayHi immediately, put it in viewDidLoad and load the app. Delete all further code outside of the class before building again:
override func viewDidLoad() {
super.viewDidLoad()
sayHi()
}

Answer 3

You can create a instance of controller inside a function or a block
when you are working in a class or Xcode projects in Xcode Playground your way of accessing the function sayHi(name: String) in
ViewController works.
For Xcode projects try the following
import UIKit
class ViewController: UIViewController{
func sayHi(name: String){
print("Hi \(name)")
}
}
class SecondViewController: UIViewController{
override func viewDidLoad() {
super.viewDidLoad()
let viewcontroller = ViewController()
viewcontroller.sayHi(name: "Bob")
}
}
When you initialise the SecondViewController you can access the ViewController()
To execute the function sayHi(name: String) immediately when the
ViewController() is initialised you can call it in viewDidLoad() or in
func viewWillAppear()
import UIKit
class ViewController: UIViewController{
override func viewDidLoad(_ animated: Bool) {
super.viewDidLoad(animated)
//Call the function hear
sayHi(name: "Bob")
}
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
// or call hear
sayHi(name: "Bob")
}
func sayHi(name: String){
print("Hi \(name)")
}
}