How to unwrap programmed UIButton (swift4) - ios

Question

I am trying to create a button using programmed constraints. I am trying to not use the storyboard. I am having trouble unwrapping the button. How do I do this?
import UIKit
var aa: [NSLayoutConstraint] = []
class ViewController: UIViewController {
var btn: UITextField!
override func viewDidLoad() {
super.viewDidLoad()
self.view.addSubview(btn)
let leadingc2 = btn.widthAnchor.constraint(equalToConstant: 80)
let trailingC2 = btn.heightAnchor.constraint(equalToConstant: 50)
let topc2 = btn.centerXAnchor.constraint(equalTo: self.view.centerXAnchor, constant: -50)
let bottomc2 = btn.centerYAnchor.constraint(equalTo: self.view.centerYAnchor, constant: -250)
aa = [leadingc2,trailingC2,topc2,bottomc2]
NSLayoutConstraint.activate(aa)
}
}

Answer 1

You do not need to unwrap it. You need to instantiate it before using it.
override func viewDidLoad() {
super.viewDidLoad()
btn = UITextField() // Create the button like this before using it.
self.view.addSubview(btn)
btn.translatesAutoresizingMaskIntoConstraints = false
let leadingc2 = btn.widthAnchor.constraint(equalToConstant: 80)
let trailingC2 = btn.heightAnchor.constraint(equalToConstant: 50)
let topc2 = btn.centerXAnchor.constraint(equalTo: self.view.centerXAnchor, constant: -50)
let bottomc2 = btn.centerYAnchor.constraint(equalTo: self.view.centerYAnchor, constant: -250)
aa = [leadingc2,trailingC2,topc2,bottomc2]
NSLayoutConstraint.activate(aa)
}
Any variable declared with ! will be force unwrapped, meaning that if you forget to create an instance and use the variable, it will throw an error and crash your app.

Answer 2

Use this code:
import UIKit
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
view.addSubview(button)
setupConstraints()
}
lazy var button: UIButton = {
let button = UIButton(type: .system)
button.setTitle("Button", for: .normal)
button.backgroundColor = .blue
//your action here
button.addTarget(self, action: #selector(buttonAction), for: .touchUpInside)
return button
}()
private func setupConstraints() {
button.translatesAutoresizingMaskIntoConstraints = false
let top = NSLayoutConstraint(item: button, attribute: .top, relatedBy: .equal, toItem: view, attribute: .top, multiplier: 1, constant: 100)
let left = NSLayoutConstraint(item: button, attribute: .left, relatedBy: .equal, toItem: view, attribute: .left, multiplier: 1, constant: 50)
let right = NSLayoutConstraint(item: button, attribute: .right, relatedBy: .equal, toItem: view, attribute: .right, multiplier: 1, constant: -50)
let height = NSLayoutConstraint(item: button, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 50)
view.addConstraints([top, left, right, height])
}
#objc func buttonAction() {
print("Button has pressed")
}
}