What does '!' (exclamation mark) next to the file name represent in Xcode
<h2>Issue:</h2>

<p>I am wondering what does <strong>'!'  (exclamation mark)</strong>, next to the name of the file mean in <strong>Xcode</strong>. I know that it is meant to tell you that a file is <strong><em>"Missing"</em></strong>, but I couldn't find any online explanation. And I also need a way to fix it. I can edit the code inside of the so-called <strong><em>"Missing"</em></strong> file. When I run it into the simulator everything works completely fine, no error and not even a warning.</p>

<h2>Technical Data:</h2>

<p>I am running <strong>Xcode 8</strong> with <strong>Swift 3</strong>.</p>

<h2>What I have tried so far:</h2>

<ul>
<li>I tried re-opening <strong><em>Xcode</em></strong> --> didn't work</li>
<li>I tried solving it with <em>source-control</em> --> nothing happened</li>
<li>I searched for it inside my <strong>trash</strong> --> no file inside the <strong>trash</strong>
with the same name</li>
<li>I have checked the <strong>connections</strong> --> <strong>everything is fine</strong></li>
</ul>

<h2>Images:</h2>

<p><strong>1. The file inside my editor:</strong></p>

<p><a href="https://i.stack.imgur.com/itB7v.png" rel="noreferrer"><img src="https://i.stack.imgur.com/itB7v.png" alt="enter image description here"></a></p>

<p><strong>2. Simulator:</strong></p>

<p><a href="https://i.stack.imgur.com/G0zVf.png" rel="noreferrer"><img src="https://i.stack.imgur.com/G0zVf.png" alt="enter image description here"></a></p>

<h2>Code:</h2>

<pre><code>import Foundation
import UIKit

class Documentation: UIViewController {

    @IBOutlet weak var Basics: UIButton!
    let defaults = UserDefaults.standard

    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.addGestureRecognizer(self.revealViewController().panGestureRecognizer())
        let menuButton = UIBarButtonItem(barButtonSystemItem: .organize, target: self.revealViewController(), action: #selector(SWRevealViewController.revealToggle(_:)))
        menuButton.tintColor = UIColor.white
        self.navigationItem.leftBarButtonItem = menuButton
        self.customizeButton(button: Basics, angle: 50, borderWidth: 2, borderColor: #colorLiteral(red: 0.0693108514, green: 0, blue: 0.2353696823, alpha: 1))

    }
    @IBAction func BasicsAction(_ sender: UIButton) {

    }

    func customizeButton(button: UIButton, angle: Int, borderWidth: Int, borderColor: UIColor){
        button.layer.cornerRadius = CGFloat(angle)
        button.layer.borderWidth = CGFloat(borderWidth)
        button.layer.borderColor = borderColor.cgColor
        let completionLabel = button.viewWithTag(1) as! UILabel!
        completionLabel?.text = "0/10"
        if defaults.value(forKey: "\(button.currentTitle)") != nil{
            let done = defaults.value(forKey: "\(button.currentTitle)") as! Int
            completionLabel?.text = "\(done)/10"
            if done == 10{
                button.layer.borderColor = #colorLiteral(red: 0.2193539292, green: 0.4209204912, blue: 0.1073316187, alpha: 1).cgColor
                button.layer.borderWidth = 5
            }
        }

    }
}
</code></pre>

<p>Any help is greatly appreciated if it comes with a good explanation!</p>