发现 nil 时崩溃，同时隐式展开非 nil 的可选值

Question

I'm developing an iOS database app using Firebase. After Authentication (which performs successfully), the userUID gets passed from my LoginViewController to my HomeViewController. Then, in my HomeViewController, I use the user's UID to get the rest of the user's info from the Cloud Firestore associated with that UID. After retrieving it, I want to show the first and last name in a label. I'm certain that the retrieval of the info is successful and that I do have two strings (firstName and lastName). When I print them in the console I get precisely what I want. But I don't know why, when I try to show such strings in a label, I get the following error:

Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value: file /Users/fafa/Documents/Advisory Program/Advisory Program/HomeViewController.swift, line 41

I've tried several things, but I can't seem to fix this. Here's part of my code:

LoginViewController.swift

@IBAction func loginTapped(_ sender: Any) {
    
    let error = validateFields()
    
    if error != nil {
        
        showErrorMessage(error!)
        
    } else {
        
        let email = emailTextField.text!.trimmingCharacters(in: .whitespacesAndNewlines)
        let password = passwordTextField.text!.trimmingCharacters(in: .whitespacesAndNewlines)
        
        Auth.auth().signIn(withEmail: email, password: password) { (result, error) in
            
            if error != nil {
                
                self.showErrorMessage(error!.localizedDescription)
            
            } else {
                
                let homeViewController = self.storyboard?.instantiateViewController(withIdentifier: "HomeVC") as? HomeViewController
                self.navigationController?.pushViewController(homeViewController!, animated: true)
                
                homeViewController!.userUID = result!.user.uid
                
            }
        }
    }
}

---

HomeViewController.swift

override func viewDidLoad() {
    super.viewDidLoad()
    
    let db = Firestore.firestore()
    db.collection("teachers").whereField("uid", isEqualTo: userUID).getDocuments { (snapshot, error) in
    
        if let error = error {
                    
            print("Error getting documents: \(error)")
                    
        } else {

            for document in snapshot!.documents {

                // Error gets printed in the following line
                TeacherHeaderCollectionReusableView().teacherNameLabel.text = "\(document.get("firstName") ?? "default value") \(document.get("lastName") ?? "default value")" 

                print("\(document.documentID) => \(document.data())")
            }
        }
    }
}

---

TeacherHeaderCollectionReusableView.swift

class TeacherHeaderCollectionReusableView: UICollectionReusableView {

    @IBOutlet weak var teacherHeaderView: UIView!
    @IBOutlet weak var teacherPhoto: UIImageView!
    @IBOutlet weak var teacherNameLabel: UILabel!

    override func awakeFromNib() {
        super.awakeFromNib()
        
        //teacherHeaderView.backgroundColor = UIColor.init(red: 140/255, green: 3/255, blue: 3/255, alpha: 1)
        let blurEffect = UIBlurEffect(style: .dark)
        let blurEffectView = UIVisualEffectView(effect: blurEffect)
        blurEffectView.frame = teacherHeaderView.bounds
        blurEffectView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        teacherHeaderView.insertSubview(blurEffectView, at: 0)
        teacherHeaderView.layer.cornerRadius = 10.0
        teacherHeaderView.layer.masksToBounds = true
    
        teacherPhoto.layer.cornerRadius =  teacherPhoto.frame.width/2.0
        teacherPhoto.clipsToBounds = true
        teacherNameLabel.textColor = .white
    }

    func configure(image:UIImage, name:String) {
        teacherPhoto.image = image
        teacherNameLabel.text = name
    }

    static func nib() -> UINib {
        return UINib(nibName: "TeacherHeaderCollectionReusableView", bundle: nil)
    }

}

Answer 1

Here the teacherNameLabel of type UILabel is nil . You have to initialize the TeacherHeaderCollectionReusableView from xib to avoid the crash. In this line: TeacherHeaderCollectionReusableView().teacherNameLabel.text you're trying to create a new instance of TeacherHeaderCollectionReusableView by using an empty initializer. Using an empty initializer would not load the subviews that you've created in the interface builder. Here's an example of how you initialize the TeacherHeaderCollectionReusableView properly.

let nib = UINib(nibName: "TeacherHeaderCollectionReusableView", bundle: nil)
let teacherHeaderCollectionReusableView = nib.instantiate(withOwner: nil, options: nil)[0] as? TeacherHeaderCollectionReusableView
teacherHeaderCollectionReusableView?.teacherNameLabel.text = "..."