Crash while unwrapping an Optional Value - swift

Question

I am getting an error. It is returning nil on a DispatchQueue call.
DispatchQueue.main.async {
self.headerImageView.image = image // <- CRASH
}
Turns out that this is where I am getting a crash.
headerImageView is just a variable which is not connected to the storyboard via IBOutlet. Here is the entire code:
import UIKit
import WebKit
let offset_HeaderStop:CGFloat = 40.0
let offset_B_LabelHeader:CGFloat = 95.0
let distance_W_LabelHeader:CGFloat = 35.0
class MovieDetailsViewController: UIViewController, UIScrollViewDelegate {
var movie: Movie?
var urlBuilder = MovieUrlBuilder()
var config = TMDBConfiguration()
#IBOutlet weak var scrollView: UIScrollView!
#IBOutlet weak var headerView: UIView!
#IBOutlet weak var headerLabel: UILabel!
#IBOutlet weak var titleLabel: UILabel!
#IBOutlet weak var genreLabel: UILabel!
#IBOutlet weak var ratingLabel: UILabel!
#IBOutlet weak var releaseLabel: UILabel!
#IBOutlet weak var descriptionText: UITextView!
#IBOutlet weak var trailerContainerView: UIView!
#IBOutlet weak var trailerLabel: UILabel!
#IBOutlet weak var videoContainer: WKWebView!
var headerImageView: UIImageView!
override func viewDidLoad() {
super.viewDidLoad()
scrollView.delegate = self
}
override func viewDidAppear(_ animated: Bool) {
headerImageView = UIImageView(frame: headerView.bounds)
headerImageView.image = UIImage(named: "Poster")
headerImageView.contentMode = .scaleAspectFill
headerView.insertSubview(headerImageView, belowSubview: headerLabel)
headerView.clipsToBounds = true
}
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
fetchPopularMovies()
}
func scrollViewDidScroll(_ scrollView: UIScrollView) {
let offset = scrollView.contentOffset.y
var headerTransform = CATransform3DIdentity
if offset < 0 {
let headerScaleFactor: CGFloat = -(offset) / headerView.bounds.height
let headerSizeVariation = ((headerView.bounds.height * (1.0 + headerScaleFactor)) - headerView.bounds.height)/2.0
headerTransform = CATransform3DTranslate(headerTransform, 0, headerSizeVariation, 0)
headerTransform = CATransform3DScale(headerTransform, 1.0 + headerScaleFactor, 1.0 + headerScaleFactor, 0)
headerView.layer.transform = headerTransform
} else {
headerTransform = CATransform3DTranslate(headerTransform, 0, max(-offset_HeaderStop, -offset), 0)
let labelTransform = CATransform3DMakeTranslation(0, max(-distance_W_LabelHeader, offset_B_LabelHeader - offset), 0)
headerLabel.layer.transform = labelTransform
}
headerView.layer.transform = headerTransform
}
override var preferredStatusBarStyle: UIStatusBarStyle{
return UIStatusBarStyle.lightContent
}
private func fetchPopularMovies() {
if let poster = movie?.backdropPath {
let baseURL = URL(string: config.baseImageURL)!
let url = baseURL.appendingPathComponent("w500").appendingPathComponent(poster)
let request = URLRequest(url: url)
let task = urlBuilder.session.dataTask(with: request) {(data, response, error) in
do {
if let image = UIImage(data: data!) {
DispatchQueue.main.async {
self.headerImageView.image = image // <- CRASH
}
}
}
}
task.resume()
}
}
}
Thank you for the support!

Answer 1

You are initializing the image view in viewDidAppear which will be called after viewWillAppear where the image view is populated. Despite the asynchronous task it's not guaranteed that the view is initialized before being populated.
I recommend to initialize the image view lazily, the benefit is that the property is non-optional and it's initialized once when it's accessed the first time.
lazy var headerImageView: UIImageView = {
let view = UIImageView(frame: self.headerView.bounds)
view.image = UIImage(named: "Poster")
view.contentMode = .scaleAspectFill
self.headerView.insertSubview(view, belowSubview: self.headerLabel)
view.clipsToBounds = true
return view
}()

Answer 2

Your async call is located in fetchPopularMovies, which is called at viewWillAppear. You initialise your image view in viewDidAppear, which is called after viewWillAppear.
Basically, headerImageView is nil.
"But I put the call in an async block! So viewDidAppear should have been called already when I set the image view's image, right?" you said.
Well, relying on async to say "I want this thing to run after a certain method returns" is not a good idea. It is not always guaranteed to do that.
To fix this, you need to initialise header image view before fetchPopularMovies is called. You can put it in viewDidLoad for example.