On-Demand Resources - ODR : AppThinning

ODR_ViewController.swift

// Steps to mark ODR resources
// Select file(image, audio, etc) - File inspector - On Demand Resource Tags - Give it a name

class ODR_ViewController: UIViewController {
    var tubbyRequest : NSBundleResourceRequest?
    @IBOutlet weak var resourceDownloadProgress: UIProgressView! //to show download progress
    
    // fileName is the name of the file's ODR tag
    // if you want to download more than 1 file, change String to [String]
    func fetchODRResourceWithName(fileName: String) {//to be called when the file needs to be accessed
        guard self.tubbyRequest == nil else {return}
        self.tubbyRequest = NSBundleResourceRequest(tags: [fileName])
        self.resourceDownloadProgress.isHidden = false
        self.resourceDownloadProgress.progress = Float((tubbyRequest?.progress.fractionCompleted)!)
        self.tubbyRequest!.progress.addObserver(self, forKeyPath: "fractionCompleted", options: .new, context: nil)
        self.tubbyRequest?.conditionallyBeginAccessingResources(completionHandler: { (resourceAvailable) in
            if !resourceAvailable { //resource not available locally
                self.tubbyRequest!.beginAccessingResources { err in
                    guard err == nil else {//an error occured while downloading
                        print(err as Any)
                        return
                    }
                    DispatchQueue.main.async {//file downloaded
                        self.resourceDownloadProgress.isHidden = true
                        //Do the next steps
                    }
                }
            } else { //resource available locally
                DispatchQueue.main.async {
                    self.resourceDownloadProgress.isHidden = true
                    //DO the next steps
                }
            }
        })
    }
    
    func stopDownloadingFile() {//to be called to cancel on-going download process, or to be called on 'viewWillDisappear' to be sure
        guard self.tubbyRequest != nil else {
            return
        }
        self.tubbyRequest!.endAccessingResources()
        self.tubbyRequest!.progress.removeObserver(self, forKeyPath: "fractionCompleted")
        self.tubbyRequest!.progress.cancel()
        self.tubbyRequest = nil
    }
    
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if keyPath == "fractionCompleted" {
            DispatchQueue.main.async { [weak self] in
                self?.resourceDownloadProgress.progress = Float((self?.tubbyRequest!.progress.fractionCompleted)!)
            }
        }
    }
}

Also, these files can be purged at any time by the system so you'll need to retain it if you expect it to be around when you want to access it later: https://stackoverflow.com/questions/50063305/ios-is-purging-on-demand-resources-downloaded-mp3-how-to-prevent-this