How to know when a PDF is rendered using PDFKit - swift

Question

I'm trying to show a loading while a PDF ins't already been shown on the screen. The problem is, my loading always stops before the document is already rendered, sometimes it can't take 2 or 3 seconds and I need to know when the PDF is already rendered to stop the activyIndicator. IS IT possible using PDFKIT? My code:
class PDFViewController: URLSessionDownloadDelegate {
#IBOutlet weak var pdfView: PDFView!
var pdfDocument: PDFDocument!
override func viewDidLoad() {
super.viewDidLoad()
setupPDFView()
setupNavigationBar()
}
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
self.startLoading()
}
override func viewDidAppear(_ animated: Bool) {
super.viewWillAppear(animated)
loadPDF()
}
private func loadPDF() {
DispatchQueue.main.async {
guard let url = URL(string: self.viewModel.pdfURL) else {
self.showAlert(with: self.viewModel.strings.invalidInvoice)
return
}
let urlSession = URLSession(configuration: .default, delegate: self, delegateQueue: OperationQueue())
let downloadTask = urlSession.downloadTask(with: url)
downloadTask.resume()
}
}
private func setupPDFView() {
self.pdfView.displayMode = .singlePageContinuous
self.pdfView.autoScales = true
}
func startLoading() {
guard let window = UIApplication.shared.keyWindow,
!window.subviews.contains(where: { $0 is LoadingView }) else { return }
let loadingView = LoadingView(frame: window.bounds)
window.addSubview(loadingView)
Thread.performUIUpdate {
loadingView.startAnimation()
}
}
func stopLoading() {
guard let window = UIApplication.shared.keyWindow,
let view = window.subviews.first(where: { $0 is LoadingView }),
let loadingView = view as? LoadingView else { return }
Thread.performUIUpdate {
loadingView.stopAnimation()
loadingView.removeFromSuperview()
}
}
func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
DispatchQueue.main.async {
self.pdfDocument = PDFDocument(url: location)
if let pdfDocument = self.pdfDocument {
self.pdfView.document = pdfDocument
}
self?.stopLoading()
}
}
}

Answer 1

This seems like a really bad idea:
func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
DispatchQueue.main.async {
self.pdfDocument = PDFDocument(url: location)
The problem is that while you are getting off the current thread and the asynchronous code starts, the method finishes and the temporary document at location can be destroyed. I would suggest writing like this:
func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
let pdf = PDFDocument(url: location)
DispatchQueue.main.async {
self.pdfDocument = pdf
I'm not saying that that will solve your issue, but it seems a lot less dangerous than what you're doing.
As for your actual issue, I would suggest registering for a notification such as this one and see whether it arrives at the right moment.

Answer 2

You have to observe the PDFViewVisiblePagesChanged notification to know when a PDFView pages are visible, add your code in the method.
NotificationCenter.default.addObserver(self, selector: #selector(pdfViewVisiblePagesChanged(sender:)), name: .PDFViewVisiblePagesChanged, object: nil)
#objc func pdfViewVisiblePagesChanged(sender: Notification) {
//Add your code here
}