从firebase存储回来的url是nil Swift4

Question

I'm trying to upload an image to firebase storage and get the download url to save it in database, but I get a nil value and the function returns upon checking it. I followed the documentation and solutions from other posts and I can't see where I'm mistaking. The function is :

func uploadImage(completed: @escaping (Bool) -> (),_ image: UIImage ){
        print("      ##############         UPLOAD STARTED         ###########")

        //        let stringURL: String?
//        guard let uid = Auth.auth().currentUser?.uid else {return} // use the userUid to sign the alert


        // Create a root reference
        let storageRef = Storage.storage().reference()

        // Create a reference to "mountains.jpg"
//        let alertsRef = storageRef.child("userAlertImages.jpg")//("user/\(uid)") // change path for userAlertImages path

        // Create a reference to 'images/mountains.jpg'
        let alertsImagesRef = storageRef.child("Alert Images/userAlertImages.jpg")

        // While the file names are the same, the references point to different files
//        alertsRef.name == alertsImagesRef.name;            // true
//        alertsRef.fullPath == alertsImagesRef.fullPath;    // false


        let imageData = UIImageJPEGRepresentation(image, 0.5)

        let metaData = StorageMetadata()
        metaData.contentType = " jpeg " // data type
        metaData.customMetadata = ["k1": "",
                                   "k2" : " ",
                                   "k3" : "",
                                   "k4" : ""]

        alertsImagesRef.putData(imageData! as Data , metadata: metaData) { metaData, error in
            if(error != nil){
                print(error as Any)
                return
            }
        }

        // Fetch the download URL
        alertsImagesRef.downloadURL { (url,error) in
            guard let downloadURL = url else {
                print("@@@@@@@@@@ downloaded url is: \(url)  @@@@@@@@@@@@@")
                return
            }
            NewMapViewController.alertImageURL = (url?.absoluteString) ?? ""
//            NewMapViewController.alertImageURL = (downloadURL)

            print("######### url is:\(String(describing: url)) #########")
            completed(true)
            //                            self.postAlertNotification()
            self.tapCounter = 0
            self.performSegue(withIdentifier: "chooseIconSegue", sender: self)

        }

}

can you see where's the error? Many thanks.

Answer 1

Thinking about it I realised that downloadURL fetch was actually done before than the image upload was complete, that's because Firebase is asynchronous. So I added a completion block to the uploading part, and in the completion scope I put the downloadURL fetching part. It's not blinking eyes fast and I will appreciate any suggestion on speeding up things, because this way there is a little lag before the segue gets performed. It's not annoying at all an I could, and probably should, add a little spinning wheel to show users that the app has not frozen, but I rather just avoid the lag altogether if possible. I left the prints written in hope that this post will help others new to Firebase as I am, giving detailed almost step-by-step guidance, as this kind of answers really helped me before, and I haven't found any on the subject. Rewritten function is:

func uploadImage(completed: @escaping (Bool) -> (),_ image: UIImage ){
        print("      ##############         UPLOAD STARTED         ###########")

        // Create a root reference
        let storageRef = Storage.storage().reference()


        // Create a reference to Images folder
        let alertsImagesRef = storageRef.child("Alert Images")
        // Create a reference for new images
        let uuid = UUID()
        let imageRef = alertsImagesRef.child("userAlertImage \(uuid).jpg")

        let imageData = UIImageJPEGRepresentation(image, 0.5)

        let metaData = StorageMetadata()
        metaData.contentType = " jpeg " // data type
        metaData.customMetadata = ["k1": "",
                                   "k2" : " ",
                                   "k3" : "",
                                   "k4" : ""]

        imageRef.putData(imageData! as Data , metadata: metaData, completion: { metaData, error in
            if(error != nil){
                print(error as Any)
                return
            }
            print("               ####### image uploaded #######")
            self.tapCounter = 0
            self.performSegue(withIdentifier: "chooseIconSegue", sender: self)
            // fetch url v2
            imageRef.downloadURL(completion: { (url, err) in
                if let err = err {
                    print("Error downloading image file, \(err.localizedDescription)")
                    return
                }

                guard let url = url else { return }
                //Now you have the download URL for the image, and can do whatever you want with it.
                NewMapViewController.alertImageURL = url.absoluteString

                print("       ######### url is:\(String(describing: url)) #########")
                completed(true)
                //                            self.postAlertNotification()
//                self.tapCounter = 0
//                self.performSegue(withIdentifier: "chooseIconSegue", sender: self)
                print("      ##############         UPLOAD ENDED         ###########")
            })
        })
    }