AVAssetExportSession.requestExportSession callback never called (swift 3, iOS10)
<p>The following code never calls the callback for export.  The export session is created just fine.  I see no error, and no progress.  The CPU is 0%.  I see no exceptions.  Status is 1 (in progress), the progress is 0, the error is nil.  The video plays in the gallery.  I can successfully get an image for the video.  I have extracted the code into a single UIViewController for testing, see below:</p>

<p>I tested using iOS10.1.1 with a video I recorded on the iPad.</p>

<pre><code>import UIKit
import Photos

class ViewController: UIViewController {

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

// MARK: Properties
@IBOutlet weak var button: UIButton!

// MARK: Actions
@IBAction func onPress(_ sender: UIButton) {
    requestGalleryPermission() {
        (_ hasPermission: Bool) in
        if hasPermission {
            // fetch the asset from Photos
            let fetchResult = PHAsset.fetchAssets(withLocalIdentifiers: ["9FFAD8B5-0941-4A95-830F-4ACFA563B71B/L0/001"], options: nil)

            // if we successfully fetched the asset...
            if let asset = fetchResult.firstObject {

                self.exportAsset(asset)
            }
        }
    }
}

func exportAsset(_ asset: PHAsset) {
    let tempFilename = "full_sized_image.mov"
    var tempURL = URL(fileURLWithPath: NSTemporaryDirectory(), isDirectory: true).appendingPathComponent(tempFilename)
    tempURL = tempURL.standardizedFileURL


    let options = PHVideoRequestOptions()
    options.deliveryMode = .highQualityFormat
    options.isNetworkAccessAllowed = true

    // remove any existing file at that location
    do {
        try FileManager.default.removeItem(at: tempURL)
    }
    catch {
        // most likely, the file didn't exist.  Don't sweat it
    }

    PHImageManager.default().requestExportSession(forVideo: asset, options: options, exportPreset: AVAssetExportPresetHighestQuality) {
        (exportSession: AVAssetExportSession?, _) in

        if exportSession == nil {
            print("COULD NOT CREATE EXPORT SESSION")
            return
        }

        exportSession!.outputURL = tempURL
        exportSession!.outputFileType = AVFileTypeQuickTimeMovie

        print("GOT EXPORT SESSION")
        exportSession!.exportAsynchronously() {
            print("COMPLETION HANDLER!!!!")
        }

        print("progress: \(exportSession!.progress)")
        print("error: \(exportSession!.error)")
        print("status: \(exportSession!.status.rawValue)")
    }
}

func requestGalleryPermission(_ completionHandler: @escaping (_ hasPermission: Bool) -&gt; Void) {

    let authorizationStatus = PHPhotoLibrary.authorizationStatus()
    if authorizationStatus == .denied || authorizationStatus == .restricted {
        completionHandler(false)
    }
    else if authorizationStatus == .authorized {
        completionHandler(true)
    }
    else {
        // ask for authorization
        PHPhotoLibrary.requestAuthorization({ (status: PHAuthorizationStatus) in

            // if the user gave us authorization...
            if status == .authorized {

                print("User gave authorization")

                completionHandler(true)
            }
            else {
                print("User denied authorization")

                completionHandler(false)
            }
        })
    }
}
}
</code></pre>