Problems with rendering uiview to uiimage

So I set up an UIImageView and a Label, and put them together to a single UIView(as my picture shown here)

And I did the following to render this view to an UIImage, and share it which will bring the UIActivityViewController.

  @IBOutlet var viewBack: UIView!
    @IBOutlet var dismissButton: UIButton!
    @IBOutlet var shareButton: UIButton!
    @IBOutlet var imageView: UIImageView!
    @IBOutlet var hello: UILabel!



    var image = UIImage()
    override func viewDidLoad() {
        super.viewDidLoad()
        shareButton.backgroundColor = UIColor.grayColor()
        shareButton.setTitleColor(UIColor.whiteColor(), forState: .Normal)
        shareButton.layer.cornerRadius = 10
        // Do any additional setup after loading the view.
        dismissButton.backgroundColor = UIColor.grayColor()
        dismissButton.layer.cornerRadius = 10
        viewBack.backgroundColor = UIColor(white: 1, alpha: 0.0)
        UIGraphicsBeginImageContextWithOptions(viewBack.bounds.size, viewBack.opaque, 0.0)
        viewBack.drawViewHierarchyInRect(viewBack.bounds, afterScreenUpdates: false)
        imageView.drawViewHierarchyInRect(imageView.frame, afterScreenUpdates: false)
        hello.drawViewHierarchyInRect(hello.frame, afterScreenUpdates: false)
        let snapshotImageFromMyView = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        print(snapshotImageFromMyView)
        image = snapshotImageFromMyView




    }
    @IBAction func dismiss(sender: AnyObject) {
        dismissViewControllerAnimated(true, completion: nil)
    }

    @IBAction func share(sender: AnyObject) {
        var shareArray : [AnyObject] = []
        shareArray.append(image)
        let activityVC = UIActivityViewController(activityItems: shareArray, applicationActivities: nil)
        if let popover = activityVC.popoverPresentationController{
            popover.sourceView = sender as? UIView
            popover.sourceRect = sender.bounds
        }

        self.presentViewController(activityVC, animated: true, completion: nil)
    }

So viewBack is the UIView contains two subviews- The UIImage and the label. And it seems to be that I set up all the rendering procedure normally. But when I actually share it, the result is really strange, a black picture. Apparently I set my image view to a specific image, and label isn't empty as well. I don't know what's wrong here, hopefully somebody could help me. Thanks in advance.