Double Tap to Zoom In/Out

I have an app that displays a bunch of pictures when the user presses the button from a text list view screen. I have a specific subclass that allows the user to pinch and zoom in, but I am curious as to what code I need to enter in my particular swift file to allow the user to double tap to zoom in and out. I get three errors using this code. Two say "Value of type 'ZoomingScrollView' has no member 'scrollview' or 'imageview' " and also " 'CGRectZero' is unavaiable in Swift" My code for the subclass that allows the user to zoom in is below along with the code I am using to double tap to zoom.

import UIKit

class ZoomingScrollView: UIScrollView, UIScrollViewDelegate {

 @IBOutlet weak var viewForZooming: UIView? = nil {
didSet {
    self.delegate = self
} }

func viewForZoomingInScrollView(scrollView: UIScrollView) -> UIView? {
return viewForZooming }

@IBAction func userDoubleTappedScrollview(recognizer:  UITapGestureRecognizer) {
if let scrollV = self.scrollview {
    if (scrollV.zoomScale > scrollV.minimumZoomScale) {
        scrollV.setZoomScale(scrollV.minimumZoomScale, animated: true)
    }
    else {
        //(I divide by 3.0 since I don't wan't to zoom to the max upon the double tap)
        let zoomRect = self.zoomRectForScale(scrollV.maximumZoomScale / 3.0, center: recognizer.locationInView(recognizer.view))
        self.scrollview?.zoomToRect(zoomRect, animated: true)
    }
} }

func zoomRectForScale(scale : CGFloat, center : CGPoint) -> CGRect {
var zoomRect = CGRectZero
if let imageV = self.imageView {
    zoomRect.size.height = imageV.frame.size.height / scale;
    zoomRect.size.width  = imageV.frame.size.width  / scale;
    let newCenter = imageV.convertPoint(center, fromView: self.scrollview)
    zoomRect.origin.x = newCenter.x - ((zoomRect.size.width / 2.0));
    zoomRect.origin.y = newCenter.y - ((zoomRect.size.height / 2.0));
}
return zoomRect; }}