title: Assign unique behaviour for two-finger swipe on scrollview
body: <p>I would like to assign a unique behaviour for two-finger swipe on WKWebView.
  Referring to <a href="https://www.cocoanetics.com/2010/06/hacking-uiscrollview-gesture-recognizers/" rel="nofollow">this site,</a> I wrote codes as below. It worked, but scrolling on webview got extremely slow.</p>
  
  <p>Is there any better way to do it by avoiding slow scrolling?</p>
  
  <pre><code>    let doubleSwipeGestureRecognizer = UISwipeGestureRecognizer.init(target: self, action: "doubleSwiped2")
      doubleSwipeGestureRecognizer.numberOfTouchesRequired = 2
      doubleSwipeGestureRecognizer.direction = UISwipeGestureRecognizerDirection.Up
      self.webView!.addGestureRecognizer(doubleSwipeGestureRecognizer)
  
      for gesture in self.webView!.scrollView.gestureRecognizers!{
          let gestureClass = gesture.classForCoder
          let gestureName = NSStringFromClass(gestureClass)
          print(gestureName)
  
          if gestureName.containsString("Swipe"){
              // do nothing
          } else {
              gesture.requireGestureRecognizerToFail(doubleSwipeGestureRecognizer)
  
          }
      }
  </code></pre>