UIScrollView已經定義了捏合和移動的手勢，正常的操作來說，還會使用雙擊圖片進行縮放，這需要我們自己收動在UIScrollView加上點擊的手勢

1.添加手勢

        let doubleTapGesture = UITapGestureRecognizer(target: self,action: #selector(FLCameraFilterTopView.handleDoubleTap(_:)))
       doubleTapGesture.numberOfTapsRequired = 2
       scrollView.addGestureRecognizer(doubleTapGesture)

2.處理點擊手勢

    func handleDoubleTap(tap:UITapGestureRecognizer) {
        
        //獲取當前縮放值
        var newScale = scrollView.zoomScale
        
        if scrollView.zoomScale == 1 {
            newScale = 0.8
        }else {
            newScale = 1
        }
        
        //獲取要縮放的大小和起始位置，這里都是居中
        let zoomRect = self.zoomRectForScale(newScale, center: CGPointMake(FL.ScreenWidth/2,FL.ScreenWidth/2))

        scrollView.zoomToRect(zoomRect, animated: true)
    }
    
    func zoomRectForScale(scale:CGFloat,center:CGPoint) -> CGRect {
        
        var zoomRect = CGRectZero
        
        zoomRect.size.height = scrollView.frame.size.height / scale
        zoomRect.size.width = scrollView.frame.size.width / scale
        zoomRect.origin.x = center.x - zoomRect.size.width/2.0
        zoomRect.origin.y = center.y - zoomRect.size.height/2.0
        
        return zoomRect
    }
```
3. UIScrollViewDelegate 里設置

```
    func scrollViewDidEndZooming(scrollView: UIScrollView, withView view: UIView?, atScale scale: CGFloat) {
        //保證下一次的縮放在前次縮放的基礎上
        scrollView.setZoomScale(scale, animated: true)
    }
    
```