         showImageView.addGestureRecognizer(singleTap)
         
         showImageView.isUserInteractionEnabled = true
+        showImageView.backgroundColor = UIColor.red
         
         mainScrollView.delegate = self
         mainScrollView.maximumZoomScale = 3.0

        
        // 双击：如果未放大，则放大两倍；否则，缩小为原图大小
        if self.mainScrollView.zoomScale == 1.0 {
//            print("imageHeight is \(self.imageHeight)")
//            print("imageViewHeight is \(self.showImageView.frame.size.height)")
//            print("touchCenter is \(touchCenter.y)")
            
            let touchCenter = tap.location(in: tap.view)
            

            return CGPoint(x: 0, y: 0)
        }
        
        let actualImageY = (self.showImageView.bounds.size.height - actualImageHeight) / 2
        
        // 双击区域在图片上方空白处，需作调整，调整touchCenter数值
        if center.y > 0 && center.y < actualImageY {
            center.y = self.showImageView.bounds.size.height / 2
        }
        
        let actualImageMaxY = (self.showImageView.bounds.size.height + actualImageHeight) / 2
        // 双击区域在图片下方空白处，需作调整，调整touchCenter数值
        if center.y > actualImageMaxY && center.y < self.showImageView.bounds.size.height {
            center.y = self.showImageView.bounds.size.height / 2
        }
        
