swift4.0 图片放缩

图片放缩原理采用在UIScrollView上面添加UIImageView。UIScrollView的具体用法可参见：UIScrollView的用法。这篇文章写的很详细。

图片放缩的重点
1.控制图片放缩后展示的frame
2.为了完整的显示图片需要计算图片的大小。
3.加载网络图片

展示图

1.计算frame

  /// 按照比例更改 ContentSize
     func changegContentSize(zoomScale:CGFloat){
        self.zoomScale = zoomScale
        let horizontalPadding = self.imageView.frame.width < self.frame.width ? (self.frame.size.width - self.imageView.frame.size.width)/2.0 : 0.0
        let verticalPadding = self.imageView.frame.height < self.frame.height ? (self.frame.size.height - self.imageView.frame.size.height)/2.0 : 0.0
        let imageScaleWidth = zoomScale * self.normalWidth
        let imageScaleHeight = zoomScale * self.normalHeight
        
        var padding:CGFloat = 0.0
        if imageScaleWidth < self.frame.width{
            padding = (self.frame.width - imageScaleWidth)/2.0
            let paddingY = (self.frame.height - imageScaleHeight)/2.0
            
            if imageScaleHeight < self.frame.height{
                self.imageView.frame = CGRect.init(x: padding, y: paddingY, width: imageScaleWidth, height: imageScaleHeight)
            }else{
                self.imageView.frame = CGRect.init(x: padding, y: verticalPadding, width: imageScaleWidth, height: imageScaleHeight)
            }
            
            
        }else{
            self.imageView.frame = CGRect.init(x: horizontalPadding , y: verticalPadding, width: imageScaleWidth, height: imageScaleHeight)
        }
        
        
    }

2.计算图片大小

extension UIImage{
    
    /// 获取
    ///
    /// - Parameter view: 展示的view
    /// - Returns: 图片大小
    func getImageSize(view:UIView) -> CGSize {
        var size = CGSize.zero
        
        let sw = view.bounds.size.width
        let sh = view.bounds.size.height
        
        
        if self.size.width <= sw && self.size.height <= sh{
            return self.size
        }else if self.size.width > sw{
            size.width = sw
            size.height = (self.size.height/self.size.width) * sw
        }else if self.size.height > sh{
            size.height = sh
            size.width = (self.size.width/self.size.height) * sh
        }
        return size
    }
}

3.加载网络图片我采用的是
let data = NSData.init(contentsOf: URL.init(string: url!)!)
优点：方便快捷
缺点：当数据过多或者图片过大时加载很慢

优化方案1
利用多线程加载

                self.createImagView(image: UIImage.init(named: "datadpload_photos") ?? UIImage())
                DispatchQueue.global().async {
                    let data = NSData.init(contentsOf: URL.init(string: self.url!)!)
                    if data == nil{
                        DispatchQueue.main.async {
                          self.createImagView(image: UIImage.init(named: "datadpload_photos") ?? UIImage())
                           self.setZoomScale()
                        }


                        return
                    }
                    let image = UIImage.init(data: data! as Data)

                    if image == nil{
                        DispatchQueue.main.async {
                            self.createImagView(image: UIImage.init(named: "datadpload_photos") ?? UIImage())
                            self.setZoomScale()
                        }
                        return
                    }
                    DispatchQueue.main.async {
                        self.createImagView(image: image!)
                        self.setZoomScale()
                    }
                }
                

优化方案2
利用SDWebImage

  self.imageView?.sd_setImage(with: URL.init(string: self.url!)!, placeholderImage: UIImage.init(named: "datadpload_photos"), options: [], completed: { (image1, error, cacheMemoryOnly, url) in
                if error == nil{
                    let size = image1?.getImageSize(view:self) ?? CGSize.zero
                    self.imageView?.bounds = CGRect.init(x: 0, y: 0, width: size.width, height: size.height)
                    self.imageView?.center = CGPoint.init(x: self.bounds.size.width/2 , y: self.bounds.size.height/2)
                    
                    self.imageView?.image = image1
                    
                    self.contentSize = self.imageView?.bounds.size ?? CGSize.zero
                    
                    self.normalWidth = self.contentSize.width
                    self.normalHeight = self.contentSize.height
                }
            })

demo下载地址:demo