图片缓存 压缩处理

import UIKit

extension UIImage{
//根据传入的宽度生成一张按照宽高比压缩的新图片
func imageWithScale(width:CGFloat) -> UIImage{
   //1.根据 宽度 计算高度
    let height = width * size.height / size.width
  //2.按照宽高比绘制一张新的图片
    let currentSize = CGSize.init(width: width, height: height)
    UIGraphicsBeginImageContext(currentSize)  //开始绘制
     draw(in: CGRect.init(origin: CGPoint.zero, size: currentSize))
    let newImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()  //结束上下文
    return newImage!
}
}