iOS中图片压缩方法

ios中系统为我们提供了两种压缩图片的方法:
UIImagePNGRepresentation(UIImage*__nonnullimage);// return image as PNG. May return nil if image has no CGImageRef or invalid bitmap format]

UIImageJPEGRepresentation(UIImage*__nonnullimage,CGFloatcompressionQuality);//return image as JPEG. May return nil if image has no CGImageRef or invalid bitmap format. compression is 0(most)..1(least)

UIImageJPEGRepresentation:方法需要两个参数:图片的引用和压缩系数;
UIImagePNGRepresentation :方法仅仅需要一个参数:图片的引用;
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UIImagePNGRepresentation:耗时更多,图片数据量更大,读取照片数据的时候可能会照成卡顿;
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两者返回的都是NSData类型;

在实际开发中,可能会遇到从手机相册中选择图片的需求,选择图片这个过程是一个消耗性能的过程,取决于手机图片的大小,如果手机像素非常高,图片的尺寸非常大,这个时候就会变得很消耗性能;
以上两种方法虽然都可能达到压缩图片的需求,但是可能会照成图片的失真;
下面提供另一种方法:根据图片从新绘制一张出来,不会失真,只是改变图片大小,比如之前是50005000的分辨率,这个时候你压缩后可能就只有500500,不会失真,只会改变大小,建议使用;

- (UIimage *)imageWithImage:(UIImage*)image
          scaledToSize:(CGSize)newSize;
{
    UIGraphicsBeginImageContext(newSize);
    [image drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];
    UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return newImage;
}

Swift版:
func useImage(image: UIImage) -> NSData {
        //实现等比例缩放
        let hfactor = image.size.width / screnWidth;
        let vfactor = image.size.height / screnHeight;
        let factor = fmax(hfactor, vfactor);
        //画布大小
        let newWith: CGFloat = image.size.width / factor
        let newHeigth: CGFloat = image.size.height / factor
        let newSize = CGSize(width: newWith, height: newHeigth)
        
        UIGraphicsBeginImageContext(newSize)
        image.drawInRect(CGRect(x: 0, y: 0, width: newWith, height: newHeigth))
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        //图像压缩
        let newImageData = UIImageJPEGRepresentation(newImage, 0.5)
        return newImageData!
    }