Objective-C修改图片尺寸/生成缩略图
+(UIImage *)imageResize :(UIImage*)img andResizeTo:(CGSize)newSize
{
    CGFloat scale = [[UIScreen mainScreen]scale];
    //UIGraphicsBeginImageContext(newSize);
    UIGraphicsBeginImageContextWithOptions(newSize, NO, scale);
    [img drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];
    UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return newImage;
}                
