UIImage创建水印的分类方法

+ (instancetype)waterImageWithBg:(NSString *)bg logo:(NSString *)logo
{
    UIImage *bgImage = [UIImage imageNamed:bg];
   
    // 1.创建一个基于位图的上下文(开启一个基于位图的上下文)
    UIGraphicsBeginImageContextWithOptions(bgImage.size, NO, 0.0);
   
    // 2.画背景
    [bgImage drawInRect:CGRectMake(0, 0, bgImage.size.width, bgImage.size.height)];
   
    // 3.画右下角的水印
    UIImage *waterImage = [UIImage imageNamed:logo];
    CGFloat scale = 0.2;
    CGFloat margin = 5;
    CGFloat waterW = waterImage.size.width * scale;
    CGFloat waterH = waterImage.size.height * scale;
    CGFloat waterX = bgImage.size.width - waterW - margin;
    CGFloat waterY = bgImage.size.height - waterH - margin;
    [waterImage drawInRect:CGRectMake(waterX, waterY, waterW, waterH)];
   
    // 4.从上下文中取得制作完毕的UIImage对象
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
   
    // 5.结束上下文
    UIGraphicsEndImageContext();
   
    return newImage;
}