加水印

- (void)viewDidLoad {
    [super viewDidLoad];
    
    UIImage *image = [self drawImageAtImageContext];
    UIImageView *imageView = [[UIImageView alloc]initWithImage:image];
    imageView.center = CGPointMake(160, 282);
    [self.view addSubview:imageView];
    
}
//添加水印
-(UIImage *)drawImageAtImageContext{
    //获得一个位图图形上下文
    CGSize size = CGSizeMake(300, 188);//画布大小
    
    UIGraphicsBeginImageContext(size);
    
    UIImage *image = [UIImage imageNamed:@"frame_shop_lovely_bg@2x.jpg"];
    //注意绘图的位置是相对于画布顶点而言，不是屏幕
    [image drawInRect:CGRectMake(0, 0, 300, 188)];
    //添加水印
    CGContextRef ref = UIGraphicsGetCurrentContext();
    //字符的长度和 font
    UIFont *font = [UIFont systemFontOfSize:15];
    NSString *str = @"大金毛";
    int strLength = str.length *font.pointSize;
    //两点确定一条直线
    //下划线的长度等于字符长度
    CGContextMoveToPoint(ref, 200, 178);
    CGContextAddLineToPoint(ref, strLength + 200, 178);
    //直线的颜色、宽度
    [[UIColor redColor]setStroke];
    CGContextSetLineWidth(ref, 2);
    //绘制图像到指定图形上下文，只有边框
    CGContextDrawPath(ref, kCGPathStroke);
    //字符的位置
    [str drawInRect:CGRectMake(200, 158, 100, 30) withAttributes:@{NSFontAttributeName:font,NSForegroundColorAttributeName:[UIColor redColor]}];
    //返回绘制的新图形
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    //最后一定不要忘了关闭上下文
    UIGraphicsEndImageContext();
    
    return newImage;
}