作者:Mitchell 

• 1、获取图片

    UIImage*image = [UIImage imageNamed:@"imageName"];

• 2、获取位图上下文

    UIGraphicsBeginImageContext(CGSizeMake(self.view.frame.size.width, self.view.frame.size.height));

• 3、绘制图片

    [image drawAtPoint:CGPointMake(0, 0)];

• 4、绘制文字

    NSString*str = @"Mitchell";
    [str drawAtPoint:CGPointMake(10, 10) withAttributes:@{NSForegroundColorAttributeName:[UIColor magentaColor],NSFontAttributeName:[UIFont systemFontOfSize:40]}];

• 5、生成图片

    image = UIGraphicsGetImageFromCurrentImageContext();

• 6、关闭上下文

    UIGraphicsEndImageContext();

• 7、获取图片数据

    NSData *data = UIImagePNGRepresentation(image);

• 8、保存图片

    _img = [UIImage imageWithData:data];