关于二维码生成，网上也是有很多，很早以前的第三方库大多数都是通过C++写，也是有的如zxing，也是挺好用，这里介绍的是通过CIFilter创建二维码。
创建二维码非常简单，只要传入简单的字符串就好了

- (CIImage *)createQRForString:(NSString *)qrString {

    NSData *stringData = [qrString dataUsingEncoding:NSUTF8StringEncoding];

    // 创建filter

    CIFilter *qrFilter = [CIFilter filterWithName:@"CIQRCodeGenerator"];

    // 设置内容和纠错级别

    [qrFilter setValue:stringData forKey:@"inputMessage"];

    [qrFilter setValue:@"M" forKey:@"inputCorrectionLevel"];

    // 返回CIImage

    return qrFilter.outputImage;

为生成的二维码是一个CIImage，我们直接转换成UIImage的话大小不好控制，所以使用下面方法返回需要大小的UIImage：

- (UIImage *)createNonInterpolatedUIImageFormCIImage:(CIImage *)image withSize:(CGFloat) size {

    CGRect extent = CGRectIntegral(image.extent);

    CGFloat scale = MIN(size/CGRectGetWidth(extent), size/CGRectGetHeight(extent));

   // 1.创建bitmap;

    size_t width = CGRectGetWidth(extent) * scale;

    size_t height = CGRectGetHeight(extent) * scale;

    CGColorSpaceRef cs = CGColorSpaceCreateDeviceGray();

    CGContextRef bitmapRef = CGBitmapContextCreate(nil, width, height, 8, 0, cs, (CGBitmapInfo)kCGImageAlphaNone);

    CIContext *context = [CIContext contextWithOptions:nil];

    CGImageRef bitmapImage = [context createCGImage:image fromRect:extent];

    CGContextSetInterpolationQuality(bitmapRef, kCGInterpolationNone);

    CGContextScaleCTM(bitmapRef, scale, scale);

    CGContextDrawImage(bitmapRef, extent, bitmapImage);

    // 2.保存bitmap到图片

    CGImageRef scaledImage = CGBitmapContextCreateImage(bitmapRef);

    CGContextRelease(bitmapRef); 

    CGImageRelease(bitmapImage);

    //原图

    UIImage *outputImage = [UIImage imageWithCGImage:scaledImage];

    UIGraphicsBeginImageContextWithOptions(outputImage.size, NO, [[UIScreen mainScreen] scale]);

    [outputImage drawInRect:CGRectMake(0,0 , size, size)];

    //水印图

    UIImage *waterimage = [UIImage imageNamed:@"icon"];

    [waterimage drawInRect:CGRectMake((size-waterImagesize)/2.0, (size-waterImagesize)/2.0, waterImagesize, waterImagesize)];

    UIImage *newPic = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return newPic;

}