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;
}