// 1.创建一个滤镜
CIFilter *filter = [CIFilter filterWithName:@"CIQRCodeGenerator"];
    
// 2.设置默认的属性
[filter setDefaults];
    
// 3.给滤镜设置数据
NSString *string = @"http://www.baidu.com";
NSData *data = [string dataUsingEncoding:NSUTF8StringEncoding];
[filter setValue:data forKeyPath:@"inputMessage"];
    
// 4.获取出输出的数据
CIImage *outputImage = [filter outputImage];
    
// 5.设置到imageView上面
// self.imageView.image = [UIImage imageWithCIImage:outputImage];
   self.imageView.image = [self createNonInterpolatedUIImageFormCIImage:outputImage withSize:200];

第5步骤中有一方法是为了得到高清的二维码图片。所用的方法
/**
 *  根据CIImage生成指定大小的UIImage
 *
 *  @param image CIImage
 *  @param size  图片宽度
 */
- (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);
    return [UIImage imageWithCGImage:scaledImage];
}