CIDetectorTypeQRCode类型的CIDetector未正确实例化

Question

我尝试使用以下代码扫描图像中的QR码。 基本上我的问题是“prepareQRCodeDetector”返回nil（至少在iPhone 5s上，还没有在另一台设备上测试）

请注意，CIDetectorTypeFace返回一个正常工作的CIDetector。

在文档中，据说该功能可从iOS 8获得

    //iOS CoreImage QR Detector
- (CIDetector *) prepareQRCodeDetector {
    return [CIDetector detectorOfType:CIDetectorTypeQRCode context:nil options:@{CIDetectorAccuracy: CIDetectorAccuracyHigh}];
}

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingImage:(UIImage *)image editingInfo:(NSDictionary *)editingInfo {
    if (image != nil){
        //AVFoundation QRCode detection
        CIImage *ciImage = [[CIImage alloc] initWithCGImage:image.CGImage options:nil];
        NSString * decodedAccountName = @"";
        CIDetector * detector = [self prepareQRCodeDetector];
        if (detector != nil) {
            NSArray<CIFeature*> *features = [detector featuresInImage:ciImage];
            for (id feature in features) {
                decodedAccountName = ((CIQRCodeFeature *)feature).messageString;
            }
            [self openUserProfileByAccountName:decodedAccountName];
        }
    }
    [picker dismissViewControllerAnimated:YES completion:nil];
}