我无法在iPhone 4中扫描QR码

Question

以下代码已经在iPhone 5c，5s和iPhone 6中使用IOS 7和IOS 8，但在iPhone 4中无效，

AVCaptureMetadataOutput *captureMetadataOutput = [[AVCaptureMetadataOutput alloc] init];

[_captureSession addOutput:captureMetadataOutput];

dispatch_queue_t dispatchQueue;
dispatchQueue = dispatch_queue_create("myQueue", NULL);
[captureMetadataOutput setMetadataObjectsDelegate:self queue:dispatchQueue];
[captureMetadataOutput setMetadataObjectTypes:[NSArray  arrayWithObject:AVMetadataObjectTypeQRCode]];

_videoPreviewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:_captureSession];

[_videoPreviewLayer setVideoGravity:AVLayerVideoGravityResizeAspectFill];
[_videoPreviewLayer setFrame:_viewPreview.bounds];
[_viewPreview.layer addSublayer:_videoPreviewLayer];
// Start video capture.
[_captureSession startRunning];

在iPhone 4中不调用didOutputMetadataObjects方法