title: AVCaptureSession scan QRCode in specific frame
body: <p>What I need todo is make a Scanner to get the QRCode values.</p>
  
  <p>And I followed the Apple developer documents, use AVCaptureDevice, AVCaptureSession, AVCaptureDeviceInput, AVCaptureVideoPreviewLayer, AVCaptureMetadataOutput to get it works.</p>
  
  <pre><code>_videoDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
  </code></pre>
  
  <p>Currently, I get the qrcode successfully, but if there are two or more qrcodes in the camera, we will get several qrcode, so I just want to scan the specific frame on the screen, for example CGRectMake{100, 100, 200, 200}, to make sure there is only one qrcode once we processed.</p>
  
  <p>So, how can we specify the frame we want in the  AVCaptureDeviceInput.</p>
  
  <p>Thanks a lot!</p>