I started to build a library management system for small organisations. This system doesn't require a pc at all. It will works in any iOS and Android devices. It used a camera in the device as a bar scanner. I used an open source framework ZXing.Net.Mobile to do that. I found when I fire the scanner activity, it got a blank back screen. I initialized the framework

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And I gave the camera permission in AndroidManifest.xml, that still got a blank screen.
After I read the document from ZXing.Net.Mobile, it need the flash light. After I gave the flashlight permission, it works.