CODEWITHSUNDEEP
androidusb
Gianpaolo
Gianpaolo

Reputation: 95

Android USB connection established only the second time the app is opened

For my application, I need to establish a connection with an attached Arduino device, here is the code:

public String openConnection(UsbManager manager, Context context) {

    // getting the driver with an external library...

    UsbSerialDriver driver = availableDrivers.get(0);

    PendingIntent pi = PendingIntent.getBroadcast(context, 0, new Intent(ACTION_USB_PERMISSION), 0);
    manager.requestPermission(driver.getDevice(), pi);

    UsbDeviceConnection connection = manager.openDevice(driver.getDevice());

    if (connection == null) {
        return "Found a device, but cannot connect";
    }

    // otherwise, continue and do stuff
}

The problem is that, when a device is attached, the first time I open the app it shows the alert asking for the permission, but if I click "OK", the connection is null, so it returns early. However, the second time it does not ask for any permission but the connection is opened and everything works fine.

Why does this happen?

I know this is not the most correct approach to open an USB connection, but I have other issues that are not inherent to the question, so I'm rather intrested to understand why does this happen than what should I do instead

I'm testing this on Android 8.1.0

Upvotes: 1

Views: 1087

Answers (1)

Alessio Luciani
Alessio Luciani

Reputation: 56

Try to previously ask for permission and start the rest of your code from a Broadcast Receiver that listens for the USB permission granted. This is showed in Google's docs:

private static final String ACTION_USB_PERMISSION =
"com.android.example.USB_PERMISSION";
private final BroadcastReceiver mUsbReceiver = new BroadcastReceiver() {

public void onReceive(Context context, Intent intent) {
    String action = intent.getAction();
    if (ACTION_USB_PERMISSION.equals(action)) {
        synchronized (this) {
            UsbDevice device = (UsbDevice)intent.getParcelableExtra(UsbManager.EXTRA_DEVICE);

            if (intent.getBooleanExtra(UsbManager.EXTRA_PERMISSION_GRANTED, false)) {
                if(device != null){
                  //call method to set up device communication
               }
            }
            else {
                Log.d(TAG, "permission denied for device " + device);
            }
        }
    }
}
};

Here's how you register the Broadcast Receiver:

UsbManager mUsbManager = (UsbManager) 
getSystemService(Context.USB_SERVICE);
private static final String ACTION_USB_PERMISSION =
"com.android.example.USB_PERMISSION";
...
mPermissionIntent = PendingIntent.getBroadcast(this, 0, new 
Intent(ACTION_USB_PERMISSION), 0);
IntentFilter filter = new IntentFilter(ACTION_USB_PERMISSION);
registerReceiver(mUsbReceiver, filter);

And then you start it all with:

UsbDevice device;
...
mUsbManager.requestPermission(device, mPermissionIntent);

In the way you do it, the device tries to connect to USB even before the permission has been granted and so it fails.

Upvotes: 1

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