Android Bluetooth app can't discover other devices

Question

I am a beginner in android, and tried a demo code Bluetoothchat in https://developer.android.com/samples/BluetoothChat/index.html. It doesn't work well on my phone(Nexus 5, android 6.0.1). the full package is on github. https://github.com/googlesamples/android-BluetoothChat. I didn't change anything in the demo. Android studio 2.1.2

It failed to discover other Bluetooth devices(an ipad and a blackberry). The visibility of the devices were on. If I use the default Bluetooth's discovery function on the nexus 5, the two devices were showed on the list.

I have also tried some other demos about Bluetooth discovery app. They did not work on my phone either. I know the basic idea about how to find a device, but couldn't find why it didn't work.

The relative code in Bluetoothchat is here:

basic setup

    mNewDevicesArrayAdapter = new ArrayAdapter<String>(this, R.layout.device_name);
    // Find and set up the ListView for newly discovered devices
    ListView newDevicesListView = (ListView) findViewById(R.id.new_devices);
    newDevicesListView.setAdapter(mNewDevicesArrayAdapter);
    newDevicesListView.setOnItemClickListener(mDeviceClickListener);

    // Register for broadcasts when a device is discovered
    IntentFilter filter = new IntentFilter(BluetoothDevice.ACTION_FOUND);
    this.registerReceiver(mReceiver, filter);

    // Register for broadcasts when discovery has finished
    filter = new IntentFilter(BluetoothAdapter.ACTION_DISCOVERY_FINISHED);
    this.registerReceiver(mReceiver, filter);

    // Get the local Bluetooth adapter
    mBtAdapter = BluetoothAdapter.getDefaultAdapter();

click the scanButton to scan devices

    // Initialize the button to perform device discovery
    Button scanButton = (Button) findViewById(R.id.button_scan);
    scanButton.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            doDiscovery();
            v.setVisibility(View.GONE);
        }
    });

    // Initialize array adapters. 
    // one for newly discovered devices

discovery part

/**
 * Start device discover with the BluetoothAdapter
 */
private void doDiscovery() {
    Log.d(TAG, "doDiscovery()");

    // Indicate scanning in the title
    setProgressBarIndeterminateVisibility(true);
    setTitle(R.string.scanning);

    // Turn on sub-title for new devices
    findViewById(R.id.title_new_devices).setVisibility(View.VISIBLE);

    // If we're already discovering, stop it
    if (mBtAdapter.isDiscovering()) {
        mBtAdapter.cancelDiscovery();
    }

    // Request discover from BluetoothAdapter
    mBtAdapter.startDiscovery();
}

Broadcast receiver

/**
 * The BroadcastReceiver that listens for discovered devices and changes the title when
 * discovery is finished
 */
private final BroadcastReceiver mReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();

        // When discovery finds a device
        if (BluetoothDevice.ACTION_FOUND.equals(action)) {
            // Get the BluetoothDevice object from the Intent
            BluetoothDevice device = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);
            // If it's already paired, skip it, because it's been listed already
            if (device.getBondState() != BluetoothDevice.BOND_BONDED) {
                mNewDevicesArrayAdapter.add(device.getName() + "\n" + device.getAddress());
            }
            // When discovery is finished, change the Activity title
        } else if (BluetoothAdapter.ACTION_DISCOVERY_FINISHED.equals(action)) {
            setProgressBarIndeterminateVisibility(false);
            setTitle(R.string.select_device);
            if (mNewDevicesArrayAdapter.getCount() == 0) {
                String noDevices = getResources().getText(R.string.none_found).toString();
                mNewDevicesArrayAdapter.add(noDevices);
            }
        }
    }
};

destroy part

@Override
protected void onDestroy() {
    super.onDestroy();

    // Make sure we're not doing discovery anymore
    if (mBtAdapter != null) {
        mBtAdapter.cancelDiscovery();
    }

    // Unregister broadcast listeners
    this.unregisterReceiver(mReceiver);
}

Could someone help me? Thanks in advance!

Answer 1

Make sure you've added these permissions to your android manifest.

<uses-permission android:name="android.permission.BLUETOOTH" />
<uses-permission android:name="android.permission.BLUETOOTH_ADMIN" />
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />

and make sure you're requesting ACCESS_COARSE_LOCATION if it doesn't do it automatically:

    // Handling permissions.
    if (ContextCompat.checkSelfPermission(this,
            Manifest.permission.ACCESS_COARSE_LOCATION)
            != PackageManager.PERMISSION_GRANTED) {

        // Permission is not granted
        if (ActivityCompat.shouldShowRequestPermissionRationale(this,
                Manifest.permission.ACCESS_COARSE_LOCATION)) {

            // Not to annoy user.
            Toast.makeText(this, "Permission must be granted to use the app.", Toast.LENGTH_SHORT).show();
        } else {

            // Request permission.
            ActivityCompat.requestPermissions(this,
                    new String[]{Manifest.permission.ACCESS_COARSE_LOCATION},
                    REQUEST_PERMISSION_BLUETOOTH);
        }
    } else {
        // Permission has already been granted.
        Toast.makeText(this, "Permission already granted.", Toast.LENGTH_SHORT).show();
    }

and specify what should happen if the user accept/decline the request:

@Override
public void onRequestPermissionsResult(int requestCode,
                                       @NonNull String permissions[], @NonNull int[] grantResults) {
    switch (requestCode) {
        case REQUEST_PERMISSION_BLUETOOTH: {
            // If request is cancelled, the result arrays are empty.
            if (grantResults.length > 0
                    && grantResults[0] == PackageManager.PERMISSION_GRANTED) {

                Toast.makeText(this, "Permission granted.", Toast.LENGTH_SHORT).show();

                // Permission granted.
            } else {
                Toast.makeText(this, "Permission must be granted to use the application.", Toast.LENGTH_SHORT).show();
            }
        }
    }
}