Android BLE- How is onScanResult method being called in ScanCallback?
<p>This is my first time doing Bluetooth Low Energy in Android project. The project that I am doing is basically to detect all Bluetooth LE devices and connect them to discover their services. </p>

<p>I would like to ask if anyone know how onScanResult(), onBatchScanResults() and onScanFailed() methods are being called in ScanCallback? </p>

<p>At First, run scanLeDevice() method.</p>

<pre><code>BluetoothLeScanner mLEScanner = mBluetoothAdapter.getBluetoothLeScanner();
ScanSettings settings = new ScanSettings.Builder()
           .setScanMode(ScanSettings.SCAN_MODE_LOW_POWER)
           .setCallbackType(ScanSettings.CALLBACK_TYPE_ALL_MATCHES)
           .build();
List&lt;ScanFilter&gt; filters = new ArrayList&lt;ScanFilter&gt;();

scanLeDevice(true);
</code></pre>

<hr>

<p>In this method, it will startScan. So I assume that the scan results are delivered using these callback.</p>

<pre><code>@TargetApi(21)
private void scanLeDevice(final boolean enable) {
    if (enable) {
        //stops scanning after a pre-defined scan period
        mHandler.postDelayed(new Runnable() {
            @Override
            public void run() {
                    System.out.println("BLE// mLEScanner.stopScan(mScanCallback) ");
                    mLEScanner.stopScan(mScanCallback);
                }
            }
        }, SCAN_PERIOD);
            System.out.println("BLE// mLEScanner.startScan(filters, settings, mScanCallback)");
            mLEScanner.startScan(filters, settings, mScanCallback);

    } else {
            System.out.println("BLE// mLEScanner.stopScan(mScanCallback)");
            mLEScanner.stopScan(mScanCallback);
    }
}
</code></pre>

<hr>

<p>However, In ScanCallback, I have no idea how it triggers onScanResult and deliver the scan result using the callback. In my testing(as shown below), neither onScanResult() nor onBatchScanResults() and onScanFailed() be called. Can someone explain the concept to me? It will help me a lot!</p>

<pre><code> /* Scan result for SDK &gt;= 21 */
 private ScanCallback mScanCallback = new ScanCallback() {

    @Override
    public void onScanResult(int callbackType, ScanResult result) {
        System.out.println("BLE// onScanResult");
        Log.i("callbackType", String.valueOf(callbackType));
        Log.i("result", result.toString());
        BluetoothDevice btDevice = result.getDevice();
        connectToDevice(btDevice);
    }

    @Override
    public void onBatchScanResults(List&lt;ScanResult&gt; results) {
        System.out.println("BLE// onBatchScanResults");
        for (ScanResult sr : results) {
            Log.i("ScanResult - Results", sr.toString());
        }
    }

    @Override
    public void onScanFailed(int errorCode) {
        System.out.println("BLE// onScanFailed");
        Log.e("Scan Failed", "Error Code: " + errorCode);
    }

};
</code></pre>

<hr>

<pre><code>02-17 10:38:38.513 878-895/? D/BluetoothManagerService: Added callback: android.bluetooth.IBluetoothManagerCallback$Stub$Proxy@8334cf4:true
02-17 10:38:38.520 782-782/? D/BluetoothAdapter: STATE_ON
02-17 10:38:38.529 21554-21590/? D/BtGatt.GattService: registerClient() - UUID=835342c6-81eb-4e09-9729-5bbe1c22bc86
02-17 10:38:38.529 21554-21570/? D/BtGatt.GattService: onClientRegistered() - UUID=835342c6-81eb-4e09-9729-5bbe1c22bc86, clientIf=5
02-17 10:38:38.530 782-793/? D/BluetoothLeScanner: onClientRegistered() - status=0 clientIf=5
02-17 10:38:38.530 21554-21599/? D/BtGatt.GattService: start scan with filters

02-17 10:38:38.532 782-782/? I/System.out: BLE// mLEScanner.startScan(filters, settings, mScanCallback)
02-17 10:38:38.532 21554-21573/? D/BtGatt.ScanManager: handling starting scan
02-17 10:38:38.534 21576-21577/? I/WCNSS_FILTER: ibs_msm_serial_clock_vote: vote UART CLK ON using UART driver's ioctl()
02-17 10:38:38.542 21554-21570/? D/BtGatt.GattService: onScanFilterEnableDisabled() - clientIf=5, status=0, action=1
02-17 10:38:38.543 21554-21570/? D/BtGatt.ScanManager: callback done for clientIf - 5 status - 0
02-17 10:38:38.543 21554-21573/? D/BtGatt.ScanManager: configureFilterParamter 500 10000 1 0
02-17 10:38:38.547 21554-21570/? D/BtGatt.GattService: onScanFilterParamsConfigured() - clientIf=5, status=0, action=0, availableSpace=15
02-17 10:38:38.547 21554-21570/? D/BtGatt.ScanManager: callback done for clientIf - 5 status - 0
02-17 10:38:38.548 21554-21573/? D/BtGatt.ScanManager: configureRegularScanParams() - queue=1
02-17 10:38:38.548 487-2827/? I/ACDB-LOADER: ACDB AFE returned = -19
02-17 10:38:38.549 21554-21573/? D/BtGatt.ScanManager: configureRegularScanParams() - ScanSetting Scan mode=0 mLastConfiguredScanSetting=-2147483648
02-17 10:38:38.549 21554-21573/? D/BtGatt.ScanManager: configureRegularScanParams - scanInterval = 8000configureRegularScanParams - scanWindow = 800
02-17 10:38:38.549 21554-21570/? D/BtGatt.GattService: onScanParamSetupCompleted : 0
02-17 10:38:38.568 21554-21574/? W/bt_hci: filter_incoming_event command complete event with no matching command. opcode: 0x0.
02-17 10:38:38.603 21554-21570/? D/bt_btif_gattc: btif_gattc_update_properties BLE device name=Polar HR Sensor len=15 dev_type=2
02-17 10:38:39.571 21576-21585/? I/WCNSS_FILTER: ibs_msm_serial_clock_vote: vote UART CLK OFF using UART driver's ioctl()

02-17 10:38:43.526 782-782/? I/System.out: BLE// mLEScanner.stopScan(mScanCallback) 
02-17 10:38:43.599 21576-21576/? I/WCNSS_FILTER: ibs_msm_serial_clock_vote: vote UART CLK ON using UART driver's ioctl()
02-17 10:38:43.967 21576-21576/? I/WCNSS_FILTER: ibs_msm_serial_clock_vote: vote UART CLK OFF using UART driver's ioctl()
</code></pre>

<p>Using Android Phone with API 23</p>

<p>The code I have written here is referred to: <a href="https://itunes.apple.com/us/app/lightblue-explorer-bluetooth/id557428110?mt=8" rel="noreferrer">http://www.truiton.com/2015/04/android-bluetooth-low-energy-ble-example/</a></p>

<hr>

<hr>

<p>[Updated Code V1]- Not working</p>

<p>Here is all my code
I've created a virtual Peripheral and it is in advertising mode. The virtual Peripheral is created through an app called LightBlue: <a href="https://itunes.apple.com/us/app/lightblue-explorer-bluetooth/id557428110?mt=8" rel="noreferrer">https://itunes.apple.com/us/app/lightblue-explorer-bluetooth/id557428110?mt=8</a>
Please help me to check my code :)</p>

<pre><code>@TargetApi(21)
public class BluetoothLE extends Fragment {

View view;

private BluetoothAdapter mBluetoothAdapter;
private int REQUEST_ENABLE_BT = 1;
private Handler mHandler;
private static final long SCAN_PERIOD = 5000;  // Stops scanning after 5 seconds
private BluetoothLeScanner mLEScanner;
private BluetoothGatt mGatt; //To provide bluetooth communication
private static final int PERMISSION_REQUEST_COARSE_LOCATION = 1;
private int permissionCheck;

public BluetoothLE(){
    //empty constructor
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment

    view = inflater.inflate(R.layout.fragment_bluetooth, container, false);
    mHandler = new Handler();

    /* check if BLE is supported in this phone */
    if (!getActivity().getPackageManager().hasSystemFeature(PackageManager.FEATURE_BLUETOOTH_LE)) {
        Toast.makeText(getActivity(), "BLE Not Supported", Toast.LENGTH_SHORT).show();
        getActivity().finish();
    }

    /* Enable bluetooth without leaving app */
    final BluetoothManager bluetoothManager =
            (BluetoothManager) getActivity().getSystemService(Context.BLUETOOTH_SERVICE);
    mBluetoothAdapter = bluetoothManager.getAdapter();

    /* Build ScanSetting */
    ScanSettings.Builder scanSetting = new ScanSettings.Builder()
            .setScanMode(ScanSettings.SCAN_MODE_LOW_POWER)
            .setReportDelay(5000);

    settings = scanSetting.build();


    return view;
}

@TargetApi(Build.VERSION_CODES.M)
@Override
public void onResume() {
    super.onResume();

    /* Ensures Bluetooth is available on the device and it is enabled. If not, displays a dialog requesting user permission to enable Bluetooth. */
    if (mBluetoothAdapter == null || !mBluetoothAdapter.isEnabled()) {  //Unable to obtain a BluetoothAdapter
        Intent enableBtIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
        startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT); //trigger onActivityResult

    } else {
        if (Build.VERSION.SDK_INT &gt;= 21) {
            mLEScanner = mBluetoothAdapter.getBluetoothLeScanner();
            settings = new ScanSettings.Builder()
                    .setScanMode(ScanSettings.SCAN_MODE_LOW_POWER)
                    .setCallbackType(ScanSettings.CALLBACK_TYPE_ALL_MATCHES)
                    .build();
            filters = new ArrayList&lt;ScanFilter&gt;();
        }

        if(Build.VERSION.SDK_INT &gt;= 23){
            checkLocationPermission();
        }

        scanLeDevice(true);
    }
}

@Override
public void onPause() {
    super.onPause();
    if (mBluetoothAdapter != null &amp;&amp; mBluetoothAdapter.isEnabled()) {
        scanLeDevice(false);
    }
}

@Override
public void onDestroy() {
    if (mGatt == null) {
        return;
    }
    mGatt.close();
    mGatt = null;
    super.onDestroy();
}

@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    System.out.println("BLE// onActivityResult");
    if (requestCode == REQUEST_ENABLE_BT) {
        if (resultCode == Activity.RESULT_CANCELED) {
            //Bluetooth not enabled.
            getActivity().finish();
            return;
        }
    }
    super.onActivityResult(requestCode, resultCode, data);
}

private void scanLeDevice(final boolean enable) {
    if (enable) {
        //stops scanning after a pre-defined scan period
        mHandler.postDelayed(new Runnable() {
            @Override
            public void run() {
                if (Build.VERSION.SDK_INT &lt; 21) {
                    System.out.println("BLE// mBluetoothAdapter.stopLeScan(mLeScanCallback) ");
                    mBluetoothAdapter.stopLeScan(mLeScanCallback);
                } else {
                    mLEScanner.stopScan(mScanCallback);
                    System.out.println("BLE// mLEScanner.stopScan(mScanCallback) ");
                }
            }
        }, SCAN_PERIOD);

        if (Build.VERSION.SDK_INT &lt; 21) {
            System.out.println("BLE// mBluetoothAdapter.startLeScan(mLeScanCallback)");
            mBluetoothAdapter.startLeScan(mLeScanCallback);
        } else {

            mLEScanner.startScan(mScanCallback);
            //mLEScanner.startScan(filters, settings, mScanCallback);
            System.out.println("BLE// mLEScanner.startScan(mScanCallback) ");
        }
    } else {
        if (Build.VERSION.SDK_INT &lt; 21) {
            System.out.println("BLE// mBluetoothAdapter.stopLeScan(mLeScanCallback)");
            mBluetoothAdapter.stopLeScan(mLeScanCallback);
        } else {
            System.out.println("BLE// mLEScanner.stopScan(mScanCallback)");
            mLEScanner.stopScan(mScanCallback);
        }
    }
}

/* Scan result for SDK &gt;= 21 */
private ScanCallback mScanCallback = new ScanCallback() {

    @Override
    public void onScanResult(int callbackType, ScanResult result) {
        System.out.println("BLE// onScanResult");
        super.onScanResult(callbackType, result);

        Log.i("callbackType", String.valueOf(callbackType));
        Log.i("result", result.toString());
        Log.i("Device Name: ", result.getDevice().getName());
        System.out.println("Signal: " + result.getRssi());

        BluetoothDevice btDevice = result.getDevice();
        connectToDevice(btDevice);
    }

    @Override
    public void onBatchScanResults(List&lt;ScanResult&gt; results) {
        System.out.println("BLE// onBatchScanResults");
        for (ScanResult sr : results) {
            Log.i("ScanResult - Results", sr.toString());
        }
    }

    @Override
    public void onScanFailed(int errorCode) {
        System.out.println("BLE// onScanFailed");
        Log.e("Scan Failed", "Error Code: " + errorCode);
    }

};

// scan results are returned here SDK &lt; 21
private BluetoothAdapter.LeScanCallback mLeScanCallback = new BluetoothAdapter.LeScanCallback() {
    @Override
    public void onLeScan(final BluetoothDevice device, int rssi,
                         byte[] scanRecord) {
        getActivity().runOnUiThread(new Runnable() {
            @Override
            public void run() {
                System.out.println("BLE// DEVICDE FOUND");

                Log.i("onLeScan", device.toString());

                connectToDevice(device);
            }
        });
    }
};

public void connectToDevice(BluetoothDevice device) {
    System.out.println("BLE// connectToDevice()");
    if (mGatt == null) {
        mGatt = device.connectGatt(getActivity(), false, gattCallback); //Connect to a GATT Server
        //scanLeDevice(false);// will stop after first device detection
    }
}

private final BluetoothGattCallback gattCallback = new BluetoothGattCallback() {
    @Override
    public void onConnectionStateChange(BluetoothGatt gatt, int status, int newState) {
        System.out.println("BLE// BluetoothGattCallback");
        Log.i("onConnectionStateChange", "Status: " + status);
        switch (newState) {
            case BluetoothProfile.STATE_CONNECTED:
                Log.i("gattCallback", "STATE_CONNECTED");
                gatt.discoverServices();
                break;
            case BluetoothProfile.STATE_CONNECTING:
                Log.i("gattCallback", "STATE_CONNECTING");
                break;
            case BluetoothProfile.STATE_DISCONNECTED:
                Log.e("gattCallback", "STATE_DISCONNECTED");
                break;
            default:
                Log.e("gattCallback", "STATE_OTHER");
        }
    }

    @Override
    //New services discovered
    public void onServicesDiscovered(BluetoothGatt gatt, int status) {
        List&lt;BluetoothGattService&gt; services = gatt.getServices();
        Log.i("onServicesDiscovered", services.toString());
        gatt.readCharacteristic(services.get(1).getCharacteristics().get
                (0));
    }

    @Override
    //Result of a characteristic read operation
    public void onCharacteristicRead(BluetoothGatt gatt,
                                     BluetoothGattCharacteristic
                                             characteristic, int status) {
        Log.i("onCharacteristicRead", characteristic.toString());
        gatt.disconnect();
    }
};

public void checkLocationPermission(){
    permissionCheck = ContextCompat.checkSelfPermission(getActivity(), android.Manifest.permission.ACCESS_COARSE_LOCATION);

    switch(permissionCheck){
        case PackageManager.PERMISSION_GRANTED:
            break;

        case PackageManager.PERMISSION_DENIED:

            if(ActivityCompat.shouldShowRequestPermissionRationale(getActivity(), android.Manifest.permission.ACCESS_COARSE_LOCATION)){
                //Show an explanation to user *asynchronouselly* -- don't block
                //this thread waiting for the user's response! After user sees the explanation, try again to request the permission

                Snackbar.make(view, "Location access is required to show Bluetooth devices nearby.",
                        Snackbar.LENGTH_LONG).setAction("Action", null).show();

            }
            else{
                //No explanation needed, we can request the permission
                ActivityCompat.requestPermissions(getActivity(), new String[]{android.Manifest.permission.ACCESS_COARSE_LOCATION}, PERMISSION_REQUEST_COARSE_LOCATION);
            }
            break;
    }
}
}
</code></pre>