BondHelper class for BLE Devices with encrypted characteristics

BondingHelper.java

import android.bluetooth.BluetoothDevice;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;

import com.polidea.rxandroidble2.RxBleDevice;

import java.lang.reflect.Method;
import java.util.concurrent.TimeUnit;

import io.reactivex.Completable;
import io.reactivex.disposables.Disposables;
import timber.log.Timber;


/**
 * In the communication with the scale device, some characteristics are encrypted. Therefore, Android
 * needs to be "Bonded" with the device. In order to be bond, usually what you need is a passkey. However,
 * in this case the Bonding mechanism is a "JustWorks" bonding, in which no passkey is required. The JustWorks
 * bonding happens automatically the first time the phone interacts with the device.
 * Depending on the bluetooth device and the android phone, an error can happen if the device had been
 * bonded in a previous connection. In that case, the first time trying to read or write on an
 * encrypted characteristic, the operation will fail after timeuot (30s by default) with a GATT_INSUF_AUTH
 * error. This will cause the device to unbond and bond again. After reseting the bonding, the ongoing
 * operations will work succesfuly. Chaining a retry() after the first read/write operation would be
 * a possible workaround, but it won't get to the retry() until the timeout has been triggered.
 *
 * To prevent that situation, I created this helper class that will do the following:
 * 1- Check if the devices is bonded. If it is not bonded, it will start the bonding process (step #4)
 * 2- If it is already bonded, it will create a BroadcastReceiver, start the unbonding operation and
 * continue once the Receiver gets a new STATE equal to BOND_NONE.
 * 3- Once it is onbonded, the Disposable will unregister the receiver.
 * 4- Afterwards, it will try to bond with the device. For that, it creates a BroadcastReceiver, start
 * the bonding operation and wait until the Receiver gets a new STATE equal to BOND_BONDED.
 * 5- The disposable will unregister the receiver.
 * 6- It will retry everything once if an error occur.
 * 7- The whole process, including the retry(), can have a time limit passed as a parameter.
 */
public class BondingHelper {

    private static int DEFAULT_TIMEOUT = 30;

    public static class BondingFailedException extends RuntimeException {
    }

    public static Completable bondWithDevice(final Context context, final RxBleDevice rxBleDevice) {
        return bondWithDevice(context, rxBleDevice, DEFAULT_TIMEOUT, TimeUnit.SECONDS);
    }

    public static Completable bondWithDevice(final Context context, final RxBleDevice rxBleDevice,
                                             long timeout, TimeUnit timeunit) {
        return removeBond(context, rxBleDevice.getBluetoothDevice())
                .andThen(Completable.create(completion -> {
                    Timber.d("Creating Bonding Broadcast Receiver.");
                    final BroadcastReceiver receiver = new BroadcastReceiver() {
                        @Override
                        public void onReceive(final Context context, final Intent intent) {
                            final BluetoothDevice deviceBeingPaired = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);
                            final int state = intent.getIntExtra(BluetoothDevice.EXTRA_BOND_STATE, BluetoothDevice.BOND_NONE);

                            Timber.d("Intent received in Bonding Broadcast Receiver. State %1$s - Device %2$s",
                                    state, deviceBeingPaired.getAddress());
                            if (deviceBeingPaired.getAddress().equals(rxBleDevice.getMacAddress())) {
                                if (state == BluetoothDevice.BOND_BONDED) {
                                    Timber.d("State is BOND_BONDED. Bonded Succeded.");
                                    completion.onComplete();
                                } else if (state == BluetoothDevice.BOND_NONE) {
                                    Timber.d("State is BOND_NONE. Bonding Failed.");
                                    completion.tryOnError(new BondingFailedException());
                                } else {
                                    Timber.d("State is something else.");
                                }
                            }
                        }
                    };

                    completion.setDisposable(Disposables.fromAction(() -> {
                        Timber.d("Disposing Bonding completable and unregistering Broadcast Receiver");
                        context.unregisterReceiver(receiver);
                    }));
                    context.registerReceiver(receiver, new IntentFilter(BluetoothDevice.ACTION_BOND_STATE_CHANGED));

                    Timber.d("Call for creating bond.");
                    //This returns false in immediate failure or true if the bonding can begin
                    final boolean createBondResult = rxBleDevice.getBluetoothDevice().createBond();
                    if (!createBondResult) {
                        Timber.d("Could not start bonding process.");
                        completion.tryOnError(new BondingFailedException());
                    }
                }))
                .retry() //I give the bonding process one more chance.
                .timeout(timeout, timeunit)
                .doOnError(throwable -> {
                    Timber.e(throwable, "Timeout of %1$i %2$s during bonding process.",
                            timeout, timeunit.toString());
                    throw new BondingFailedException();
                });
    }

    private static Completable removeBond(final Context context, BluetoothDevice device) {
        return Completable.create(completion -> {
            // If it was already bonded. I delete the bonding because it needs to be re-bonded.

            Timber.d("Creating Unbonding Broadcast Receiver.");
            final BroadcastReceiver receiver = new BroadcastReceiver() {
                @Override
                public void onReceive(final Context c, final Intent intent) {
                    final BluetoothDevice deviceBeingUnpaired = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);
                    final int state = intent.getIntExtra(BluetoothDevice.EXTRA_BOND_STATE, BluetoothDevice.BOND_NONE);

                    Timber.d("Intent received in Unbonding Broadcast Receiver. State %1$s - Device %2$s",
                            state, deviceBeingUnpaired.getAddress());
                    if (deviceBeingUnpaired.getAddress().equals(device.getAddress())) {
                        if (state == BluetoothDevice.BOND_NONE) {
                            Timber.d("State received is BOND_NONE. Unbonding succeded.");
                            //context.unregisterReceiver(this);
                            completion.onComplete();
                        } else if (state == BluetoothDevice.BOND_BONDED) {
                            Timber.d("State received is BOND_BONDED. Unbonding failed.");
                            //context.unregisterReceiver(this);
                            completion.tryOnError(new BondingFailedException());
                        } else {
                            Timber.d("State is something else.");
                        }
                    }
                }
            };

            completion.setDisposable(Disposables.fromAction(() -> {
                Timber.d("Disposing Unbonding completable and unregistering Broadcast Receiver");
                context.unregisterReceiver(receiver);
            }));
            context.registerReceiver(receiver, new IntentFilter(BluetoothDevice.ACTION_BOND_STATE_CHANGED));

            Timber.d("Checking bond status.");
            if (device.getBondState() == BluetoothDevice.BOND_BONDED) {
                Timber.d("The device was already bonded. Removing bond.");
                try {
                    Method m = device.getClass()
                            .getMethod("removeBond", (Class[]) null);
                    m.invoke(device, (Object[]) null);
                } catch (Exception e) {
                    completion.tryOnError(new BondingFailedException());
                }
            } else {
                Timber.d("The device was not bond.");
                completion.onComplete();
            }
        });
    }
}

Thank you!

Could you provide sample code demonstrating usage?

@RobLewis I am sorry for the super late reply, but here you have some sample code.

You can also take a look to this answer in Stackoverflow.

What I do, is scan for devices and once I find the device I want to connect with, but before establishing the connection with scaleDevice.establishConnection(autoconnectFlag) I make a call to BondingHelper.bondWithDevice(this, scaleDevice, 10, TimeUnit.SECONDS).

 public void scanBleDevices() {
        Timber.d("Start scanning.");
        mScanDisposable = rxBleClient.scanBleDevices(
                new ScanSettings.Builder()
                        .setScanMode(ScanSettings.SCAN_MODE_LOW_LATENCY)
                        .setCallbackType(ScanSettings.CALLBACK_TYPE_ALL_MATCHES)
                        .build(),
                new ScanFilter.Builder()
                        .setDeviceName(mScaleBleNameString)
                        .build()
        )
                .timeout(10, TimeUnit.SECONDS)
                .observeOn(AndroidSchedulers.mainThread())
                .doFinally(this::disposeScanning)
                .subscribe(this::connectToBleDevice, this::throwException);
    }

    private void connectToBleDevice(ScanResult scanResult) {
        RxBleDevice scaleDevice = scanResult.getBleDevice();

        mConnectionDisposable =
                BondingHelper.bondWithDevice(this, scaleDevice, 10, TimeUnit.SECONDS)
                        .andThen(Completable.fromAction(() -> mConnectionState.postValue(Constants.CONNECTING)))
                        .andThen(() -> scaleDevice.establishConnection(autoconnectFlag))
                        // Continue chaining as you please
                        .observeOn(AndroidSchedulers.mainThread())
                        .subscribeOn(Schedulers.io())
                        .doFinally(this::disposeConnection)
                        .subscribe(this::onSuccess, this::throwException);
    }