Totally Disconnect a Bluetooth Low Energy Device

Question

I connect to a BLE device with the connectGatt() method in Android. This works great.

When I disconnect I use the following:

private void disconnectDevice() {
    gatt.disconnect();
}

When I receive the callback I do a close.

private BluetoothGattCallback gattCallback = new BluetoothGattCallback() {
    @Override
    public void onConnectionStateChange(BluetoothGatt gatt, int status, int newState) {
        switch (newState) {
            case BluetoothProfile.STATE_CONNECTED:
                Log.d("BLED-GATT", "STATE_CONNECTED");
                setUIConnectionStatus("Discover services on device...", Color.YELLOW);
                checkEnableAddButton(simpleTrackEditText.getText().toString());
                gatt.discoverServices();
                break;
            case BluetoothProfile.STATE_DISCONNECTED:
                Log.d("BLED-GATT", "STATE_DISCONNECTED");
                setUIConnectionStatus("Not Connected!", Color.RED);
                gatt.close();
                break;
            default:
                Log.d("BLED-GATT", "STATE_OTHER");
        }
    }
}

This is executed and I can no longer control the device after calling disconnectDevice(). The device itself seems to think that it is still connected since I cant put it in broadcasting visibility mode (which happens if it already has a connection). However, if I kill the application and open it again then I can set the device in broadcasting mode. This tells me the app was not properly disconnected.

Any idea what I missed here?

Answer 1

The problem was that I during scanning was connecting to the same device multiple times causing my application to have many connections open at the same time. Adding !isConnected() solved the problem:

/**
 * Connects to the device. Does nothing if already connected.
 * @param macAddress the address of the device.
 */
private void connectDevice(String macAddress) {
    if (isConnected()) {
        return;
    }
    
    device = bluetoothAdapter.getRemoteDevice(macAddress);

    if (device == null) {
        showToast("Device not available");
    } else {
        showToast("Connecting...");
        gatt = device.connectGatt(this, true, gattCallback);
    }
}

Answer 2

That's totally logic because you are not disconnecting before closing Gatt.

public void disconnect() {
        if (mBluetoothAdapter == null || mBluetoothGatt == null) {
            Log.w(TAG, "BluetoothAdapter not initialized");
            return;
        }
        mBluetoothGatt.disconnect();
    }

try to call this method before closing your GATT

Answer 3

I experienced same problem and I found two possible root cause as follows: 1. Was same as @nilsi answer, connect to the same device more than once in one scanning. → solved by lock & List 2. connect to the device in one scanning and didn't get onConnectionStateChange, than I scanned again, it resulted in second connection to the same device. → solved by keeping BluetoothGatt object when I call device.connectGatt(), and call object.disconnect(), object.close() before scanning.

My test phone is 4 years old so sometimes it needs more than 20 seconds to return onConnectionStateChange....

Answer 4

As previously stated, the problem is caused by calling connectGatt multiple times. Each one of those calls create a new BluetoothGatt instance and they are all kept alive, while you have only the last one. Given the fact that sometimes it is needed to call connectGatt multiple times, I just keep all the instances that it returns and call disconnect/close on all of them when I'm done. This fixed the disconnection issue instantly

private val gattInstances = LinkedList<BluetoothGatt>()


fun connect() {
    bluetoothGatt = device?.connectGatt(
        context,
        false, gattCallback, TRANSPORT_LE
    )
    bluetoothGatt?.let { gattInstances.add(it) }
}

fun finish() {
    bluetoothGatt?.close()
    while (gattInstances.isNotEmpty()) {
        gattInstances.pop().apply {
            disconnect()
            close()
        }
    }
}