How to programmatically tell if a Bluetooth device is connected?

Question

I understand how to get a list of paired devices but how can I tell if they are connected?

It must be possible since I see them listed in my phone's Bluetooth device list and it states their connection status.

Answer 1

Add bluetooth permission to your AndroidManifest,

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

Then use intent filters to listen to the ACTION_ACL_CONNECTED, ACTION_ACL_DISCONNECT_REQUESTED, and ACTION_ACL_DISCONNECTED broadcasts:

public void onCreate() {
    ...
    IntentFilter filter = new IntentFilter();
    filter.addAction(BluetoothDevice.ACTION_ACL_CONNECTED);
    filter.addAction(BluetoothDevice.ACTION_ACL_DISCONNECT_REQUESTED);
    filter.addAction(BluetoothDevice.ACTION_ACL_DISCONNECTED);
    this.registerReceiver(mReceiver, filter);
}

//The BroadcastReceiver that listens for bluetooth broadcasts
private final BroadcastReceiver mReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();
        BluetoothDevice device = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);

        if (BluetoothDevice.ACTION_FOUND.equals(action)) {
           ... //Device found
        }
        else if (BluetoothDevice.ACTION_ACL_CONNECTED.equals(action)) {
           ... //Device is now connected
        }
        else if (BluetoothAdapter.ACTION_DISCOVERY_FINISHED.equals(action)) {
           ... //Done searching
        }
        else if (BluetoothDevice.ACTION_ACL_DISCONNECT_REQUESTED.equals(action)) {
           ... //Device is about to disconnect
        }
        else if (BluetoothDevice.ACTION_ACL_DISCONNECTED.equals(action)) {
           ... //Device has disconnected
        }           
    }
};

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A few notes:

• There is no way to retrieve a list of connected devices at application startup. The Bluetooth API does not allow you to QUERY, instead it allows you to listen to CHANGES.
• A hoaky work around to the above problem would be to retrieve the list of all known/paired devices... then trying to connect to each one (to determine if you're connected).
• Alternatively, you could have a background service watch the Bluetooth API and write the device states to disk for your application to use at a later date.