How to detect Bluetooth state change using a broadcast receiver?

Unfortunately, for apps targeting api 26 or higher, manifest declared broadcast receivers don't work anymore (reference here: https://developer.android.com/guide/components/broadcast-exceptions), with some exceptions. android.bluetooth.adapter.action.STATE_CHANGED is not in that list.

For bluetooth, you can only listen for changes on:

ACTION_CONNECTION_STATE_CHANGED, ACTION_ACL_CONNECTED, ACTION_ACL_DISCONNECTED

AS far as permissions go, to detect the state change of bluetooth you need to add this to your AndroidManifest.xml.

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

An example receiver would look like this, you add this code to where you want to handle the broadcast, for example an activity:

    private final BroadcastReceiver mReceiver = new BroadcastReceiver() {
            public void onReceive (Context context, Intent intent) {
                String action = intent.getAction();

                if (BluetoothAdapter.ACTION_STATE_CHANGED.equals(action)) {
                    if(intent.getIntExtra(BluetoothAdapter.EXTRA_STATE, -1) 
    == BluetoothAdapter.STATE_OFF)
    // Bluetooth is disconnected, do handling here
}

}

        };

To use the receiver, you need to register it. Which you can do as follows. I register the receiver in my main activity.

registerReceiver(this, new IntentFilter(BluetoothAdapter.ACTION_STATE_CHANGED));

You could also decide to add all of it to your AndroidManifest.xml. This way you can make a special class for the receiver and handle it there. No need to register the receiver, just make the class and add the below code to the AndroidManifest

<receiver
        android:name=".packagename.NameOfBroadcastReceiverClass"
        android:enabled="true">
    <intent-filter>
        <action android:name="android.bluetooth.adapter.action.STATE_CHANGED"/>
    </intent-filter>
</receiver>

You have to take following permission.

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

and you have to write this as your intent filter in receiver tag.

<action android:name="android.bluetooth.adapter.action.STATE_CHANGED" />