Set up Bluetooth

Before your app can communicate over Bluetooth or Bluetooth Low Energy, you need to verify that Bluetooth is supported on the device, and if it is, ensure that it is enabled. Note that this check is only necessary if the android:required attribute in the <uses-feature.../> manifest file entry is set to false.

If Bluetooth isn't supported, then you should gracefully disable any Bluetooth features. If Bluetooth is supported, but disabled, then you can request that the user enable Bluetooth without leaving your app.

The first step is adding the Bluetooth permissions to your manifest file in order to use the following APIs.

Once the permissions are in place, Bluetooth setup is accomplished in two steps using the BluetoothAdapter:

1. Get the BluetoothAdapter.

   The BluetoothAdapter is required for any and all Bluetooth activity. The BluetoothAdapter represents the device's own Bluetooth adapter (the Bluetooth radio). To get a BluetoothAdapter, you first need to have a Context. Use this context to obtain an instance of the BluetoothManager system service. Calling BluetoothManager#getAdapter will give you a BluetoothAdapter object. If getAdapter() returns null, then the device doesn't support Bluetooth.

   For example:

   Kotlin

   val bluetoothManager: BluetoothManager = getSystemService(BluetoothManager::class.java)
   val bluetoothAdapter: BluetoothAdapter? = bluetoothManager.getAdapter()
   if (bluetoothAdapter == null) {
     // Device doesn't support Bluetooth
   }

   Java

   BluetoothManager bluetoothManager = getSystemService(BluetoothManager.class);
   BluetoothAdapter bluetoothAdapter = bluetoothManager.getAdapter();
   if (bluetoothAdapter == null) {
     // Device doesn't support Bluetooth
   }

2. Enable Bluetooth.

   Next, you need to ensure that Bluetooth is enabled. Call isEnabled() to check whether Bluetooth is currently enabled. If this method returns false, then Bluetooth is disabled. To request that Bluetooth be enabled, call startActivityForResult(), passing in an ACTION_REQUEST_ENABLE intent action. This call issues a request to enable Bluetooth through the system settings (without stopping your app).

   For example:

   Kotlin

   if (bluetoothAdapter?.isEnabled == false) {
     val enableBtIntent = Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE)
     startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT)
   }

   Java

   if (!bluetoothAdapter.isEnabled()) {
     Intent enableBtIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
     startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);
   }

A dialog appears requesting user permission to enable Bluetooth, as shown in figure 1. If the user grants permission, the system begins to enable Bluetooth, and focus returns to your app once the process completes (or fails).

Figure 1. The enabling Bluetooth dialog.

The REQUEST_ENABLE_BT constant passed to startActivityForResult() is a locally-defined integer that must be greater than or equal to 0. The system passes this constant back to you in your onActivityResult() implementation as the requestCode parameter.

If enabling Bluetooth succeeds, your activity receives the RESULT_OK result code in the onActivityResult() callback. If Bluetooth was not enabled due to an error (or the user responded "Deny") then the result code is RESULT_CANCELED.

Optionally, your app can also listen for the ACTION_STATE_CHANGED broadcast intent, which the system broadcasts whenever the Bluetooth state changes. This broadcast contains the extra fields EXTRA_STATE and EXTRA_PREVIOUS_STATE, containing the new and old Bluetooth states, respectively. Possible values for these extra fields are STATE_TURNING_ON, STATE_ON, STATE_TURNING_OFF, and STATE_OFF. Listening for this broadcast can be useful if your app needs to detect runtime changes made to the Bluetooth state.

Once Bluetooth is enabled on the device, you can use both Bluetooth classic and Bluetooth Low Energy.

For Bluetooth classic, you can find Bluetooth devices and connect to Bluetooth devices.

For Bluetooth Low Energy, you can find BLE devices, connect to a GATT server, and transfer BLE data.