使设备可被发现

private static final int REQUEST_DISCOVERABLE_BT = 2; // Unique request code
private static final int DISCOVERABLE_DURATION = 120; // Discoverable duration time in seconds
                                                      // 0 means always discoverable
                                                      // maximum value is 3600

// ...

Intent discoverableIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_DISCOVERABLE);
discoverableIntent.putExtra(BluetoothAdapter.EXTRA_DISCOVERABLE_DURATION, DISCOVERABLE_DURATION);
startActivityForResult(discoverableIntent, REQUEST_DISCOVERABLE_BT);

// ...

@Override
protected void onActivityResult(final int requestCode, final int resultCode, final Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    if (requestCode == REQUEST_DISCOVERABLE_BT) {
        if (resultCode == RESULT_OK) {
            // Device is discoverable
        } else if (resultCode == RESULT_CANCELED) {
            // Device is not discoverable
        }
    }
}