Bluetooth LE Scanner gives no results

Question

I have this problem: I'm creating a bluetooth scanner app for LE devices, in order to scan ESP32 devices. The app works perfectly, but, in some devices (like my own), the scanner gives no results. There's no hurry, because I could test it in my father's phone and it's essentially completed. The problem is how frustrating it is, specially not knowing the cause.

Here's the ScanActivity:

import android.Manifest;
import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.bluetooth.BluetoothManager;
import android.bluetooth.le.BluetoothLeScanner;
import android.bluetooth.le.ScanCallback;
import android.bluetooth.le.ScanResult;
import android.content.Context;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Handler;
import android.view.View;

import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatActivity;
import androidx.constraintlayout.widget.ConstraintLayout;
import androidx.core.content.res.ResourcesCompat;
import androidx.fragment.app.DialogFragment;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import com.google.android.material.floatingactionbutton.FloatingActionButton;
import com.google.android.material.snackbar.BaseTransientBottomBar;
import com.google.android.material.snackbar.Snackbar;

import java.util.ArrayList;
import java.util.Objects;

public class ScanActivity extends AppCompatActivity implements ResultsListAdapter.OnResultClickListener {

    private final static int REQUEST_ENABLE_BT = 69;
    private static final int PERMISSION_REQUEST_COARSE_LOCATION = 1;
    private static final int SIGNAL_STRENGTH = -75;

    private static boolean scanning = false;
    private ConstraintLayout parent;
    private RecyclerView recyclerView;
    private FloatingActionButton startScanButton, stopScanButton;

    private ArrayList<ScanResult> results = new ArrayList<>();
    private ArrayList<BluetoothDevice> devices = new ArrayList<>();

    private BluetoothManager btManager;
    private BluetoothAdapter btAdapter;
    private BluetoothLeScanner btScanner;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_scan);

        getWindow().setStatusBarColor(ResourcesCompat.getColor(getResources(), R.color.colorAccentDark, null));
        parent = findViewById(R.id.parent_activity_scan);
        recyclerView = findViewById(R.id.scanner_recycler_view);
        startScanButton = findViewById(R.id.start_scan_button);
        stopScanButton = findViewById(R.id.stop_scan_button);

        initRecyclerView();
        startScanButton.setOnClickListener(v -> {
            start();
        });

        stopScanButton.setOnClickListener(v -> {
            stop();
        });

        btManager = (BluetoothManager) getSystemService(Context.BLUETOOTH_SERVICE);
        btAdapter = Objects.requireNonNull(btManager).getAdapter();
        btScanner = btAdapter.getBluetoothLeScanner();

        if (btAdapter != null && !btAdapter.isEnabled()) {
            Intent enableIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
            startActivityForResult(enableIntent, REQUEST_ENABLE_BT);
        }

        if (this.checkSelfPermission(Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
            final AlertDialog.Builder builder = new AlertDialog.Builder(this);
            builder.setTitle(R.string.coarse_location_access_title);
            builder.setMessage(R.string.coarse_location_access_message);
            builder.setPositiveButton(R.string.concede, null);
            builder.setOnDismissListener(dialog -> requestPermissions(new String[]{Manifest.permission.ACCESS_COARSE_LOCATION}, PERMISSION_REQUEST_COARSE_LOCATION));
            builder.show();
        }

    }

    private void start() {
        if (btAdapter != null && !btAdapter.isEnabled()) {
            Intent enableIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);
            startActivityForResult(enableIntent, REQUEST_ENABLE_BT);
            stop();
        } else {
            startScanButton.setVisibility(View.INVISIBLE);
            stopScanButton.setVisibility(View.VISIBLE);
            devices.clear();
            results.clear();
            recyclerView.getAdapter().notifyDataSetChanged();
            scan(true);
        }
    }

    private void stop() {
        btScanner.stopScan(leScanCallback);
    }

    private void scan(final boolean enable) {

        if (enable && !scanning) {
            Handler handler = new Handler();
            final Runnable runnable = () -> {
                scanning = false;
                Snackbar.make(parent, "Encontrados " + results.size() + " dispotivos.", BaseTransientBottomBar.LENGTH_LONG)
                        .setAction("", null)
                        .setBackgroundTint(ResourcesCompat.getColor(ScanActivity.this.getResources(), R.color.colorAccentDark, null))
                        .show();
                startScanButton.setVisibility(View.VISIBLE);
                stopScanButton.setVisibility(View.INVISIBLE);
                AsyncTask.execute(() -> btScanner.stopScan(leScanCallback));
            };
            handler.postDelayed(runnable, 7500);
            scanning = true;
            btScanner.startScan(leScanCallback);
        } else {
            scanning = false;
            btScanner.stopScan(leScanCallback);
        }
    }

    private void initRecyclerView() {
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        ResultsListAdapter adapter = new ResultsListAdapter(results, this);
        recyclerView.setAdapter(adapter);
    }

    private final ScanCallback leScanCallback = new ScanCallback() {
        @Override
        public void onScanResult(int callbackType, ScanResult result) {
            if (!devices.contains(result.getDevice()) && result.getRssi() > SIGNAL_STRENGTH) {
                new Handler().post(() -> {
                    if (result.getDevice().getAddress().startsWith("A4:CF:12")) {
                        results.add(result);
                        devices.add(result.getDevice());
                        recyclerView.getAdapter().notifyDataSetChanged();
                    }
                });
            }
        }
    };

    @Override
    public void onResultClick(int position) {
        BluetoothDevice device = results.get(position).getDevice();
        DialogFragment dialogFragment = new ConfirmActionDialog();
        Bundle args = new Bundle();
        args.putString("confirm_action_dialog_message",
                "¿Conectar con el dispositivo con dirección " + device.getAddress() + "?");
        args.putInt("type", ConfirmActionDialog.CONNECTION_CODE);
        args.putParcelable("object", device);
        dialogFragment.setArguments(args);
        dialogFragment.show(getSupportFragmentManager(), "Confirm connection");
    }

    @Override
    public void onResultInfoClick(int position) {
        DialogFragment dialogFragment = new InfoDialog();
        Bundle args = new Bundle();
        args.putParcelable("object", results.get(position).getDevice());
        dialogFragment.setArguments(args);
        dialogFragment.show(getSupportFragmentManager(), "InfoDialog");
    }
}

As you can see, I request location and Bluetooth permissions. Any idea?

PS:

My app goes to min sdk 27 (Android 8.1). My device is Xiaomi Mi 9T, with Android 10. It also gives no results on Xiaomi Mi A2, Android 9&10 (it was recently updated). The devices where it works perfectly are Samsung Galaxy J7 (Android 9) and Xiaomi Mi 8 Pro (Android 9)

Answer 1

You should implement onScanFailed in your ScanCallback . It might give you the reason why the scan is not working.