蓝牙扫描正常，但找不到可用的蓝牙设备

Question

我正在使用Android 9 Pie API级别。我正在通过多种方式使用广播从下面的代码向其他活动发送意图使用的可用设备。

配对的设备列表显示完美，但是搜索完成后找不到附近的可用设备。但是不知道为什么它不起作用。

private ActivityMainBinding binding;
private ArrayList<BluetoothDevice> mDeviceList = new ArrayList<>();
private BluetoothAdapter mBluetoothAdapter;
private ProgressDialog mProgressDlg;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    binding = DataBindingUtil.setContentView(this, R.layout.activity_main);

    mBluetoothAdapter = BluetoothAdapter.getDefaultAdapter();
    mProgressDlg = new ProgressDialog(this);
    mProgressDlg.setMessage("Scanning Device");
    mProgressDlg.setCancelable(false);
    mProgressDlg.setButton(DialogInterface.BUTTON_NEGATIVE, "Cancel", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            dialog.dismiss();

            mBluetoothAdapter.cancelDiscovery();
        }
    });

    if (mBluetoothAdapter == null) {
        showUnsupported();
    } else {
        binding.btnViewPaired.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Set<BluetoothDevice> pairedDevices = mBluetoothAdapter.getBondedDevices();

                if (pairedDevices == null || pairedDevices.size() == 0) {
                    showToast("No Paired Devices Found");
                } else {
                    ArrayList<BluetoothDevice> list = new ArrayList<>();

                    list.addAll(pairedDevices);

                    Intent intent = new Intent(MainActivity.this, DeviceListActivity.class);

                    intent.putParcelableArrayListExtra("device.list", list);

                    startActivity(intent);
                }
            }
        });

        binding.btnScan.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View arg0) {
                mBluetoothAdapter.startDiscovery();
            }
        });

        binding.btnEnable.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (mBluetoothAdapter.isEnabled()) {
                    mBluetoothAdapter.disable();

                    showDisabled();
                } else {
                    Intent intent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);

                    startActivityForResult(intent, 1000);
                }
            }
        });

        if (mBluetoothAdapter.isEnabled()) {
            showEnabled();
        } else {
            showDisabled();
        }
    }

    ///////////////////////////////////////////////////////////////////////////////////////////
    IntentFilter filter = new IntentFilter();
    filter.addAction(BluetoothAdapter.ACTION_STATE_CHANGED);
    filter.addAction(BluetoothDevice.ACTION_FOUND);
    filter.addAction(BluetoothAdapter.ACTION_DISCOVERY_STARTED);
    filter.addAction(BluetoothAdapter.ACTION_DISCOVERY_FINISHED);
    registerReceiver(mReceiver, filter);

}

@Override
public void onPause() {
    if (mBluetoothAdapter != null) {
        if (mBluetoothAdapter.isDiscovering()) {
            mBluetoothAdapter.cancelDiscovery();
        }
    }

    super.onPause();
}

@Override
public void onDestroy() {
    unregisterReceiver(mReceiver);

    super.onDestroy();
}

private void showEnabled() {
    binding.tvStatus.setText("Bluetooth is On");
    binding.tvStatus.setTextColor(Color.GREEN);
    binding.btnEnable.setText("Disable");
    binding.btnEnable.setEnabled(true);
    binding.btnViewPaired.setEnabled(true);
    binding.btnScan.setEnabled(true);
}

private void showDisabled() {
    binding.tvStatus.setText("Bluetooth is Off");
    binding.tvStatus.setTextColor(Color.RED);
    binding.btnEnable.setText("Enable");
    binding.btnEnable.setEnabled(true);
    binding.btnViewPaired.setEnabled(false);
    binding.btnScan.setEnabled(false);
}

private void showUnsupported() {
    binding.tvStatus.setText("Bluetooth is unsupported by this device");
    binding.btnEnable.setText("Enable");
    binding.btnEnable.setEnabled(false);
    binding.btnViewPaired.setEnabled(false);
    binding.btnScan.setEnabled(false);
}

private void showToast(String message) {
    Toast.makeText(getApplicationContext(), message, Toast.LENGTH_SHORT).show();
}

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

        if (BluetoothAdapter.ACTION_STATE_CHANGED.equals(action)) {
            final int state = intent.getIntExtra(BluetoothAdapter.EXTRA_STATE, BluetoothAdapter.ERROR);

            if (state == BluetoothAdapter.STATE_ON) {
                showToast("Bluetooth Enabled");

                showEnabled();
            }
        } else if (BluetoothAdapter.ACTION_DISCOVERY_STARTED.equals(action)) {
            mDeviceList = new ArrayList<>();

            mProgressDlg.show();
        } else if (BluetoothAdapter.ACTION_DISCOVERY_FINISHED.equals(action)) {
            mProgressDlg.dismiss();

            Intent newIntent = new Intent(MainActivity.this, DeviceListActivity.class);

            newIntent.putParcelableArrayListExtra("device.list", mDeviceList);

            startActivity(newIntent);
        } else if (BluetoothDevice.ACTION_FOUND.equals(action)) {
            BluetoothDevice device = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);

            mDeviceList.add(device);

            showToast("Found device " + device.getName());
        }
    }
};