Can't find devices.

[mthnzbk]

I don't get any error in the code below, but the StartScanningForDevicesAsync() method does not find open bluetooth devices. So Adapter.DeviceDiscovered += async (s, a) => {} I don't get any output in this part. Nor error.

Configuration

Version of the Plugin: 3.0.0

**Platform:**Android 10-11

Device: Xiaomi Note 7
`using Plugin.BLE;
using Plugin.BLE.Abstractions.Contracts;
using System.Collections.ObjectModel;
using System.Diagnostics;

namespace Kumanda
{
public partial class MainPage : ContentPage
{
readonly IBluetoothLE Ble = CrossBluetoothLE.Current;
readonly IAdapter Adapter = CrossBluetoothLE.Current.Adapter;

    public ObservableCollection<String> Devices;
    public ObservableCollection<IDevice> Device;

    public MainPage()
    {
        InitializeComponent();

        Ble.StateChanged += (s, e) =>
        {
            Debug.WriteLine("State: " + e.NewState);
        };



        Adapter.DeviceDiscovered += async (s, a) =>
        {
            Debug.WriteLine("Device: " + a.Device.ToString());
            try
            {
                Devices.Add(a.Device.ToString());
                Device.Add(a.Device);
                await Adapter.ConnectToDeviceAsync(a.Device);
                var service = await a.Device.GetServicesAsync();
                Debug.WriteLine(service.ToString());
                Debug.WriteLine(Devices + " " + Device);
            }
            catch (Exception ex)
            {
                Debug.WriteLine(ex);
            }
            
        };

        Adapter.DeviceBondStateChanged += (s, e) =>
        {
            Debug.WriteLine(e.State + " " + e.Address);
        };

        Adapter.DeviceConnected += (s, e) =>
        {
            Debug.WriteLine(e.Device.ToString());
        };
    }

    protected override void OnAppearing()
    {
        base.OnAppearing();
        
    }

    private async void OnCounterClicked(object sender, EventArgs e)
    {
        Debug.WriteLine(Ble.State);
        try
        {
            await Adapter.StartScanningForDevicesAsync();
        }
        catch (Exception ex)
        {
            Debug.WriteLine(ex);
        }
    }
}

}`

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[janusw]

I don't get any error in the code below, but the StartScanningForDevicesAsync() method does not find open bluetooth devices. So Adapter.DeviceDiscovered += async (s, a) => {} I don't get any output in this part. Nor error.

Possibly you're failing to request permissions properly? See https://github.com/dotnet-bluetooth-le/dotnet-bluetooth-le#permissions

You need to add the relevant permissions to your manifest and also request them at runtime.

Which SDK level are you targeting? Do you use Xamarin or MAUI?

[mthnzbk]

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" /> <uses-permission android:name="android.permission.INTERNET" /> <uses-permission android:name="android.permission.BLUETOOTH" /> <uses-permission android:name="android.permission.BLUETOOTH_ADMIN" /> <uses-permission android:name="android.permission.BLUETOOTH_CONNECT" /> <uses-permission android:name="android.permission.BLUETOOTH_ADVERTISE" /> <uses-permission android:name="android.permission.BLUETOOTH_SCAN" /> <uses-permission android:name="android.permission.READ_USER_DICTIONARY" /> <uses-permission android:name="android.permission.VIBRATE" /> <uses-permission android:name="android.permission.WRITE_USER_DICTIONARY" /> <uses-sdk android:minSdkVersion="26" android:targetSdkVersion="33" />

`protected override void OnCreate(Bundle savedInstanceState)
{
base.OnCreate(savedInstanceState);
if (ContextCompat.CheckSelfPermission(this, Manifest.Permission.BluetoothScan) != Permission.Granted)
{
ActivityCompat.RequestPermissions(this, new[] {Manifest.Permission.BluetoothScan }, 0);
}

if (ContextCompat.CheckSelfPermission(this, Manifest.Permission.BluetoothAdvertise) != Permission.Granted)
{
    ActivityCompat.RequestPermissions(this, new[] { Manifest.Permission.BluetoothAdvertise }, 0);
}
if (ContextCompat.CheckSelfPermission(this, Manifest.Permission.BluetoothConnect) != Permission.Granted)
{
    ActivityCompat.RequestPermissions(this, new[] { Manifest.Permission.BluetoothConnect }, 0);
}
if (ContextCompat.CheckSelfPermission(this, Manifest.Permission.BluetoothAdmin) != Permission.Granted)
{
    ActivityCompat.RequestPermissions(this, new[] { Manifest.Permission.BluetoothAdmin }, 0);
}

}`

.net 7 Maui

[Moes81]

Hm, same problem on Android 12 (Pixel 4).

these are my Permissions:

public class Android11BluetoothPermission : Microsoft.Maui.ApplicationModel.Permissions.BasePlatformPermission
{
#if ANDROID
    public override (string androidPermission, bool isRuntime)[] RequiredPermissions =>
        new List<(string permission, bool isRuntime)>
        {
            ("android.permission.ACCESS_COARSE_LOCATION", true),
            ("android.permission.ACCESS_FINE_LOCATION", true),
            ("android.permission.BLUETOOTH", true),
            ("android.permission.BLUETOOTH_ADMIN", true)
        }.ToArray();
#endif
}

public class Android12BluetoothPermission : Microsoft.Maui.ApplicationModel.Permissions.BasePlatformPermission
{
#if ANDROID
    public override (string androidPermission, bool isRuntime)[] RequiredPermissions =>
        new List<(string permission, bool isRuntime)>
        {
            ("android.permission.BLUETOOTH_SCAN", true),
            ("android.permission.BLUETOOTH_CONNECT", true)
        }.ToArray();
#endif
}

Permissions are requested properly, are granted, but still no scan results. :-( Not sure what to do or how to debug.
This is the start scan method, using Rx.Net to publish the list of found devices. However the OnAdapterOnDeviceDiscovered is never called.

public IObservable<IEnumerable<DeviceCandidate>> StartBluetoothScan()
    {
        if (_scanObservable != null)
        {
            return _scanObservable;
        }

        var observable = Observable.Create<IEnumerable<DeviceCandidate>>(observer =>
        {
            _scanObserver = observer;
            var deviceCandidates = new List<DeviceCandidate>();

            _adapter.ScanMode = ScanMode.LowLatency;
            _adapter.ScanMatchMode = ScanMatchMode.AGRESSIVE;
            _adapter.ScanTimeout = 30000;
            _adapter.DeviceAdvertised += OnAdapterOnDeviceDiscovered;
            _adapter.DeviceDiscovered += OnAdapterOnDeviceDiscovered;

            return Disposable.Create(() =>
            {
                _adapter.DeviceAdvertised -= OnAdapterOnDeviceDiscovered;
                _adapter.DeviceDiscovered -= OnAdapterOnDeviceDiscovered;
                Task.Run(async () => await StopBluetoothScanAsync());
            });

            void OnAdapterOnDeviceDiscovered(object? _, DeviceEventArgs args)
            {
                deviceCandidates.Add(new DeviceCandidate(args.Device.Id, args.Device.Name));
                observer.OnNext(deviceCandidates.DistinctBy(candidate => candidate.Id));
            }
        });

        _scanObservable = observable;
        Task.Run(async () => await _adapter.StartScanningForDevicesAsync(allowDuplicatesKey: true)
        );
        return observable;
    }

Btw. How to I remove the time out so I can scan forever?

[janusw]

See my comment below: #766 (comment)

[janusw]

Btw. How to I remove the time out so I can scan forever?

simply set CrossBluetoothLE.Current.Adapter.ScanTimeout to a very large value

[Moes81]

I'm now able to get scan results with the following configuration:

//AndroidManifest.xml
  <uses-feature android:name="android.hardware.bluetooth_le" android:required="true"/>

  <!-- Request legacy Bluetooth + location permissions on older devices. -->
  <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" android:maxSdkVersion="30"/>
  <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" android:maxSdkVersion="30"/>
  <uses-permission android:name="android.permission.BLUETOOTH"/>
  <uses-permission android:name="android.permission.BLUETOOTH_ADMIN"/>

  <uses-permission android:name="android.permission.BLUETOOTH_SCAN" android:usesPermissionFlags="neverForLocation"/>
  <uses-permission android:name="android.permission.BLUETOOTH_CONNECT"/>

public class Android12BluetoothPermission : Microsoft.Maui.ApplicationModel.Permissions.BasePlatformPermission
{
#if ANDROID
    public override (string androidPermission, bool isRuntime)[] RequiredPermissions =>
        new List<(string permission, bool isRuntime)>
        {
            ("android.permission.BLUETOOTH_SCAN", true),
            ("android.permission.BLUETOOTH_CONNECT", true)
        }.ToArray();
#endif
}

public class Android11BluetoothPermission : Microsoft.Maui.ApplicationModel.Permissions.BasePlatformPermission
{
#if ANDROID
    public override (string androidPermission, bool isRuntime)[] RequiredPermissions =>
        new List<(string permission, bool isRuntime)>
        {
            ("android.permission.ACCESS_COARSE_LOCATION", true),
            ("android.permission.ACCESS_FINE_LOCATION", true)
        }.ToArray();
#endif
}

[janusw]

Great! I assume you fixed the problem by adding the neverForLocation flag in the manifest.

[janusw]

From https://developer.android.com/develop/connectivity/bluetooth/bt-permissions:

If your app uses Bluetooth scan results to derive physical location, declare the ACCESS_FINE_LOCATION permission. Otherwise, you can strongly assert that your app doesn't derive physical location.

[mthnzbk]

I also added the missing permission, but again when the device search method works, it cannot find the pc and the other phone.

[janusw]

Well, hard to say what exactly is wrong, without a proper reproducer. I'd still bet on some kind of permission problem. We have a MAUI sample client that can act as a blueprint for your own client.

[janusw]

Or are you saying the scanning as such works now, and you only miss specific devices?