ulapp/FieldLogger/Services/Ble/BleScanner.cs

using Plugin.BLE;
using Plugin.BLE.Abstractions.Contracts;
using Plugin.BLE.Abstractions.EventArgs;

namespace FieldLogger.Services.Ble;

/// <summary>A device found during a BLE scan.</summary>
public sealed record DiscoveredDevice(Guid Id, string Name, int Rssi);

/// <summary>Thin scan wrapper over Plugin.BLE with per-platform permission handling.</summary>
public sealed class BleScanner
{
    private readonly IAdapter _adapter = CrossBluetoothLE.Current.Adapter;

    public event EventHandler<DiscoveredDevice>? DeviceDiscovered;

    public bool IsScanning => _adapter.IsScanning;

    /// <summary>
    /// Scans for BLE devices for the given duration, raising DeviceDiscovered as devices appear.
    /// Only named devices are reported.
    /// </summary>
    public async Task ScanAsync(TimeSpan duration, CancellationToken cancellationToken = default)
    {
        await EnsurePermissionsAsync();

        if (!CrossBluetoothLE.Current.IsOn)
            throw new InvalidOperationException("Bluetooth is turned off.");

        void OnDiscovered(object? sender, DeviceEventArgs e)
        {
            if (!string.IsNullOrWhiteSpace(e.Device.Name))
                DeviceDiscovered?.Invoke(this, new DiscoveredDevice(e.Device.Id, e.Device.Name, e.Device.Rssi));
        }

        _adapter.DeviceDiscovered += OnDiscovered;
        _adapter.DeviceAdvertised += OnDiscovered;
        try
        {
            using var timeout = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
            timeout.CancelAfter(duration);
            _adapter.ScanTimeout = (int)duration.TotalMilliseconds;
            try
            {
                await _adapter.StartScanningForDevicesAsync(cancellationToken: timeout.Token);
            }
            catch (OperationCanceledException)
            {
                // Scan window elapsed or caller cancelled - both are normal.
            }
        }
        finally
        {
            _adapter.DeviceDiscovered -= OnDiscovered;
            _adapter.DeviceAdvertised -= OnDiscovered;
            if (_adapter.IsScanning)
                await _adapter.StopScanningForDevicesAsync();
        }
    }

    /// <summary>Requests the runtime permissions BLE scanning needs (Android only; no-op elsewhere).</summary>
    public static async Task EnsurePermissionsAsync()
    {
#if ANDROID
        if (OperatingSystem.IsAndroidVersionAtLeast(31))
        {
            var status = await Permissions.RequestAsync<Permissions.Bluetooth>();
            if (status != PermissionStatus.Granted)
                throw new PermissionException("Bluetooth permission was denied.");
        }
        else
        {
            var status = await Permissions.RequestAsync<Permissions.LocationWhenInUse>();
            if (status != PermissionStatus.Granted)
                throw new PermissionException("Location permission (required for BLE scanning) was denied.");
        }
#else
        await Task.CompletedTask;
#endif
    }
}