Android Bluetooth Helper (power up/down adapter, build config based on SDK level etc.)

BluetoothHelper.cs

public static class BluetoothHelper
  {
    private static readonly BluetoothManager _manager;
    private static readonly BluetoothAdapter _adapter;

    static BluetoothHelper()
    {
      var context = Application.Context;
      _manager = (BluetoothManager)context.GetSystemService("bluetooth");
      _adapter = _manager.Adapter;
    }

    /// <summary>
    /// ONLY works for API Level >= 21. Less than 21 needs the old OnLeScan.
    /// Builds scan settings based on the API level of the device based on
    /// https://developer.android.com/reference/android/bluetooth/le/ScanSettings.html
    /// (see/set the Api Level tag to see what's available).
    /// </summary>
    /// <returns></returns>
    public static ScanSettings GetScanSettingsForApiSdkLevel()
    {
      switch (Build.VERSION.SdkInt)
      {
        // disable this until we actually have a marshmallow device to test it on
        // until then, treat 'M' devices as 'L' devices.
        //case BuildVersionCodes.M: // 23, marshmallow
        //  {
        //    const int MATCH_NUM_ONE_ADVERTISEMENT = 1;
        //    const int MATCH_NUM_FEW_ADVERTISEMENT = 2;
        //    const int MATCH_NUM_MAX_ADVERTISEMENT = 3;
        //    long reportDelayMs = 3000;
        //    return new ScanSettings.Builder().SetScanMode(Android.Bluetooth.LE.ScanMode.LowLatency)
        //      .SetCallbackType(ScanCallbackType.AllMatches)
        //      .SetMatchMode(BluetoothScanMatchMode.Aggressive)
        //      .SetNumOfMatches(MATCH_NUM_MAX_ADVERTISEMENT)
        //      .SetReportDelay(reportDelayMs)
        //      .Build();
        //  }
        case BuildVersionCodes.M: // 23, marshmallow
        case BuildVersionCodes.Lollipop: // 21
        case BuildVersionCodes.LollipopMr1: // 22
        default:
          {
            return new ScanSettings.Builder().SetScanMode(Android.Bluetooth.LE.ScanMode.LowLatency)
              .SetCallbackType(ScanCallbackType.AllMatches)
              .Build();
          }
      }
    }

    /// <summary>
    /// Some devices take a little while longer to spin up the bluetooth adapter (e.g. Moto X, see issue #23)
    /// This will wait up to 2000ms for it to transition to <see cref="State.On"/>, tho it should come up a lot 
    /// more quickly than that.
    /// </summary>
    /// <returns></returns>
    public static async Task WaitForBluetoothToPowerOn(BluetoothAdapter adapter = null)
    {
      adapter = adapter ?? _adapter;

      if (adapter.State == State.On) return;

      if (adapter.State != State.On)
      {
        adapter.Enable();
      }

      int i = 1;
      // check 5 times a second for it to power on, maybe a bit aggressive.
      while (adapter.State != State.On && i < 10)
      {
        await Task.Delay(200);
        i++;
      }
      LogHost.Default.Debug($"Waited {i * 200}ms for Bluetooth/BleScanner to power on");
    }

    /// <summary>
    /// Waits up to 2,000ms for Bluetooth adapter's power state to transition to <see cref="State.Off"/>, tho it should
    /// go away a lot more quickly than that.
    /// </summary>
    /// <returns></returns>
    public static async Task WaitForBluetoothToPowerOff(BluetoothAdapter adapter = null)
    {
      adapter = adapter ?? _adapter;
      int i = 1;

      if (adapter.State == State.Off) return;

      if (adapter.State != State.Off)
      {
        adapter.Disable();
      }

      while (adapter.State != State.Off && i < 10)
      {
        await Task.Delay(200);
        i++;
      }
      LogHost.Default.Debug($"Waited {i * 200}ms for Bluetooth/BleScanner to turn off");
    }

    /// <summary>
    /// Works for devices like headsets, handsfree car kits, etc., but fails miserably
    /// at detecting bluetooth-tethered internet connections. On the plus side, if the profile 
    /// type is not supported, it means the device won't be connected to any of those types 
    /// either.
    /// </summary>
    /// <param name="manager"></param>
    /// <returns></returns>
    public static List<BluetoothDevice> GetConnectedDevices(BluetoothManager manager = null)
    {
      // BondedDevices just returns the list of devices the device has paired with, previously,
      // but doesn't say whether any of them are currently connected.
      // see http://developer.android.com/reference/android/bluetooth/BluetoothDevice.html#BOND_BONDED
      //var bondedDevices = manager.Adapter.BondedDevices;
      //if (bondedDevices.Any())
      //{
      //  this.Log().Debug(string.Format("Paired with {0}", string.Join(", ", bondedDevices.Select(bd => bd.Name))));
      //}
      manager = manager ?? _manager;

      var devices = new List<BluetoothDevice>();

      devices.AddRange(GetConnectedDevicesOfType(manager, ProfileType.Headset));
      if (devices.Any()) return devices.ToList();

      devices.AddRange(GetConnectedDevicesOfType(manager, ProfileType.A2dp));
      if (devices.Any()) return devices.ToList();

      devices.AddRange(GetConnectedDevicesOfType(manager, ProfileType.Health));
      if (devices.Any()) return devices.ToList();

      devices.AddRange(GetConnectedDevicesOfType(manager, ProfileType.Gatt));
      if (devices.Any()) return devices.ToList();

      devices.AddRange(GetConnectedDevicesOfType(manager, ProfileType.GattServer));
      if (devices.Any()) return devices.ToList();

      devices.AddRange(GetConnectedDevicesOfType(manager, ProfileType.Sap));
      return devices;
    }

    /// <summary>
    /// Not all Bluetooth profile types are supported on all devices for lookup,
    /// so they throw an illegal argument exception instead of just returning 
    /// 'no devices', so that's what we do here.
    /// </summary>
    /// <param name="manager"></param>
    /// <param name="profileType"></param>
    /// <returns></returns>
    private static IEnumerable<BluetoothDevice> GetConnectedDevicesOfType(BluetoothManager manager = null, ProfileType profileType = ProfileType.Headset)
    {
      manager = manager ?? _manager;

      var devices = new List<BluetoothDevice>();
      try
      {
        devices.AddRange(manager.GetConnectedDevices(profileType));
        return devices;
      }
      catch (Java.Lang.IllegalArgumentException ex)
      {
        LogHost.Default.Debug(ex.Message);
      }
      return devices;
    }

    public static bool IsCurrentlyConnected(BluetoothManager manager = null)
    {
      manager = manager ?? _manager;
      return GetConnectedDevices(manager).Any();
    }

    public static List<BluetoothDevice> ListConnectedDevices(this BluetoothManager manager)
    {
      manager = manager ?? _manager;
      return GetConnectedDevices(manager);
    }

}