Services

Services are reusable feature.

For example, let's say we wanted to listen to when USB devices were attached and disconnected, and update the drop-down device menu. To do this, we could create a UsbService, register it under the App.axaml.cs (so we can find it in code!), and "inject" it into the Home View.

Here's what this might look like:

namespace Baballonia.Contracts;

public interface IUsbService
{
    public event Action<string> OnUsbConnected;
    public event Action<string> OnUsbDisconnected;
}

info

Note the above interface we have created. By creating an interface, we can mock a UsbService which makes unit testing a whole lot easier!

namespace Baballonia.Services;

public sealed class UsbService : IUsbService
{
    public event Action<string>? OnUsbConnected;
    public event Action<string>? OnUsbDisconnected;

    private static readonly IUsbEventWatcher UsbEventWatcher = new UsbEventWatcher(
        startImmediately: true,                 // True - This part is obvious
        addAlreadyPresentDevicesToList: false,  // False - This part is less obvious.
                                                // Don't check devices that are already plugged in! This will spam the refresh queue on launch
        usePnPEntity: false,                    // False - PnP entity is slower, overkill for our use case
        includeTTY: true);                      // True - Legacy Babble Tracker show up under /dev/ttyACM*

    private readonly TimeSpan _eventThrottleInterval = TimeSpan.FromSeconds(1);
    private DateTime _lastEventTime = DateTime.MinValue;
    private readonly object _eventLock = new();

    public UsbService()
    {
        UsbEventWatcher.UsbDeviceAdded += UsbDeviceAdded;
        UsbEventWatcher.UsbDeviceRemoved += UsbDeviceRemoved;
    }

    private void UsbDeviceAdded(object? sender, UsbDevice? device)
    {
        if (device == null)
            return;

        RateLimitedAction(device.DeviceName, OnUsbConnected);
    }

    private void UsbDeviceRemoved(object? sender, UsbDevice? device)
    {
        if (device == null)
            return;

        RateLimitedAction(device.DeviceName, OnUsbDisconnected);
    }

    private void RateLimitedAction(string deviceName, Action<string>? action)
    {
        var now = DateTime.UtcNow;
        var timeSinceLastEvent = now - _lastEventTime;

        if (timeSinceLastEvent < _eventThrottleInterval)
        {
            return;
        }

        _lastEventTime = now;
        
        action?.Invoke(deviceName);
    }

    ~UsbService()
    {
        UsbEventWatcher.UsbDeviceAdded -= UsbDeviceAdded;
        UsbEventWatcher.UsbDeviceRemoved -= UsbDeviceRemoved;
        UsbEventWatcher.Dispose();
    }
}

Now, we inject this under the App.axaml.cs:

...
services.AddTransient<IUsbService, UsbService>();
...

And now we can pull it in from the constructor in any of our registered classes.