Events and IObservable

Meadow has two complementary approaches to notifications when interrupts (changes) happen on inputs. The first is the classic .NET Eventing pattern, in which an event is raised on change, and the second is an IObservable pattern, which is a more powerful and flexible reactive-style approach.

Classic Events

The event pattern is a familiar and simple pattern for subscribing to change notifications; interrupts (changes) on inputs such as digital or analog inputs can raise a Changed notification, that can be subscribed to via a classic handler (delegate):

public class ButtonEventsApp : App<F7Micro, ButtonEventsApp>
{
    IDigitalInputPort input;

    public ButtonEventsApp()
    {
        input = Device.CreateDigitalInputPort(Device.Pins.D02);
        input.Changed += (s,e) => {
            Console.WriteLine($"Changed: {result.New.State.ToString()}, Time: {result.New.Time.ToString()}");
        };
    }
}

IObservable/Reactive Pattern

However, for more advanced filtering we've exposed System.IObservable support, along with a IObservable<IChangeResult<UNIT>> that allows you to subscribe to an observable, with an optional predicate to filter on the events, as well as a handler shortcut. Consider the following code:

public class InputObservableApp : App<F7Micro, InputObservableApp>
{
    IDigitalInputPort input;

    public InputObservableApp()
    {
        // create an input port on D02. 
        input = Device.CreateDigitalInputPort(Device.Pins.D02);

        // Note that the filter is an optional parameter. If you're
        // interested in all notifications, don't pass a filter/predicate.
        var observer = IDigitalInputPort.CreateObserver(
            handler: result => {
                Console.WriteLine($"Observer filter satisfied, time: {result.New.Time.ToShortTimeString()}");
            },
            // Optional filter parameter, showing a 1 second filter, i.e., only notify
            // if the new event is > 1 second from last time it was notified.
            filter: result => {
                if (result.Old is { } old) { // C# 8 null pattern matching for not null
                    return (result.New.Time - old.Time) > TimeSpan.FromSeconds(1);
                } else return false;
            });
        input.Subscribe(observer);
    }
}

Ports in Meadow.Core and peripherals in Meadow.Foundation expose a static CreateObserver() convenience method for creating filterable observers that can listen for change notifications. That observer takes two arguments:

• Action<IChangeResult<UNIT>> handler - The subscriber/handler that will be called when the notification is raised.
• Predicate<IChangeResult<UNIT>>? filter = null - An optional filter expression, or predicate, that tests for a particular condition is passed in to the FilterableChangeObserver constructor, which is used to test whether the change satisfies a particular condition. Any expression that evaluates to a boolean (true/false), can be used.

Examining the filter predicate in the above code, it filters out events that occur less than a second after the last notification:

return (result.New.Time - old.Time) > TimeSpan.FromSeconds(1);

General Use

However, the Subscribe method will take any IObservable, so you can also use whatever Reactive pattern and framework you choose.

Advantages of FilterableChangeObserver

The FilterableChangeObservable pattern has the distinct advantage of handling event filtering internally, meaning that consuming code will only be notified with the notifications satisfy the filter expression passed in.