Event Sources

The EventSource<TEventHandler, TOut> component is used to generate a stream of messages triggered by an event to which it subscribes. Messages are generated whenever the event fires via an event handler of type TEventHandler that posts messages of type TOut.

Construction

Since it is not possible to pass the underlying event itself, the EventSource<TEventHandler, TOut> is constructed by passing to it two delegates of type Action<TEventHandler> that will take care of subscribing and unsubscribing to the event. The event handler is specified by a converter function of type Func<Action<TOut>, TEventHandler> which is supplied an action delegate to post a message on the output stream and creates an event handler of type TEventHandler compatible with the event being subscribed to. This event handler will typically just call the post delegate to post a message, possibly translating the underlying event arguments of type TEventArgs into values of type TOut. Note that output messages may simply be the underlying event arguments themselves, though this is not a requirement.

public EventSource(
        Pipeline pipeline,
        Action<TEventHandler> subscribe,
        Action<TEventHandler> unsubscribe,
        Func<Action<TOut>, TEventHandler> converter)
        ...

Usage

Use EventSource when you need to create a generator component tied to some underlying .NET event. A typical use case would be feeding some UI event as a source in the \psi pipeline. This may be accomplished by creating an EventSource where TEventHandler is of the same type as the UI event, and the messages to be posted on the output stream are of type TOut, whose values are derived from the event arguments. Define the event handler in the return value of the converter function, and hook/unhook the handler to the underlying event in the subscribe and unsubscribe delegates that are passed to the constructor.

The following example illustrates how an EventSource may be used to generate messages containing the value of a slider control whenever the value changes.

// Hook up slider control ValueChanged event to an EventSource
var sliderValue = new EventSource<EventHandler<int>, double>(
        pipeline,
        handler => this.SliderControl.ValueChanged += handler, 
        handler => this.SliderControl.ValueChanged -= handler, 
        post => new RoutedPropertyChangedEventHandler<double>((sender, e) => post(e.NewValue))));

// Output value whenever there is a new message
sliderValue.Do(s => Console.WriteLine("Slider value: {0}", s));