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~~Title: Events Programming Guide in C#~~

# Events Programming Guide in C# #

EFL is event-driven. This means that execution usually takes place within an internal EFL *Main Loop*. The application receives notifications through function callbacks. These can apply to virtually any event which occurs on a computer. 

Events play a central role in EFL. In this guide, you'll learn more about the required methods to handle them.

You can also find usage examples in the [EFL examples repository](https://git.enlightenment.org/tools/examples.git/tree/).

## Listening to Events from Objects ##

All EFL objects can emit events. You can discover more about them in the **Events** section of their respective [API Reference documentation](/develop/api/) (only in C, C# *coming soon*).

In C# you register a callback method to be called when an object emits a given event using the `+=` operator:

object.event += callback;

Substitute *object* for any EFL object and *event* for the identifier of the event (such as ``PollHighEvt`` or ``TickEvt``). Set *callback* to the method to be called when the event occurs.

The method signature for the callback is:

void callback(object sender, EventArgs e);

*sender* is the object that emitted the event and *e* contains any additional information that the event sent, after casting it to the required type (for example, `efl.input.Interface.KeyDownEvt_Args` when connecting to the `efl.input.Interface.KeyDownEvt` event).

> **NOTE:**
> The [API Reference documentation](/develop/api/) for each event tells you what type to cast *e* to (*Not available for C# yet*).
> See [EFL_EVENT_POINTER_DOWN](/develop/api/efl/input/interface/event/pointer_down) for example.

To stop receiving notifications for a particular event, unregister the callback using the `-=` operator:

object.event -= callback;

Note that in order to unregister the callback you have to provide the callback method again. This is because you can register different callback methods to the same event.

## Pausing and Resuming Event Notifications ##

All event emissions from a given object can be paused (*frozen*) using `FreezeEvent()` and resumed with `ThawEvent()`:


While an object is frozen only high-priority events (marked as *hot* in the documentation) will be emitted. Hot events cannot be stopped.

Remember that ALL events emitting from a object are stopped if it's frozen, except for hot events. If you need to stop individual events you can unregister their callback temporarily and then re-register later.

## Example ##

Below is the `core_event.cs` example taken from [the examples repository](https://git.enlightenment.org/tools/examples.git/tree/reference/csharp/core/src/core_event.cs):

public class Example
    // Polling callback
    private static void PollCb(object sender, EventArgs e)
        Console.WriteLine("  Poll from {0}", ((efl.IObject)sender).GetName());

    public static void Main()
        // Initialize EFL and all UI components

        // Retrieve the application's main loop
        var mainloop = efl.App.GetLoopMain();

        // This event gets triggered continuously
        mainloop.PollHighEvt += PollCb;

        // This timer will control events fired by the main loop
        new efl.Loop_Timer(mainloop, (efl.ILoop_Timer etimer) => {
            // Trigger every 100ms
            // To count number of timer triggers
            int tick_count = 0;
            etimer.TickEvt += (object sender, EventArgs e) => {
                string message = "Tick {0} from {1}: ";
                // Depending on the number of timer ticks, it does a different thing
                switch (tick_count) {
                    case 0:
                        message += "Freezing Mainloop events";
                    case 1:
                        message += "Thawing Mainloop events";
                        message += "Quitting";
                        mainloop.Quit(new eina.Value(0));
                Console.WriteLine(message, tick_count, ((efl.IObject)sender).GetName());

        Console.WriteLine("Waiting for Timer to call back...");

        // Start the EFL main loop (and the experiment)

        // Shutdown EFL

        Console.WriteLine("Application is over");

A handler is connected to the `PollHighEvt` event of the application's main loop, which triggers continuously, at an undefined frequency of several shots per second (See the Main Loop Programming Guide, *coming soon*). At every shot, a line is printed on the console.

At the same time, a timer is instantiated, firing every 100ms, which does a different thing at every shot:

* First it freezes (pauses) all main loop events (except hot ones).
* Then it thaws (resumes) all main loop events.
* Finally, it quits the application.

When you run the application, it should produce something like this on the console:

Waiting for Timer to call back...
  Poll from Mainloop
  Poll from Mainloop
  Poll from Mainloop
  Poll from Mainloop
  Poll from Mainloop
  Poll from Mainloop
Tick 0 from Timer: Freezing Mainloop events
Tick 1 from Timer: Thawing Mainloop events
  Poll from Mainloop
  Poll from Mainloop
  Poll from Mainloop
  Poll from Mainloop
  Poll from Mainloop
  Poll from Mainloop
Tick 2 from Timer: Quitting

As you can see, the line `Poll from Mainloop` is printed continuously except in the period between Tick 0 and Tick 1 of the Timer, where main loop events are frozen.

The exact amount of `Poll from Mainloop` messages you get depends on the frequency of the `PollHighEvt` event, which is chosen by EFL. The important thing is that there should be no such messages in between timer ticks 0 and 1, since main loop events are frozen,

## Further Reading ##

[`core_event.cs` example](https://git.enlightenment.org/tools/examples.git/tree/reference/csharp/core/src/core_event.cs)
:    C# Source code for this example.

[`Efl.Object` API Reference](https://www.enlightenment.org/develop/api/efl/object)
:    Detailed documentation for the EFL object, which implements the events mechanism.