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.

NOTE Some C# classes are currently in BETA state
They should only be used for experimenting and NOT for any product development.
These classes are marked as BETA in the reference documentation.
The source code for the tutorials is subject to change in the future.

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 (only in C, C# coming soon).

In C#, you register a callback method for a given event by using the += operator:

object.event += callback;

Substitute object with any EFL object and event with the identifier of the event (such as PollHighEvt or TickEvt). Set callback to the method to be invoked 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. Events emitting additional information require that you use their own EventArgs class, for example, Efl.Input.InterfaceKeyDownEvt_Args when connecting to the Efl.Input.Interface.KeyDownEvt event.

NOTE: The API Reference documentation for each event tells you what type to cast e to (Not available for C# yet). See EFL_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 for 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.


Below is the core_event.cs example taken from the examples repository:

public class Example
    // Polling callback
    private static void PollCb(object sender, EventArgs e)
        Console.WriteLine("  Poll from {0}", ((Efl.Object)sender).GetName());
    public static void Main()
        // Initialize EFL and all UI components
        // Retrieve the application's main loop
        var mainloop = Efl.App.AppMain;
        // This event gets triggered continuously
        mainloop.PollHighEvt += PollCb;
        // This timer will control events fired by the main loop
        var timer = new Efl.LoopTimer(mainloop, 0.1);
        // To count number of timer triggers
        int tick_count = 0;
        timer.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.Object)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). 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
C# Source code for this example.
Efl.Object API Reference
Detailed documentation for the EFL object, which implements the events mechanism.