Handle events

Section overview

• Use event handler of Context
• Retrieve window associated events via Event.
• Use pub-sub event system

Review the last section

In the last section, "Create a Window", we created a simple base app that shows only a black blank window. The code that we created is shown below.

#include "Terreate/Terreate.hpp"

int main() {
  Terreate::Context context;

  auto window = context.createWindow(700, 500, "My first window");

  while (context.valid()) {
    window->fill(0.85, 0, 0);
    window->clear();

    window->update();
    context.tick(120);
  }

  return 0;
}

In this section, we will handle some events that is associated to window.

Get event handler

The first thing we need to do is to get an event handler from the Context:

...

int main() {
  ...

  auto window = context.createWindow(700, 500, "My first window");
  auto event = context.getEventHandler();

  ...
}

EventHandler has various events that is essential to create applications like mouse, clipboard, keyboard, text input(including IME), and more! If you want to know more about the events, see Event type documentation for more information.

Subscribe to an event

Earlier we got the event handler from the context, but this is not enough to get the event itself - in order to get the event, we need to register a callback to the event:

#include <Terreate/Terreate.hpp>
#include <iostream>

int main() {
  ...

  auto event = context.getEventHandler();

  auto mousePosCallback = [](u64 timestamp, shared<Window> window, shared<Mouse> mouse, vec2 const &pos, vec2 const &rel) {
    std::cout << "Mouse moved to: (" << pos.x << ", " << pos.y << ")" << std::endl;
  };
  event->onMouseMotion.subscribe(mousePosCallback);

  ...
}

This code outputs the mouse position when the mouse cursor is moved. Let's take a look at this step by step. First, we get a handler from context. Next, we define the callback for retrieving the event. The arguments of function is defined for each event type. This is shown in the Event callback type documentation. Finally, we called subscribe to register the callback to event.