Managing events with the minilibx

By Aurélien Brabant

Wanderer, beware!

Hey! If this post is the first you're reading from me about the minilibx, you'd better check out the first post, which explains all the basics you need to know. That said, let's discuss how we are going to handle events using the minilibx!

How minilibx handles events

To make use of events, it is really important to understand how the minilibx event system works globally.

Remember about the naive while loop we used last time, when we wanted to keep showing the window on the screen ? Well, the minilibx offers us the mlx_loop function, which basically starts an infinite loop for us, but inside this loop things happen. And by things, I essentialy mean events.

Clicking anywhere in the window with the mouse is an event. Pressing a key is another. Events are basically everything the user can do to interact with the program.

To make the program interactive, events are a must have. Before the mlx_loop function is called, the minilibx allows us to register events that are able to be triggered after the loop has started. The minilibx calls these functions hooks.

Take a look at the flowchart below. I tried to summarize briefly how the whole thing works by reading the source code directly.

As you can see, it is somewhat simple. Now that you've an understanding of how the event loop works, let's figure out how to register the events using the minilibx API.

Registering events with minilibx's hooks

In order for us to register events, the minilibx provides us a set of functions called hooks that we'll be able to use to register events before mlx_loop is called.

There are five different hook functions we can actually make use of:

int	mlx_mouse_hook (void *win_ptr, int (*funct_ptr)(), void *param);
int	mlx_key_hook (void *win_ptr, int (*funct_ptr)(), void *param);
int	mlx_expose_hook (void *win_ptr, int (*funct_ptr)(), void *param);
int	mlx_loop_hook (void *mlx_ptr, int (*funct_ptr)(), void *param);
int	mlx_hook(void *win_ptr, int x_event, int x_mask, int (*funct)(), void *param);

You can see that excepting the last prototype, the parameters are identical. Let's discuss what are these parameters quickly:

The mlx_mouse, mlx_key, and mlx_expose hooks

These hooks are self explanatory. You can register keyboard and mouse related event, as well as expose events. Expose events are triggered when the content of a window gets lost (for example, when the minilibx's window is covered partially or entirely by another) and needs to be re-drawn.

Let's take the code we've done in the last post. We're going to add a new feature to it: whenever the escape key is pressed, the window will disappear and all the memory allocated for the program will be freed correctly. Here is a way to do it using mlx_key_hook:

#include <X11/keysym.h>

typedef struct s_data
{
	void	*mlx_ptr;
	void	*win_ptr;
}	t_data;

int	handle_no_event(void *data)
{

Don't feel overwhelmed by this code. There's a lot of things we added here. Let's figure out what it does.

Organizing data to pass it to the hook functions

The first big change is the way we are organizing data. We are now using a t_data object to store all the important stuff. This is a structure defined at the very top of our C file. But why would we want to do that ?

The answer is related to the way we need to pass arguments to the hook functions. We are only able to pass a single void pointer. Therefore, to pass multiple arguments, we obviously need a structure, in order to pass a pointer to this structure. That's the reason why we took this approach.

Hooking into the KeyRelease event using mlx_key_hook

In order for us to get the proper event, we use the mlx_key_hook function. The function that will be executed each time a key is released is handle_input. This is the duty of this function to check which key has been released, and to do things accordingly. We also pass the address of data which is the t_data object that contains all the stuff that we need to access inside handle_input.

The handle_input function checks if the key's symbol corresponds to the escape key. We included the X11/keysym.h header in order to get the values of all the available symbols.

Notice how I keep saying "key symbol" and not "key code". That's because a key code is not the same thing than a key symbol. The key code for the "A" key on an AZERTY keyboard layout will be the same than the key code for the "Q" key on a QWERTY layout. However, what we want to deal with is the symbol. If the key I expect to be pressed is "A", I want it to be "A" whatever the keyboard layout of the user is. I want to get it by the symbol. This conversion is done by the minilibx (to be exact, Xlib handles it) internally. What we're getting in handle_input is the correct key symbol.

If the symbol check evaluates to true, then that means the escape key has been released, so we destroy the window. Because the window gets destroyed, the mlx_loop ends as there's no window left.

Try it out!

mlx_loop_hook

There's another hook we used in the previous example, but we didn't take the time to talk about it.

Well, the mlx_loop_hook is one hook that is triggered when there's no event processed. It is especially useful to draw things continuously on the screen, even if we didn't really need it in our example. The only reason we've used it is because without it, the loop would have never ended. That's directly related to how the mlx_loop is implemented. You can look at the implementation if you're that curious.

Register ANY X event with mlx_hook

Let's talk about the last one now. Maybe you realized it, but mlx_key_hook and mlx_mouse_hook are limited. Did you notice how I kept saying "released" when I was talking about the key hook ?

I didn't say "pressed", simply because a key press and a key release are different events. One problem with these basic hooks is that they only handle one event related to the keyboard and the mouse. For the keyboard, it is the KeyRelease event.

Try to run the program and press the escape key without releasing it. You'll notice that nothing happens unless you release the key. That's because we registered to a KeyRelease event. However, we need an alternative for other events that the X Window System handles. That's exactly what the mlx_hook is used for.

The mlx_hook function takes five arguments. We've already explained what win_ptr, func_ptr and param are, but there are two new parameters:

Let's change the program to add an event handler for the KeyPress event:

#include <X11/X.h>

int	handle_keypress(int keysym, t_data *data)
{
	if (keysym == XK_Escape)
		mlx_destroy_window(data->mlx_ptr, data->win_ptr);

	printf("Keypress: %d\n", keysym);
	return (0);
}

It is needed to add the X11/X.h header in order to get the macros releated to the event names/masks. I decided to replace the mlx_key_hook call by another call to the more generic mlx_hook because I find this clearer.

Each hook as its own handler, and this is working perfectly, with no leaks at all! With mlx_hook, it is possible to hook into any event X provides, so feel free to implement cool stuff on your own!

That said, that's it for this post ! In the next post, we're going to tackle the fun part: drawing things on the screen!

You can find the final code here.