Writing games in Perl - Part 2 - Controlling the Ball
Following the first post on the subject of writing games in Perl, where we created a bouncing ball (I know, it is a rectangle, but I trust your imagination), this post is going to add something very important when dealing with games: input.
Silveira Neto suggested that I should include more specific instructions on how to start the game (and maybe a video), so I recalled that I didn't mention that all the sources for this posts (including the text) is currently hosted at a github repository (if you plan to contribute, please just ask me for commit permissions instead of forking the repo).
So if you want to run the codes posted here, you first need to:
$ git clone http://github.com/ruoso/games-perl.git
You can check for updates by calling
$ git pull origin master
from inside the games-perl directory. Each directory inside games-perl starts with the number of the post. The first post is inside the 1-bouncing-ball directory and the second is in 2-controlling. To run the the first code just get inside the first directory and call:
$ perl ball.pl
The second example code is based on the first, so the script name is the same, so just get into the other directory and run the same line. If you get an error like:
Can't locate SDL/Video.pm in @INC (@INC contains: /etc/perl /usr/local/lib/perl/5.10.0 /usr/local/share/perl/5.10.0 /usr/lib/perl5 /usr/share/perl5 /usr/lib/perl/5.10 /usr/share/perl/5.10 /usr/local/lib/site_perl .) at ball.pl line 8.
It means you probably don't have the newest SDL, take a look at the first post to see how to get the newest redesigned SDL.
Controlling the Ball
Enough for the introduction, let's get to the actual code. The first thing we need is understanding SDL Events. If you ever programmed GUI applications or even if you wrote some javascript you are aware of how an event framework looks like. SDL is no exception, you need to wait (or poll) for the events, and each event will contain the information you need to figure out what happened.
In our case, we want to apply additional acceleration to the ball whenever the arrow keys are pressed. But if we have an event-based system, the way to figure out which of those four keys is currently pressed is keeping a state mask and update it when you receive keydown and keyup events.
So what we're going to do is to manipulate the acc_h and acc_v ball attributes depending on the keydown and keyup events. It might look complicated, but the only change we need is (this is inside ball.pl main loop):
while (SDL::Events::poll_event($event)) {
if ($event->type == SDL_QUIT) {
exit;
} elsif ($type == SDL_KEYDOWN &&
$sevent->key_sym() == SDLK_LEFT) {
$ball->acc_h(-1);
} elsif ($type == SDL_KEYUP &&
$sevent->key_sym() == SDLK_LEFT) {
$ball->acc_h(0);
} elsif ($type == SDL_KEYDOWN &&
$sevent->key_sym() == SDLK_RIGHT) {
$ball->acc_h(1);
} elsif ($type == SDL_KEYUP &&
$sevent->key_sym() == SDLK_RIGHT) {
$ball->acc_h(0);
} elsif ($type == SDL_KEYDOWN &&
$sevent->key_sym() == SDLK_UP) {
$ball->acc_v(1);
} elsif ($type == SDL_KEYUP &&
$sevent->key_sym() == SDLK_UP) {
$ball->acc_v(0);
} elsif ($type == SDL_KEYDOWN &&
$sevent->key_sym() == SDLK_DOWN) {
$ball->acc_v(-1);
} elsif ($type == SDL_KEYUP &&
$sevent->key_sym() == SDLK_DOWN) {
$ball->acc_v(0);
}
}
So, this is it. Follows a small video of the game.