A4 - Timers

Often in a game, we want something to happen after a certain period of time. Beginner game programmers will often try to put a time.sleep statement somewhere to achieve this, only to realize this completely freezes the game.

in OArcade, the solution is to use the oa.set_timer function. It accepts two arguments:

• The time in seconds to wait
• A reference to a function/method to call after that amount of time has passed.

We can see an example of this in the Fly sprite:

    def on_step(self):

        # ... (Irrelevant stuff) ...

        if colliding_player:
            colliding_player.take_damage(2)
            self.movement_state = "retreat"
            oa.set_timer(3, self.reset_movement_state)

        # ... (Irrelevant stuff) ...

    def reset_movement_state(self):
        self.movement_state = "towards player"

Here, after the fly has collided with a player, it moves into the "retreat" state and sets a timer to called the reset_movement_state method after three seconds.

Main Task

Download this fireball image and put it into the images folder within your game.

Add the following code to the end of sprites.py:

class FireBall(oa.Sprite):

    def on_create(self):
        self.visual = "images/fireball.png"
        self.layer = cc.Layers.MOVING_GAME_OBJECTS
        self.change_x = 8

    def on_step(self):
        # If we've gone off screen, remove ourself from the game.
        if refs.cur_view.world_camera.point_in_view(int(self.x), int(self.y)) == False:
            self.kill()

In level_1.py, add the following method to your Level_1 class:

    def spawn_fireball(self):
        # Choose a y-value for the fireball within a 200 pixel range of the player
        y = int(self.player.y + random.randint(-200, 200))
        FireBall(-10, y)

In the on_create method in the Level_1 class, set a timer to call self.spawn_fireball after two seconds.

Finally, copy the same line to the end of the spawn_fireball method you pasted in earlier, which should then create a loop of fireballs spawning every two seconds.

Extended Task

Find another way to integrate a timer into the game. For example, maybe an enemy will do something every ___ number of seconds.