Prototyping The Game - Developing Games With Ruby - page 58

96 angle -= 45.0 if $window . button_down?(left)
97 angle += 45.0 if $window . button_down?(right)
98 elsif $window . button_down?(left)
99 angle = 90.0
100 angle += 45.0 if $window . button_down?(up)
101 angle -= 45.0 if $window . button_down?(down)
102 elsif $window . button_down?(right)
103 angle = 270.0
104 angle -= 45.0 if $window . button_down?(up)
105 angle += 45.0 if $window . button_down?(down)
106 end
107 angle || previous_angle
108 end
109 end

Tank has to be aware of the Map to check where it's moving, and it uses Camera to find
out where to aim the guns. When it shoot s, it produces instances of Bullet , that are
simply returned to the caller. Tank won't keep track of them, it's “fire and forget”.

Implementing Bullets And Explosions

Bullets will require some simple vector math. You have a point that moves along the vector
with some speed. It also needs to limit the maximum vector length, so if you try to aim too
far, the bullet will only go as far as it can reach.
03-prototype/entities/bullet.rb

1 class Bullet
2

COLOR = Gosu :: Color :: BLACK

MAX_DIST = 300 3

START_DIST = 20 4

5
6 def initialize (source_x, source_y, target_x, target_y)
7 @x , @y = source_x, source_y
8 @target_x , @target_y = target_x, target_y
9 @x , @y = point_at_distance( START_DIST )
10 if trajectory_length > MAX_DIST
11 @target_x , @target_y = point_at_distance( MAX_DIST )
12 end
13 sound . play
14 end
15
16 def draw
17 unless arrived?
18 $window . draw_quad( @x - 2 , @y - 2 , COLOR ,