Feature Reference - The Game Maker's Companion: Game Development - page 349

4. Add a Step , Step event and include an Execute Code action. Insert the following lines:
1: {
2: var dx, dy;
3: hspeed = keyboard_check(vk_right)-keyboard_check(vk_left);
4: vspeed = keyboard_check(vk_down)-keyboard_check(vk_up);
5: if ( hspeed != 0 ) vspeed = 0;
6: speed *= 4;
7: if ( speed != 0 )
8: {
9: switch ( direction )
10: {
11: case 0: sprite_index = spr_explorer_right; break;
12: case 90: sprite_index = spr_explorer_up; break;
13: case 180: sprite_index = spr_explorer_left; break;
14: case 270: sprite_index = spr_explorer_down; break;
15: }
16: image_speed = 0.5;
17: } else image_speed = 0;
18: dx = lengthdir_x(speed,direction);
19: dy = lengthdir_y(speed,direction);
20: if ( place_meeting(x+dx,y+dy,obj_wall) ) speed = 0;
21: }
This script requires some explanation. Line 2 first defines two variables that we wish to
use only in this script. Lines 3-4 set the vertical and horizontal speed, dependent on
the state of the arrow keys. We've taken advantage of the result of keyboard_check ,
which either returns 1 ( true ) if the key is pressed or 0 ( false ) otherwise. These values
are used to calculate a vertical and horizontal speed. For example, if vk_right (the right
arrow key) is being pressed while vk_left (the left arrow key) is not, then the outcome
is 1-0 = 1 . The other way around, if vk_left is being pressed but vk_right isn't, then
the outcome is 0-1 = -1 . If neither are being pressed, then the outcome is 0-0 = 0 and
if both are being pressed, then the result is 1-1 = 0 . This is correct as it shouldn't go
left or right on either occasion.
Line 5 limits the speed we have computed to only horizontal or vertical
movement, just in case the player is pressing both a left/right key and an up/down key.
Line 6 multiplies whatever speed we ended up with by four so the explorer actually
moves faster than one pixel per step.
Lines 7-17 change the sprite and animation speed, depending on the current
speed . One cool thing about Game M aker is that when we change hspeed and vspeed ,
then the variables speed and direction are automatically adjusted. So, Line 7 first
makes sure that speed doesn't animate it, because we only want to change the sprite if
a key was pressed. Lines 9-15 then check the direction and change the sprite
accordingly ( 0 for right, 90 for upward, 180 for left, and 270 for downward).
Line 16 sets image_speed to 0.5 to animate the sprite, while the else statement
doesn't animate it if the speed was 0 to begin with.
Lines 18-20 deal with running into walls. The lengthdir_x and lengthdir_y
functions help us calculate how much the instance would travel horizontally and
vertically with the current speed and direction . We then check if there is a wall in that
direction. If there is, we set the speed to 0 to prevent the explorer from going that way.