Feature Reference - The Game Maker's Companion: Game Development - page 362

If you need more than one ship to have a cannon (for example, when enemies have gun
mounts as well), you'll notice a problem with the current setup: only one of them would carry all
cannons! This happens because Game Maker's actions address all instances of obj_cannon at
once, rather than the one we've created specifically for that ship. To fix this, we need to keep track
of the cannon instance each ship creates for itself.

To Make Each Spaceship Keep Its Own Cannon
1. First, open obj_cannon , select the End Step event, and click Delete .
2. Now, open obj_spaceship , select the Create event, and delete the Create Instance
action. Include an Execute Code action and insert the following lines:
1: {
2: cannon = instance_create(0,0,obj_cannon);
3: }

This script snippet stores the ID of the instance in a local variable called cannon . We
can now use this variable to reference only the cannon we've created for this ship.
3. Because only the spaceship knows which cannon is its own, we'll make it responsible
for keeping it in the correct place. Add a Step , End Step event and include an Execute
Code action. Insert the following lines:
1: {
2: cannon.x = x;
3: cannon.y = y;
4: }

4. Now, open the Destroy event and remove the Destroy Instance action. Include an
Execute Code action and insert the following lines:
1: {
2: with ( cannon ) instance_destroy();
3: }

5. If you wish to test this, place a few more instances of obj_spaceship in the room. Keep

in mind they're all controlled by the same mouse and keys! Maybe you could make a
game of that…

Result: Reference/Result/mouse_aim_and_fire2.gmk

Mouse Cursor
Do you think the Windows mouse cursor is rather dull? Or do you need a game where the mouse
cursor consists of crosshairs to shoot with? Here is how you can replace the default mouse cursor
with a sprite of your own.