Fighting Talk: The Empower Strikes Back - The Game Maker's Companion: Game Development - page 142

Left and Right
The basic problem with horizontal lift segments is that we expect them to move together in a long
line, but the segment at the back of the line has no idea when the segment at the front of the line
has collided with obj_solid . Our lift segments are children of obj_solid themselves, so you might
think that when the segment at the front stops, the next segment would collide into it, and so on,
until all the segments come to a halt. This is kind of true, but these collisions would actually
happen in subsequent steps, producing a messy “train crash” effect. As a result, the Alarm event
would get set in different steps and the line of segments would start breaking apart as they began
moving again (try it if you like).
So what we need is the ability to instantly let all the segments in the line know that there has
been a collision, so that they all can stop moving in the same step. One potential way to do this
would be to apply a Set Alarm action to all lift objects at the same time (just by using the Applies
to option on the top of the action form). Unfortunately, this would mean that all the lifts in the
same room would receive the alarm and come to a halt, so unless we only have one horizontal lift
on each level, then that won't work either.
A better solution is to use a loop to let all the adjacent segments know about the collision.
We've used a Repeat action before to create multiple obj_enemy_die objects when an enemy dies,
but it has all sorts of uses. We will use it here to repeatedly check along a line looking for other
instances of the same lift object that are linked to the one that has collided. We then just need to
change each one into an instance of the lift object that moves in the opposite direction.
Unfortunately, although Game Maker provides a Check Object action that we could use to
check for adjacent instances, there is no way of calling Change Instance on that adjacent
instance. What we need is an equivalent of the Check Object action that also provides us with the
actual instance found. We then need an equivalent of the Change Instance action that can be
called for another instance. Fortunately, both of these are easy to create using scripts that we can
then apply in the horizontal lift objects to achieve the desired effect.

Creating Horizontal Lifts
1. Use the Import Scripts option ( Scripts menu) to load check_object_returned.gml and

change_instance_for.gml from the Chapter06/Resources directory on the CD. This will
add these scripts to the Scripts folder in the resource tree.

2.

Create a new object called obj_lift_hlimit (in the Platforms , Lifts group), give it the
spr_lift_hlimit sprite, and disable the Visible option.

3. Create two new objects called obj_lift_left and obj_lift_right . Give them the

appropriate lift sprites, set their Depth to -1001 , and their Parent to obj_lift .

4. Add a Create event to obj_lift_left and include a Speed Horizontal action ( move

tab) that sets Hor. Speed to 0 . The horizontal platforms will also stand still for a while
as they change direction.

Include a Set Alarm action ( main2 tab) that sets Alarm 0 to 90 Steps . 5.

6.

Add an Alarm, Alarm 0 event and include a Speed Horizontal action ( move tab) that
sets Hor. Speed to -LIFT_SPEED .

7. Add a Collision event with obj_solid and include a Set Variable action ( control tab)

that sets horizontal_offset to 16 . This is a new variable that we will use to keep track
of how far to the left or right we are looking for platform segments.

Include a Repeat action with Times set to 10 . Setting this number to 10 means that we
will stop looking for adjacent platform segments after we have found 10—so platforms
can't be longer than 10 segments. If you want longer platforms, then increase this.

8.