GML: From Ninja to Pirate - The Game Maker's Companion: Game Development - page 265

If there wasn't a collision, then lines 18 and 19 will move the instance forward one pixel
and line 20 will add one to the dist variable, accordingly. We now return to the start of the
while loop, which will keep repeating this process until dist is greater than max_dist or the
script returns true as a result of finding a collision. Line 23 will therefore only ever be reached
if no collision was found and it returns false to indicate that this is the case.
Figure 11-5. In this diagram, the black line has a length of one pixel and is rotated by the angle dirn. The
function lengthdir_x( 1, dirn ) provides the horizontal component of that line (in red) and lengthdir_y
provides the vertical component of that line (in blue)
Changing Lifts
There were also two scripts we created in Zool so that we could get hold of all adjacent lift
instances and change them all into another kind of lift object during the same step. The
check_object_returned script provided the id of adjacent instances and the change_instance_for
script used that id to change those instances into another kind of object. However, both of these
scripts can be simplified to a couple of lines of code in GML, so it isn't worth having them as
scripts at all anymore.
You can write the check_object returned script as:
returned = instance_place( x+dx, y+dy, find_obj );
Where dx and dy are the offsets required and find_obj is the type of object we're looking for.
Similarly, you can write the change_instance_for script as just two lines:
with( returned )
instance_change( change_obj, true );
Where change_obj is the type of object we want the instance to change into. Both of these
scripts have therefore been removed from zoolgml.gmk , and replaced with these lines where
they were called. However, another script has been added to handle the entire process of
switching lift instances called change_lift_instances and this uses both of these processes to
handle the majority of the lift changing process in one place.