Platform Beginnings: An Idea with Legs - The Game Maker's Companion: Game Development - page 33

Adding a Collision Event to the Walking State Object
1. Reopen obj_pod_walking and add a Collision event with obj_platform . Recall that all
the platform objects have obj_platform as their parent, so this collision event will be
triggered for collisions between obj_pod_walking and any kind of platform.
2. Include a Set Variable action ( control tab). Type x into Variable and xprevious into
Value . This sets the x position back to the value it had in the last step.
3. Include another Set Variable action. Type y into Variable and yprevious into Value .
This then does the same for the y position.
4. Include a Move to Contact action ( move tab). Type direction into Direction, leave
Maximum as -1 , and select all objects for Against . This will find the contact point in
the direction Fishpod is moving. The Maximum value indicates the distance (in pixels)
that the action will check for a collision before giving up. A value of -1 means it will
check for collisions an arbitrary distance away, but sometimes it is useful to be able to
limit this.
5. Include a Change Instance action ( main1 tab). Select obj_pod_standing from the
Change Into menu and yes for Perform Events . This will put the object into the
standing state after a horizontal collision.
Keep the walking state object open, as we now need to handle the transition from the
walking state to the falling state when there is no longer a platform supporting the character's
feet. We can achieve this using the Check Object action.
Adding a Step Event to the Walking State Object
1. Add a Step, Step event to obj_pod_walking . We're going to use a Step event because the
walking object is constantly moving and the ground could stop supporting it at any
point (a Begin Step or End Step would have an almost identical effect in this case).
2. Include a Check Object action ( control tab). Select obj_platform for Object , leave X as
0 , and set Y to 1 . Check both the Relative and NOT options. This checks to see if there
would be a collision with an instance of obj_platform if Fishpod was moved down one
pixel and only performs the next action if this is not true (that is, if Fishpod is standing
in thin air).
3. Include a Change Instance action ( main1 tab). Select obj_pod_falling from the
Change Into menu and yes for Perform Events . This will now put the object into the
falling state when it is no longer supported by a platform.
That completes the walking state object, which now has all the state transitions originally
shown in Figure 2-9. Now we've just got the jumping and falling state objects to complete.