Advanced Game Logic - 3D Game Development Practical Introduction - page 342

Before discussing the details of this script, notice RequireComponent annotation we used
before declaring the class. This annotation requires the game object to which SlidingDoor
script is attached to have a GeneralDoor script as well. If you attach SlidingDoor script
to an object, Unity automatically attaches GeneralDoor script if does not exist. Similarly,
if you try to remove GeneralDoor script from an object while SlidingDoor is attached to
it, the removal is refused by Unity. We benefit from this mechanism because SlidingDoor
completely depends on GeneralDoor and cannot be used alone.
Th slidingDirection vector determines the distance the door moves along its local axes when
it is opened. For example, if the sliding direction is (0, 2, 0), the door is going to move two
meters up when it is opened. The variable speed controls the speed of the door when it opens
or closes. Opening and closing the door is in fact a process of smoothly moving it between
originalPosition and slidingPosition . The initial position of the door when Start() is called is
taken as originalPosition (close position). On the other hand, slidingPosition (open position) is
computed by adding slidingDirection to originalPosition . In addition to door which refer-
ences the attached GeneralDoor , we implement an internal state management by using the
enumerator SlidingDoorState (line 107). The variable state of type SlidingDoorState tells us
what the sliding door is doing at any given moment (opened, closed, opening, or closing).
The initial state is set according to door. initallyOpen . Illustration 80 shows a double sliding
door that has two parts with opposite sliding directions.