Using Advanced Physics Concepts - Learn Unity3D Programming with UnityScript - page 188

Rather than assign explicit coordinates to platformStartPosition with the variable declaration,
your script assigns whatever the position of the platform game object is in the Start() function.
By writing it this way, you can reposition the platform in the Scene view without having to come back
and edit the script to update the start position coordinates.
(2) private var platformEndPosition : Vector3;
public var lowLevel : float = 6;
function Start () {
...
platformEndPosition = (transform.position - Vector3(0,
lowLevel, 0));
}
In the same fashion, the platformEndPosition variable is declared but not assigned explicit
coordinates. The platform will move straight down along the Y-axis for the distance designated
by the lowLevel variable. A value of 6 is explicitly assigned to lowLevel to begin with, but more
importantly the lowLevel variable is public so you can adjust the range of the platform's movement
directly in the editor. The platformEndPosition is assigned in the Start() function the result of
subtracting the lowLevel value from the y coordinate of the platformStartPosition .
(3) public var duration : float = 5.0f;
The duration variable determines how fast the platform moves from platformStartPosition to
platformEndPosition , so making it public allows you to adjust the speed of the platform movement
directly from the editor.
(4) private var step : float = 0.0f;
private var movingBack : boolean = false;
Both step and movingBack are used for changing the direction of the platform to maintain a repeating
up-and-down movement during gameplay.
(5) function FixedUpdate() {
Since this is physics-based rigidbody movement, the FixedUpdate() function is used instead of
Update() . A common beginner mistake is to forget the Rigidbody component for the game object.
Make sure the Platform 4 × 1 × 4 has a Rigidbody component. If not, select it in the Hierarchy, then
in the Inspector select Add Component ➤ Physics ➤ Rigidbody. When the Rigidbody component
appears in the Inspector, uncheck Use Gravity since this isn't needed and check Is Kinematic so the
platform movement is unaffected by the player character jumping onto it.
(6) var delta : float = (Time.deltaTime / duration);
Time.deltaTime is the time in seconds it took to complete the last frame, so dividing by duration
assigns a normalized fraction to delta so that
(7) if(movingBack) step -= delta;