ActionScript 3.0 Design Patterns

228 | Chapter 6: Composite Pattern

Moving a Kinematic Pair

As discussed before, our snake will be a composite object consisting of several con-

nected components that represent a kinematic chain. Kinematic chains are com-

posed of kinematic pairs: two rigid segments connected by a joint. How do

kinematic pairs move and orient themselves? The basic idea is that the parent seg-

ment pulls the child segment. In a simple kinematic pair, the only restriction on

movement is the joint. Because it’s the free end, the parent segment will move and

rotate independent of the child. The child will strive to rotate and orient itself to face

the joint, and move to keep up with its parent.

Figure 6-7 shows a four-step sequence by which a parent segment will pull a child in

a kinematic pair. Even though the parent pulls, the child segment does all the work.

Figure 6-7. Child segment keeping up with parent in a kinematic pair

1

Child

Parent

C 1

P 1

d

Child

C 1

P 1

Parent

P 2

Parent

Child P 1

C 1

P 2

Child

Parent

P 2

C 1

R

C 2

2

3 4