Effect: Physics - JavaFX Special Effects - page 123

public override function update():Void{
shaft.endX = bodies[1].getPosition().getX() - translateX;
shaft.endY = bodies[1].getPosition().getY() - translateY;
bottomNode.translateX = bodies[1].getPosition().getX() - translateX;
bottomNode.translateY = bodies[1].getPosition().getY() - translateY;
}
}
In the init method of the class Pendulum in Listing 6-7, two bodies are created. The first Body stored
in bodies[0] is a StaticBody that defines the top, fixed circle. The second Body , stored in bodies[1] is the
big ball at the bottom. Since the class Pendulum allows the caller to specify the distance between the top
and bottom bodies and the starting angle for the bottom Body , a little math is required to find the starting
position of the bottom Body , as shown in Figure 6-5.
Figure 6-5. Starting positions
Here we introduce the concept of a Joint . A Joint in Phys2D is an object that defines a constraint
between two Bodies. In this case, we want the two bodies to stay a particular distance from each other to
simulate that the lower body is hung from the upper body by a string or wire. In this way, the lower Body
will swing around the upper Body when it is pulled by gravity or impacted by another object.
There are three JavaFX Nodes that are used to draw each Pendulum , one for each body and a Line used
to draw the connection between the two. The position of these Nodes is updated in much the same way
as the other WorldBodies defined in this chapter—simply update the location for each step. For this class
we are ignoring rotation.