PTOLEMY II - CiteSeerX

Designing Actors
type constraints are not cloned automatically because of the difficulty of ensuring that the other object
being referred to in a relative constraint is the intended one. Thus, in figure 5.5, the clone() method
repeats the relative type constraints that were specified in the constructor:
newObject.output.setTypeAtLeast(newObject.input);
newObject.output.setTypeAtLeast(newObject.factor);
Note that at no time during cloning is any constructor invoked, so it is necessary to repeat in the
clone() method any initialization in the constructor. For example, the clone() method in the Expression
actor sets the values of a few private variables:
newObject._iterationCount = 1;
newObject._time = (Variable)newObject.getAttribute("time");
newObject._iteration =
(Variable)newObject.getAttribute("iteration");
5.3 Action Methods
Figure 5.1 shows a set of public methods called the action methods because they specify the action
performed by the actor. By convention, these are given in alphabetical order in Ptolemy II Java files,
but we will discuss them here in the order that they are invoked. The first to be invoked is the preinitialize()
method, which is invoked exactly once before any other action method is invoked. The preinitialize()
method is often used to set type constraints. After the preinitialize() method is called, type
resolution happens and all the type constraints are resolved. The initialize() method is invoked next,
and is typically used to initialize state variables in the actor, which generally depends on type resolution.
After the initialize() method, the actor experiences some number of iterations, where an iteration is
defined to be exactly one invocation of prefire(), some number of invocations of fire(), and at most one
invocation of postfire().
5.3.1 Initialization
The initialize() method of the Average actor is shown in figure 5.6. This data- and domain-poly-
public class Average extends Transformer {
...
public void initialize() throws IllegalActionException {
super.initialize();
_count = 0;
_sum = null;
}
...
}
///////////////////////////////////////////////////////////////////
//// private members ////
private Token _sum;
private int _count = 0;
FIGURE 5.6. Code segment from the Average actor, showing the initialize() method.
162 Ptolemy II