Minimalist Abstract State Pattern | 403Using Delegation with the Context
TheContextclass shown in Example 11-1 is an aggregator—it owns the Strategy
object. So in every Context class where Strategy patterns are found, you will find a ref-
erence to the strategy class—in this example a variable taking the lowercase name of
the Strategy interface. (Example 11-1 to Example 11-5 make up the first application.)
TheContextclass is pretty simple. One reason is that all the methods it needs are del-
egates implemented from theStrategy interface. Let’s now look at such an interface.
Adding a Strategy
In a Strategy design pattern, you’re likely to see more than one interface, depending
on the different types of methods required. Example 11-2 shows a single method in
the interface.
This represents the first step in encapsulating a behavior. The subsequent concrete
Strategy implementations add detail.
Details of the Strategy
Next, in Example 11-3, a concrete strategy adds a method,think( ), which does
something. In this case, it just sends out a trace message.
Example 11-1. Context.as
package
{
class Context
{
protected var strategy:Strategy;
public function doStrategy( ):void
{
strategy.think( );
}
}
}
Example 11-2. Strategy.as
package
{
interface Strategy
{
function think( ):void;
}
}