Minimalist Example of a Composite Pattern | 211root
composite 1
leaf 1
leaf 2
composite 2
leaf 3
leaf 4
leaf 5
leaf 6It is important to note that theclientsees only the interface defined by theComponent
class. The client doesn’t need to differentiate between composite and leaf nodes, and
isn’t tied to how the operation( )method is implemented. It can simply call
operation( ) on any leaf or composite node and get consistent results.
Accessing Child Nodes
You may have noticed that we didn’t override and implement thegetChild(n:int)
method in theCompositeclass. However, this method’s very important to the compos-
ite pattern, as it allows the client to develop a composite structure by not declaring
variables, as in the previous example. This ensures proper garbage collection when
removing nodes. Garbage collection allows the application to recover memory and
resources allocated to deleted nodes, and will be discussed in more detail later in the
chapter. ThegetChild( )method allows the client to access the children of any com-
posite node. The followinggetChild( ) method is implemented in theComposite class.
override public function getChild(n:int):Component
{
if ((n > 0) && (n <= aChildren.length))
{
return aChildren[n-1];
} else {
return null;
}
}The parameterizedgetChild( )method returns the child object by the position indi-
cated by the parametern(inde xstarts from 1). After doing a range check, the method
returns the child node ornullif no children exist. The client can now leverage the
getChild( ) method and build the same composite structure more efficiently.
// create root node
var root:Composite = new Composite("root");// add a node to root
root.add(new Composite("node 1"));
root.getChild(1).add(new Leaf("leaf 1"));
root.getChild(1).add(new Leaf("leaf 2"));// add another node
root.add(new Composite("node 2"));