ActionScript 3.0 Design Patterns

Extended Example: Color Printing | 83

Clients

Clients need to instantiate the new print center classes to access the color printers.

Theprint( )method takes a parameter that specifies a color or black and white print

job. They operate on different print job objects based on the passed parameters.

var pcNewHighVol:NewPrintCenter = new NewHighVolPrintCenter( );

var pcNewLowhVol:NewPrintCenter = new NewLowVolPrintCenter( );

pcNewHighVol.print("LongThesis.doc", NewHighVolPrintCenter.BW);

pcNewHighVol.print("SalesReport.pdf", NewHighVolPrintCenter.COLOR);

pcNewLowhVol.print("ShortVita.doc", NewLowVolPrintCenter.BW);

pcNewLowhVol.print("SalesChart.xlc", NewLowVolPrintCenter.COLOR);

The old print center classes,LowVolPrintCenter and HighVolPrintCenter, which

implement a non-parameterized factory method, will continue to work. They’ll con-

tinue to use theWorkgroupPrintjobandInkjetPrintjobprint job classes without dis-

ruption. The power of the factory method design pattern to handle changing

requirements is very evident in this example.

Parallel Class Hierarchies

Take a look at the class diagram for the extended print center application

(Figure 2-7). You will see two distinct concrete product class hierarchies. The

WorkgroupPrintjobandColorLaserPrintjobclasses represent the high volume print

jobs. Likewise, theInkjetPrintjobandColorInkjetPrintjobclasses represent the low

volume print jobs. In addition, knowledge about these class hierarchies is encapsu-

lated within their corresponding creator classes. Note that the product classes can-

not be accessed directly. They can only be accessed through the creator classes. The

NewLowVolPrintCenterandNewHighVolPrintCenterknow about their corresponding

product class hierarchy. We can think of the factory method pattern as sets ofparal-

lel class hierarchies: the concrete creator classes and their corresponding products.

This is a powerful way of encapsulating knowledge and managing dependencies

within software applications.

Figure 2-7. Class diagram for the extended print shop example

Printjob

InkjetPrintjob

ColorInkjetPrintjob

InkjetPrintjob

ColorInkjetPrintjob

NewPrintCenter

print()

createPrintjob()

NewLowVoIPrintCenter

createPrintjob()

NewHighVoIPrintCenter

createPrintjob()