Design Patterns Explained

Chapter 15 • The Decorator Pattern 243
In this solution, I have put the control in SalesTicket, with flags
saying whether it is to print the headers or the footers.
This works quite well if I do not have to deal with a lot of options or if
the sales orders using these headers do not change.
If I have to deal with many different types of headers and footers,
printing only one each time, then I might consider using one Strategy
pattern for the header and another Strategy pattern for the footer.
What happens if I have to print more than one header and/or footer
at a time? Or what if the order of the headers and/or footers needs to
change? The number of combinations can quickly overwhelm.
In situations like this, the Decorator pattern can be very useful.
Instead of controlling added functionality by having a control
method, the Decorator pattern says to control it by chaining
together the functions desired in the correct order needed. The Decorator
pattern separates the dynamic building of this chain of functionality
from the client that uses it, in this case, the SalesOrder.
The Decorator Pattern
According to Gang of Four, the Decorator pattern's intent is to
Attach additional responsibilities to an object dynamically.
Decorators provide a flexible alternative to subclassing
for extending functionality. 1
The Decorator pattern works by allowing me to create a chain of
objects that starts with the decorator objects— the objects responsible
for the new function— and ends with the original object. Figure 15-3
illustrates this.
1. Gamma, E., Helm, R., Johnson, R., Vlissides, J., Design Patterns: Elements of Reusable
Object-Oriented Software, Reading, Mass.: Addison-Wesley, 1995, p. 315.
The approach is not
flexible
The Decorator
pattern helps
The intent, according
to the Gang of
Four
How it works