Chapter 11

State machines can be pictured visually using symbols from the Unified  Mod 

eling Language (UML), which was introduced briefly at the end of Chapter 3. 

Figure 11 5 shows a state machine for the BBS application. The four boxes rep 

resent the four screens of the application. The arrows pointing from one state to 

the next are the events. From any given state, there are a fixed number of available 

events, or transitions. The diagram begins with an initial state, shown as a filled 

circle. The default event list leads from the initial state to the first real state. 

This means that when users first arrive at the application, they should be shown a 

list of messages.

Figure 11 5. State machine for BBS application

By using a state machine to design your CGIs, you are able to view the appli 

cation as a whole. Each event that is accessible from a particular state will generally 

show up in the final application as a button or link. With this knowledge, you can 

use state transition diagrams to assist you in designing both your user interface 

and your program code. Also, the state transition diagram can notify you if you're 

missing a button on a particular screen.

The CGI::Application module recognizes that Web applications are best 

understood as state machines. As you are about to see, this module provides a 

reusable alternative to rolling your own ad hoc flow control from if elsif blocks.

CGI::Application to the Rescue

CGI::Application offers a cleaner solution to this CGI project. Listings 11 1 and 11 2 

show the full CGI::Application version browse through it and then I'll walk you 

through the code.

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