Implementing Navigation and Pageflows with Seam

jPDL pageflows

jPDL is the process description language you will use to describe a process or a stateful pageflow within Seam. To get a flavor of the syntax and see it working in real life we will use an example. We will select an example where state transitions form the basis of all interactions. Our example is an online interactive game that involves two players where each player plays the alternate move. The outcome can be either one of the players winning or the game ending in a deadlock. Games like chess, tic-tac-toe, nim, dots and checkers fit into this type of interaction model. The source code for download implements the game of tic-tac-toe but it could equally well apply to any of the others on the list. Figure 3 shows the state transitions in such a game.

Figure 3: State transitions in a two player game where either one of them wins or the game ends in a draw

The states are start, make a move, victory or impasse. Alternate moves continue till either one wins or a stalemate is reached. Such a flow can be written very effectively using jPDL. Listing 6 illustrates the state transitions for our example application using jPDL.

Listing 6 jPDL shows state transitions for a two player game where each player plays alternatively and either one wins or the game ends in a draw. This is a very elementary example as compared to what one may encounter in real business situations. However, it gives a flavor of process definition and state transitions in a stateful flow scenario.

Listing 6: Process definition of the two player game
<process-definition name="twoPlayerGame" initial="start>
 <state name="start">
  <transition name="AStartsGame" to="APlays">
  <transition name="BStartsGame" to="BPlays">
 </state>

 <state name="APlays">
  <transition name="gameActive" to="BPlays">
  <transition name="victory" to="AWins">
  <transition name="impasse" to="draw">
 </state>

 <state name="BPlays">
  <transition name="gameActive" to="APlays">
  <transition name="victory" to="BWins">
  <transition name="impasse" to="draw">
 </state>
</process-definition>

The state transitions of this type could be implemented using Java within the action or event handler code. However, doing that would be neither easier nor as flexible. jPDL state transition changes can be made easily at one place in an XML file that resembles the natural language syntax. Changes to the code that manages state transitions in far more complicated and often not as decoupled. Also because it involves changes to the code rather than to a process description, it has to be done by a programmer. A person who has never programmed can also make changes to the XML file, since they are easy to understand.

Summary

This article was about showing the implementation strategies as they are leveraged in practice. Now that the basics are well understood its time to start picking up complex cases and determine which option to choose to define navigation. Making a decision between stateless and stateful models is not always clear and straightforward. Experience above all usually helps make this decision effectively.

Understanding navigation models means getting a sense of the dynamics of interaction and flow control under varied scenarios. Once the interaction strategies are well understood it’s easier to deal with the user interface elements themselves and see how the interaction impacts their behavior and presentation logic. In the case of rich interactions as with AJAX and RIA, the navigation models not only pervade across layers but include multiple languages and sometimes an additional client side framework with Seam.

Therefore getting hold of the navigation model early on is going to help you go a long way in building a robust and scalable web application.