7.2.3. Page nodes and transitions
Each <page> node represents a state where the system is waiting for user input:
<page name="displayGuess" view-id="/numberGuess.jsp">
<redirect/>
<transition name="guess" to="evaluateGuess">
<action expression="#{numberGuess.guess}" />
</transition>
</page>
The view-id is the JSF view id. The <redirect/> element has the same effect as <redirect/> in a JSF navigation rule: namely, a post-then-redirect behavior, to overcome problems with the browser's refresh button (note that Seam propagates conversation contexts over these browser redirects).
The transition name is the name of a JSF outcome triggered by clicking a command button or command link in numberGuess.jsp.
<h:commandButton type="submit" value="Guess" action="guess"/>
When the transition is triggered by clicking this button, jBPM will activate the transition action by calling the guess() method of the numberGuess component. Notice that the syntax used for specifying actions in the jPDL is just a familiar JSF EL expression, and that the transition action handler is just a method of a Seam component in the current Seam contexts. So we have exactly the same event model for jBPM events that we already have for JSF events! (The
One Kind of Stuff
principle.)
In the case of a null outcome (for example, a command button with no action defined), Seam will signal the transition with no name if one exists, or else simply redisplay the page if all transitions have names. So we could slightly simplify our example pageflow and this button:
<h:commandButton type="submit" value="Guess"/>
Would fire the following un-named transition:
<page name="displayGuess" view-id="/numberGuess.jsp">
<redirect/>
<transition to="evaluateGuess">
<action expression="#{numberGuess.guess}" />
</transition>
</page>
It is even possible to have the button call an action method, in which case the action outcome will determine the transition to be taken:
<h:commandButton type="submit" value="Guess" action="#{numberGuess.guess}"/>
<page name="displayGuess" view-id="/numberGuess.jsp">
<transition name="correctGuess" to="win"/>
<transition name="incorrectGuess" to="evaluateGuess"/>
</page>
However, this is considered an inferior style, since it moves responsibility for controlling the flow out of the pageflow definition and back into the other components. It is much better to centralize this concern in the pageflow itself.
