7.7. Navigation
You can use standard JSF navigation rules defined in
faces-config.xml in a Seam application. However, these rules have several limitations:
- It is not possible to specify that request parameters are used when redirecting.
- It is not possible to begin or end conversations from a rule.
- Rules work by evaluating the return value of the action method; it is not possible to evaluate an arbitrary EL expression.
Another problem is that "orchestration" logic is scattered between
pages.xml and faces-config.xml. It is better to unify this logic under pages.xml.
This JSF navigation rule:
<navigation-rule>
<from-view-id>/editDocument.xhtml</from-view-id>
<navigation-case>
<from-action>#{documentEditor.update}</from-action>
<from-outcome>success</from-outcome>
<to-view-id>/viewDocument.xhtml</to-view-id>
<redirect/>
</navigation-case>
</navigation-rule>
Can be rewritten as follows:
<page view-id="/editDocument.xhtml">
<navigation from-action="#{documentEditor.update}">
<rule if-outcome="success">
<redirect view-id="/viewDocument.xhtml"/>
</rule>
</navigation>
</page>
However, this method pollutes
DocumentEditor with string-valued return values (the JSF outcomes). Instead, Seam lets us write:
<page view-id="/editDocument.xhtml">
<navigation from-action="#{documentEditor.update}"
evaluate="#{documentEditor.errors.size}">
<rule if-outcome="0">
<redirect view-id="/viewDocument.xhtml"/>
</rule>
</navigation>
</page>
Or even:
<page view-id="/editDocument.xhtml">
<navigation from-action="#{documentEditor.update}">
<rule if="#{documentEditor.errors.empty}">
<redirect view-id="/viewDocument.xhtml"/>
</rule>
</navigation>
</page>
The first form evaluates a value binding to determine the outcome value used by the subsequent rules. The second approach ignores the outcome and evaluates a value binding for each possible rule.
When an update succeeds, we probably want to end the current conversation, like so:
<page view-id="/editDocument.xhtml">
<navigation from-action="#{documentEditor.update}">
<rule if="#{documentEditor.errors.empty}">
<end-conversation/>
<redirect view-id="/viewDocument.xhtml"/>
</rule>
</navigation>
</page>
Since the conversation has ended, any subsequent requests will not know which document we are interested in. We can pass the document ID as a request parameter, which also makes the view bookmarkable:
<page view-id="/editDocument.xhtml">
<navigation from-action="#{documentEditor.update}">
<rule if="#{documentEditor.errors.empty}">
<end-conversation/>
<redirect view-id="/viewDocument.xhtml">
<param name="documentId" value="#{documentEditor.documentId}"/>
</redirect>
</rule>
</navigation>
</page>
Null outcomes are a special case in JSF, and are interpreted as instructions to redisplay the page. The following navigation rule matches any non-null outcome, but not the null outcome:
<page view-id="/editDocument.xhtml">
<navigation from-action="#{documentEditor.update}">
<rule>
<render view-id="/viewDocument.xhtml"/>
</rule>
</navigation>
</page>
To perform navigation when a null outcome occurs, use the following:
<page view-id="/editDocument.xhtml">
<navigation from-action="#{documentEditor.update}">
<render view-id="/viewDocument.xhtml"/>
</navigation>
</page>
The view ID can be given as a JSF EL expression:
<page view-id="/editDocument.xhtml">
<navigation>
<rule if-outcome="success">
<redirect view-id="/#{userAgent}/displayDocument.xhtml"/>
</rule>
</navigation>
</page>