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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>
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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>
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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>
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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>
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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>
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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>
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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>
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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>
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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>
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