Navigating between pages

Page flow

The site logic or page flow describes the conditions that trigger the navigation from one page to the other. It also describes how arguments are passed from one page to the other. In a simple web application simulating an ATM, the navigation logic could look like the one described in the diagram on the right. In this diagram, the square boxes represent pages and diamond-shaped boxes represent actions performed by the end-user.

With the PFC, page flow is expressed declaratively and externally to the pages. Consequently, pages can be designed independently from each other. The benefits of a clear separation between site logic and page logic and layout include:

Simplicity: the site logic is declared in one place and in a declarative way. You don't need to write custom logic to perform redirects between pages or pass arguments from page to page.
Maintainability: having different developers implementing independent page is much easier. Since the relationship between pages is clearly stated in the page flow, it also becomes much easier to modify a page in an existing application without affecting other pages.

Actions and results

An example

Consider a view-account page in the ATM web application. The page displays the current balance and lets the user enter an amount of money to withdraw from the account. The page looks like this:

This page is composed of different parts illustrated in the figure below:

The page model retrieves the current balance.
The page view displays the balance, and presents a form for the user to enter the amount to withdraw.
An action executed when the user enters an amount in the text field. This action checks if the amount entered is inferior or equal to the account balance. If it is, the balance is decreased by the amount entered and the transaction is considered valid. Otherwise, the transaction is considered illegal. Depending on the validity of the transaction, a different page is displayed. If the transaction is valid, the anything-else page is displayed; otherwise the low-balance page is displayed.

This behavior is described in the Page Flow with:

<page
  id="view-account"
  path="/view-account"
  model="view-account-get-balance-model.xpl"
  view="view-account-get-balance-view.xsl">
    <action when="/amount != ''" action="view-account-action.xpl">
        <result id="success" when="/success = 'true'" page="anything-else"/>
        <result id="failure" when="/success = 'false'" page="low-balance"/>
    </action>
</page>

The `<page>` element

On the <page> element, as documented above:

The path attribute tells the PFC what relative URL corresponds to this page. The URL is relative to the application context path.
The model attribute points to the page model XPL pipeline.
The view attribute points to the page view XSLT template.

The `<action>` element

The <page> element contains an <action> element. It is named action because it is typically executed as a consequence of an action performed by the end-user, for example by clicking on a button or a link which causes a form to be submitted. There may be more than one <action> element within a <page> element element. On an <action> element:

The when attribute contains an XPath 2.0 expression executed against the XML submission. The first <action> element with a when attribute evaluating to true() is executed. The when attribute is optional: a missing when attribute is equivalent to when="true()". Only the last <action> element is allowed to have a missing when attribute. This allows for defining a default action which executes if no other action can execute.
When the action is executed, if the optional action attribute is present, the XPL pipeline it points to is executed.

The `<result>` element

The <action> element can contain zero or more <result> elements.

If an action attribute is specified on the <action> element, the <result> element can have a when attribute. The when attribute contains an XPath 2.0 expression executed against the data output of the action XPL pipeline. The first <result> evaluating to true() is executed. The when attribute is optional: a missing when attribute is equivalent to when="true()". Only the last <result> element is allowed to have a missing when attribute. This allows for defining a default action result which executes if no other action result can execute.
A <result> element optionally has a page attribute. The page attribute contains a page id pointing to a page declared in the same page flow. When the result is executed and the page attribute is present, the destination page is executed, and the user is forwarded to the corresponding page.
NOTE: In this case, a page model or page view specified on the enclosing <page> element does not execute! Instead, control is transferred to the page with the identifier specified. This also means that the page flow epilogue does not execute.
A <result> element can optionally contain an XML submission. The submission can be created using XSLT, XQuery, or the deprecated XUpdate. You specify which language to use with the transform attribute on the <result> element. The inline content of the <result> contains then a transformation in the language specified. Using XSLT is recommended.
The transformation has automatically access to:
- An instance input, containing the current XML submission. From XSLT, XQuery and XUpdate, this input is available with the doc('input:instance') function. If there is no current XML submission, a "null" document is available instead:
- An action input, containing the result of the action XPL pipeline if present. From XSLT, XQuery and XUpdate, this input is available with the doc('input:action') function. If there is no action result, a "null" document is available instead:
- The default input contains the current current XML submission as available from the instance input.
  The result of the transformation is automatically submitted to the destination page. If there is no destination page, it replaces the current XML submission document made availabe to the page model and page view.

An action XPL pipeline supports an optional instance input containing the current XML submission, and produces an optional data output with an action result document which may be used by a <result> element's when attribute, as well as by an XML submission-producing transformation. This is an example of action XPL pipeline:

<p:config
  xmlns:p="http://www.orbeon.com/oxf/pipeline"
  xmlns:oxf="http://www.orbeon.com/oxf/processors">

    <p:param name="instance" type="input"/>
    <p:param name="data" type="output"/>

    <!-- Call the data access layer -->
    <p:processor name="oxf:pipeline">
        <p:input name="config" href="../data-access/delegate/read-document.xpl"/>
        <p:input name="document-id" href="#instance#xpointer(/*/document-id)"/>
        <p:output name="document-info" ref="data"/>
    </p:processor>

</p:config>

Notice the instance input, the data output, as well as a call to a data access layer which uses information from the XML submission and directly returns an action result document.

The following is an example of using XSLT within <result> element in order to produce an XML submission passed to a destination page:

<action
  when="/form/action = 'show-detail'"
  action="../bizdoc/summary/find-document-action.xpl">
    <result page="detail" transform="oxf:xslt">
        <form xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xsl:version="2.0">
            <document-id><xsl:value-of select="doc('input:action')/document-info/document-id"/></document-id>
            <document><xsl:copy-of select="doc('input:action')/document-info/document/*"/></document>
        </form>
    </result>
</action>

Notice the transform attribute set to oxf:xslt, and the use of the doc('input:action') to refer to the output of the action XPL pipeline specified by the action attribute on the <action> element. The current XML submission can also be accessed withdoc('input:instance').

Also see the Orbeon Forms Tutorial.

Controlling internal XML submissions

You can control what method is used to perform an internal XML submission described within a <result> element. Consider this page flow:

<page id="a" path="/a" model="..." view="...">
    <action when="...">
        <result page="b"/>
    </action>
</page>

<page id="b" path="/b" model="..." view="...">

Going from page "a" to page "b" can be done with either a "forward" or a "redirect" method:

The benefit of the "redirect" method is that after being redirected to page b, the end-user will see a URL starting with /b in the browser's address bar. He will also be able to bookmark that page and to come back to it later. However, a drawback is that the request for page b is sent by the browser with a GET method. Since browsers impose limits on the maximum amount of information that can be sent in a GET (URL length), this method might not work if the amount of information that needs to be passed to page b from page a is too large. This typically happens when working with fairly large XML submissions. In those cases, you must use the "forward" method, which does not limit the amount of information passed from page to page. The "forward" method also reduces the number of roundtrips with the server.

NOTE: A third instance passing option, redirect-exit-portal, behaves like the redirect method but sends a redirection which exits the portal, if any. This is mainly useful for the Orbeon Forms examples portal.

You can configure the method:

At the application level, in properties.xml with:

 <property as="xs:string" processor-name="oxf:page-flow" name="instance-passing" value="forward|redirect"/>

At the page flow level with the instance-passing attribute on the page flow root element:
```
 <controller instance-passing="forward|redirect">...</controller>
```

In the page flow at the "result" level, with the instance-passing attribute on the <result> element:

 <page id="a" path="/a" model="..." view="...">
     <action when="...">
         <result page="b" instance-passing="forward|redirect"/>
     </action>
 </page>

A configuration at the application level (properties.xml) can be overridden by a configuration at the page flow level (instance-passing on the root element), which can in its turn be overridden by a configuration at the result level (instance-passing on the <result> element).

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