tapestry-core/src/site/apt/guide/pagenav.apt

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 Page Navigation
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Page Navigation

  In essense, a Tapestry application is a number of related pages, working together.  To some degree, each page is like an application unto itself.
  
  Any individual request will be targetted at a single page.  Requests come in two forms: 
  
  * <action> requests target a specific component on a specific page, triggering an event within that component
  
  * <render> requests target a specific page, and stream the HTML markup for that page back to the client
  
  []
  
  This dictomy between action requests 
  and render requests is new in Tapestry 5. It is in some ways based on ideas from the Portlet specification and differentiating
  the two types of requests alleviates a number of problems in traditional web applications related to the browser back button, or to the user hitting the
  refresh button in their browser.
  
Action Requests
  
  Action requests may take the form of hyperlinks
  ({{{../component-parameters.html#org.apache.tapestry5.corelib.components.actionlink}ActionLink}}) or form submissions
  ({{{../component-parameters.html#org.apache.tapestry5.corelib.components.form}Form}}).
  
  In both cases, the value returned from an {{{event.html}event handler method}} controls the response sent to the client web browser.
  
  The URL for an action request identifies the name of the page, the nested id of the component, and the name of the event to trigger on the component (this is usually "action").
  Further, an action request may contain additional context information, which will be provided to the event handler method.  
  
  These URLs expose a bit of the internal structure of the application.  Over time, as an application grows and is maintained, the ids of components may change. This means that
  action request URLs should not be bookmarked.  Fortunately, users will rarely have the chance to do so (see below).
  
* Null response

  If the event handler method returns no value, or returns null, then the current page (the page containing the component) will render the response.
  
  A page render link for the current page is created and sent to the client as a client side redirect.  The client browser will automatically submit a new request
  to generate the page.
  
  The user will see the newly generated content in their browser. In addition, the URL in the browser's address bar will be a render request URL.  Render request URLs are
  shorter and contain less application structure (for instance, they don't include component ids or event types).  Render requests URLs are what your users will bookmark. The action request URLs
  are transitory, meaningful only while the application is actively engaged, and not meant to be used in later sessions.
  
* String response
  
  When a string is returned, it is expected to be the logical name of a page (as opposed to the page's
  fully qualified class name).  As elsewhere, the name of the page is case insensitive.
  
  Again, a render request URL will be constructed and sent to the client as a redirect.
  
* Class response
  
  When a class is returned, it is expected to be a page class. Returning a page class from an event handler is safer for refactoring
  than returning a page name.
  
  As with other response types, a render request URL will be constructed and sent to the client as a redirect.
  
* Page response
  
  You may also return an instance of a page, rather than the name or class of a page.
  
  A page may be injected via the {{{../../apidocs/org/apache/tapestry5/annotations/InjectPage.html}InjectPage}} annotation.
  
  Often, you will configure the page in some way before returning the page (examples below).
  
  You can also return a component within the page, but this will generate a runtime warning.
  
* Link response

  An event handler method may return a 
  {{{../../apidocs/org/apache/tapestry5/Link.html}Link}} instance directly.  The Link is converted into a URL and a client redirect to that URL is sent to the client.
  
  The {{{../../apidocs/org/apache/tapestry5/ComponentResources.html}ComponentResources}} object that is injected into your pages (and components) has methods
  for creating action and page links (they are actually defined in
  {{{../../apidocs/org/apache/tapestry5/ComponentResourcesCommon.html}ComponentResourcesCommon}}).
  
* Stream response

  An event handler can also return a {{{../../apidocs/org/apache/tapestry5/StreamResponse.html}StreamResponse}} object, which encapsulates a stream to
  be sent directly to the client browser.  This is useful for compnents that want to, say, generate an image or PDF and provide it to the client.


* URL response

  A URL is handled as a client redirect to an external URL.
  
* Object response

  Any other type of object returned from an event handler method is an error.
  
Page Render Requests

  Render requests are simpler in structure and behavior than action requests. In the simplest case, the URL is simply the
  logical name of the page.
  
  Pages may have an <activation context>.  The activation context represents persistent information about the state of the page.  In practical terms,
  the activation context is usually the id of some database-persistent object.
  
  When a page has an activation context, the values of the context are appended to the URL path.  
  
  Not all pages have an activation context.
  
  The activation context may be explicitly set when the render request link is created (the PageLink component has a context parameter for this purpose).
  When no explicit activation context is provided, the page itself is queried for its activation context.
  
  This querying takes the form of an event trigger. The event name is "passivate" (as we'll see shortly, there's a corresponding "activate").  The 
  return value of the method is used as the context.  For example:
  
+---+
public class ProductDetail
{
  private Product product;
  
  . . .
  
  long onPassivate() { return product.getId(); }
}
+----+

  The activation context may consist of a series of values, in which case the return value of the method should be an array or a List.

  <<Note: If you are using the {{{../tapestry-hibernate/}tapestry-hibernate}} integration library and your passivate context
  is a Hibernate entity, then you can just use the entity itself, not its id.  Tapestry will automatically extract the entities' id into the URL,
  and convert it back for the "activate" event handler method.>>
  
* Page activation

  When a page render request arrives, the page is activated before it is rendered.
  
  Activation serves two purposes:
  
  * It allows the page to restore its internal state from data encoded into the URL (the activation context discussed above).
  
  * It provides coarse approach to validating access to the page.
  
  []
  
  The later case, validation, is generally concerned with user identity and access; if you have pages that may only be accessed by certain users,
  you may use the page's activate event handler responsible for verifying that access.
  
  A page's activate event handler mirrors its passivate handler:
  
+----+
  . . .
  
  void onActivate(long productId)
  {
     product = productDAO.getById(productId);
  }
  
  . . .
+-----+

  Here's the relevant part: when the page renders, it is likely to include more action request URLs (links and forms). The action requests
  for those links and forms will <also> start by activating the page, before performing other work. This forms an unbroken chain of requests
  that include the same activation context.
  
  To some degree, this same effect could be accomplished using a {{{persist.html}persistent page value}}, but that requires an active session,
  and the result is not bookmarkable.

  The activate event handler may also return a value, which is treated identically to a return value of an action request event trigger.  This will typically
  be used in an access validation scenario.
  
Page Navigation Patterns

  This combination of action links and context and page context can be put together in any number of ways.
  
  Let's take a typical master/detail relationship using the concept of a product catalog page.  In this example, the 
  ProductListing page is a list of products, and the ProductDetails page must display the details for a specific product.
  
* Action Requests / Persistent Data

  In this pattern, the ProductListing page uses action events and a persistent field on the ProductDetails page.
  
  ProductListing.html:
  
+---+
  <t:loop source="products" value="product">
    <a t:type="actionlink" t:id="select" context="product.id">${product.name}</a>
  </t:loop>
+---+

  ProductListing.java:  
  
+---+
  @InjectPage
  private ProductDetails details;
  
  Object onActionFromSelect(long productId)
  {
    details.setProductId(productId);
    
    return details;
  }
+---+

  ProductDetails.java:

+----+
  @Inject
  private ProductDAO dao;
  
  private Product product;
  
  @Persist
  private long productId;
  
  public void setProductId(long productId) { this.productId = productId; }
  
  void onActivate()
  {
    product = dao.getById(productId);
  }
+----+

  This is a minimal approach, perhaps good enough for a prototype. 
  
  When the user clicks a link, the action request URL will initially be something like "http://.../productlisting.select/99" and the final
  render request URL will be something like "http://.../productdetails".  Notice that the product id ("99") does not appear in the render request URL.
  
  It has some minor flaws:
  
  * It requires a session (to store the _productId field between requests).
  
  * It may fail if the ProductDetails page is accessed before a valid product id is set.
  
  * The URL does not indicate the identity of the product; if the user bookmarks the URL and comes back later, they will trigger the previous case (no valid product id).
  
* Action Requests / Persistent Data

  We can improve the previous example without changing the ProductListing page,  using a passivation and activation context
  to avoid the session and make the links more bookmarkable.
  
  ProductDetails.java:

+----+
  @Inject
  private ProductDAO dao;
  
  private Product product;
  
  private long productId;
  
  public void setProductId(long productId) { productId = productId; }
  
  void onActivate(long productId)
  {
    this.productId = productId;
    
    product = dao.getById(productId);
  }
  
  long onPassivate() { return productId; }
+----+   

  This change ensures that the render request URL will include the product id, i.e., "http://.../productdetails/99".
  
  It has the advantage that the connection from page to page occurs in typesafe Java code, inside the onActionFromSelect method of ProductListing.
  It has the disadvantage that clicking a link requires two round trips to the server.
  
* Render Requests Only

  This is the most common version of this master/detail relationship.
  
  ProductListing.html:
  
+---+
  <t:loop source="products" value="product">
    <a t:type="pagelink" page="productdetails" context="product.id">${product.name}</a>
  </t:loop>
+---+  

  ProductListing.java:
  
  No code is needed to support the link.
  
  ProductDetails.java:

+----+
  @Inject
  private ProductDAO dao;
  
  private Product product;
  
  private long productId;
   
  void onActivate(long productId)
  {
    this.productId = productId;
    
    product = dao.getById(productId);
  }
  
  long onPassivate() { return productId; }
+----+     
  
  The setProductId() method is no longer needed.
  
* Limitations

  As your application's workflow expands, you may find that there is not a reasonable way to avoid storing some data persistently between requests, outside
  of the page activation context.  For example, if from the ProductDetails page, the user is allowed to navigate to related pages and then back to ProductDetails, it
  starts to become necessary to keep passing that product id around from page to page to page.
  
  At some point, persistent values make more sense.  In the near future, there will be a client-side persistence strategy that will encode persistent data, such as
  the product id field, into query parameters (and hidden form fields) automatically.