Ajax and Zones

Tapestry provides easy-to-use support for Ajax, the technique of using JavaScript to dynamically update parts of a web page with content from the server without redrawing the whole page. With Tapestry, you can do simple Ajax updates without having to write any JavaScript code at all.

Ajax support is included in many built-in components and component mixins via the async parameter (in Tapestry 5.4+) and the zone parameter (for earlier versions). Here we use an EventLink component to trigger an Ajax update of another area of the page:

<t:eventlink event="updateTime" async="true">update</t:eventlink>
...
<t:zone t:id="timeArea" id="timeArea">
    The current time is ${currentTime}
</t:zone>

The corresponding Java code might look like this:

@Inject
private AjaxResponseRenderer ajaxResponseRenderer;

@InjectComponent
private Zone timeArea;

@Property
private Date currentTime;
 ...
/** respond to the "updateTime" event */
void onUpdateTime()
{
    currentTime = new Date();
    ajaxResponseRenderer.addRender(timeArea);
} 

That onUpdateTime method is just an ordinary Tapestry event handler, except that it uses an injected AjaxResponseRenderer to tell Tapestry what zone to update when the link is clicked.

Since Tapestry 5.4.2, you can also easily invoke server-side event handlers using the @PublishEvents annotation and the t5/core/ajax JavaScript function, as explained in the Invoking server-side event handler methods from JavaScript section below.

Zones

Zones are Tapestry's approach to performing partial page updates. A Zone component renders as an HTML element, typically a <div>, and serves as a marker for where dynamically-updated content should be replaced. A zone is recognizable in the DOM because it will have the attribute data-container-type=zone. The client-side support for Zones is keyed off of this attribute and value.

Starting in Tapestry 5.4 you can use any HTML element in your template as a zone marker, by passing its client-side id to the two-argument version of the addRender method.

A Zone updated can be triggered by an EventLink, ActionLink or Select component, or by a Form. All of these components support the async and/or zone parameters. Clicking such a link will invoke an event handler method on the server as normal ... except that a partial page response is sent to the client, and the content of that response is used to update the Zone's <div> in place.

Event Handler Return Types

In a traditional request, the return value of an event handler method may used to determine which page will render a complete response, and a redirect may sent to the client to render the new page (as a new request).

In contrast, with a Zone update, the return value may used to render a partial response within the same request.

If you only have one zone to update and don't want to use AjaxResponseRenderer, you can instead return a value from your event handler method. The simplest case is just to return the zone's own body:

@Inject
private Request request;

@InjectComponent
private Zone myZone;
...
Object onActionFromSomeLink()
{
   return myZone.getBody(); // AJAX request, return zone's own body
}

The possible return values are:

• An injected Block or Component to render as the response. The response will be a JSON hash, with a "content" key whose value is the rendered markup. This is the basis for updates with the Zone component.
• The zone's own body (using Zone's getBody() method)
• null (to redraw the current page)
• A JSONObject or JSONArray, which will be sent as the response.
• A StreamResponse, which will be sent as the response.
• A Link, which will send a redirect to the client.
• A page name (as a String), or a page class, or a page instance, which will send a redirect to the indicated page.

See Page Navigation for full descriptions of the above.

Graceful Degradation

Users who do not have JavaScript enabled may click EventLinks (or ActionLinks, or Forms) that are configured to update a Zone. When that occurs, the request will still be sent to the server, but Tapestry will handle it as a traditional request.

This happens automatically when your event handler method has a void return type.

However, to support graceful degradation when your event handler method has a non-void return type, you should detect non-Ajax requests and return a traditional response, typically null to redraw the whole page. This is accomplished by injecting the Request object, and invoking the isXHR() method. This value will be true for Ajax requests, and false for traditional request.

@Inject
private Request request;

@InjectComponent
private Zone myZone;
...
Object onActionFromSomeLink()
{
    // return either the zone body (ajax) or whole page (non-ajax)
    return request.isXHR() ? myZone.getBody() : null;
} 

Multiple Zone Updates

An event handler often needs to update multiple zones on the client side. To accomplish this, use an AjaxResponseRenderer, indicating the zones to update. You must know the client-side id for each zone to update (the best way for this is to lock down the zone's id using the id parameter of the Zone component).

AjaxResponseRenderer was introduced in Tapestry 5.3. For Tapestry 5.2 and earlier, return a MultiZoneUpdate object instead.

The renderer for each zone can be the zone itself, a block, a component, a Renderable or a RenderCommand ... or an object, such as String, that can be coerced to either of these.

@InjectComponent
private Zone userInput;

@InjectComponent
private Zone helpPanel;

@Inject
private AjaxResponseRenderer ajaxResponseRenderer;

void onActionFromRegister()
{
    ajaxResponseRenderer.addRender("userInput",
        userInput).addRender("helpPanel", helpPanel);
}

This example assumes that there are two zones, "userInput" and "helpPanel", somewhere in the rendered page, waiting to receive the updated content.

This demonstrates why it is necessary for the developer to specify a particular client-side id for Zone components; if they were dynamically allocated ids, as is typical in most other elements, it would be impossible for this code to know what client-side id was used for the Zone.

Zone Component Id vs. Zone Element Id

Like all Tapestry components, Zones have a component id, specified using the t:id attribute. If you do not assign a component id, a unique id is assigned by Tapestry.

However, to coordinate things on the client side, it is necessary for components that wish to update the zone know the client-side element id. This is specified with the id parameter of the Zone component. If the id parameter is not bound, then a unique value (for the current page and render) is generated by Tapestry and this value is difficult to predict. (The actual value will be available as the clientId property of the Zone component itself.)

Remember that the component id (t:id) is used to inject the Zone component into the containing page or component. The client-side id (id) is used on the client side to orchestrate requests and updates. You will often seen the following construct:

<t:zone t:id="myZone" id="myzone"> ... </t:zone>

The Containing Zone (zone="^")

Since 5.2

If the Form or Link is enclosed by the Zone itself, and you're using the zone parameter instead of the async parameter, then the zone parameter may be set to the special value  ^ (the carat). The zone is found – on the client side – by searching up form the form or link element for the first enclosing element that is a Zone. In this way, the client-side coordination can occur without having to know what the specific client-side id of the Zone is. Because of this, in some cases it is no longer necessary to specify the Zone's id parameter.

An Update div within a Zone div (Tapestry 5.3 and earlier)

Deprecated : This feature is removed starting with Tapestry 5.4

In many situations, a Zone is a kind of "wrapper" or "container" for dynamic content; one that provides a look and feel ... a bit of wrapping markup to create a border. In that situation, the Zone <div> may contain an update <div>.

An Update <div> is specifically a <div> element marked with the CSS class "t-zone-update", inside the Zone's <div>.

If an Update div exists within a Zone div, then when Tapestry updates a zone only the update <div>'s content will be changed, rather than the entire Zone <div>.

The show and update functions (see Zone Functions, below) apply to the Zone <div>, not just the update <div>.

Zone Effect Functions (Tapestry 5.3 and earlier)

Deprecated: This feature refers to client-side logic only present in Tapestry 5.3 or earlier. For 5.4, there are client-side events that are triggered before and after changes to the Zone; listeners on those events can trigger whatever animations they like.

A Zone may be initially visible or invisible. When a Zone is updated, it is made visible if not currently so. This is accomplished via a function on the Tapestry.ElementEffect client-side object. By default, the show() function is used for this purpose. If you want Tapestry to call a different Tapestry.ElementEffect function when updates occur, specify its name with the zone's show parameter.

If a Zone is already visible, then a different effect function is used to highlight the change. By default, the highlight() function is called, which performs a yellow fade to highlight that the content of the Zone has changed. Alternatively, you can specify a different effect function with the Zone's update parameter:

To have Tapestry update a zone without the usual yellow highlight effect, just specify "show" for the update parameter:

<t:zone t:id="myZone" t:update="show">

You may also define and use your own JavaScript effect function (with lower-case names), like this:

Tapestry.ElementEffect.myeffectname = function(element){ YourJavascriptCodeGoesHere; };

Zone Limitations

Unlike many other situations, Tapestry relies on you to specify useful and unique ids to Zone components, then reference those ids inside EventLink (or ActionLink, or Form) components. Using Zone components inside any kind of loop may cause additional problems, as Tapestry will uniqueify the client id you specify (appending an index number).

If you create a component that contains a zone, and you use that component in a loop, you'll likely need to set the client-side id like this:

<t:zone t:id="myzone" id="prop:componentResources.id">

See this JumpStart Example for details.

The show and update function names (Tapestry 5.3 and earlier only) are converted to lower case; all the methods of Tapestry.ElementEffect should have all lower-case names. Because client-side JavaScript is so fluid (new methods may be added to existing objects), Tapestry makes no attempt to validate the function names ... however, if the names are not valid, then the default show and highlight methods will be used.

Zones may only be used inside the body of a page, not the head.

More Information

For examples of extending a Form with a Zone and updating multiple zones at once, see the Ajax Components FAQ.

There are also a number of Ajax-related examples at the   Tapestry JumpStart site.

Autocomplete Mixin

The Autocomplete mixin exists to allow a text field to query the server for completions for a partially entered phrase. It is often used in situations where the field exists to select a single value from a large set, too large to successfully download to the client as a drop down list; for example, when the number of values to select from is numbered in the thousands.

Autocomplete can be added to an existing text field:

  <t:textfield t:id="accountName" t:mixins="autocomplete" size="100"/>

The mixin can be configured in a number of ways, see the component reference.

When the user types into the field, the client-side JavaScript will send a request to the server to get completions.

You must write an event handler to provide these completions. The name of the event is "providecompletions". The context is the partial input value, and the return value will be converted into the selections for the user.

For example:

  List<String> onProvideCompletionsFromAccountName(String partial)
  {
    List<Account> matches = accountDAO.findByPartialAccountName(partial);

    List<String> result = new ArrayList<String>();

    for (Account a : matches)
    {
      result.add(a.getName());
    }

    return result;
  }

This presumes that findByPartialAccountName() will sort the values, otherwise you will probably want to sort them. The Autocomplete mixin does not do any sorting.

You can return an object array, a list, even a single object. You may return objects instead of strings ... and toString() will be used to convert them into client-side strings.

Invoking server-side event handler methods from JavaScript

Tapestry 5.4.2 introduced an API which makes it easy for server-side events to be invoked from JavaScript. On the server-side, you first need to annotate the event handler methods you want to expose with the @PublishEvent annotation. Then, in JavaScript, all you need to do is to call the existing t5/core/ajax function, but with slightly different parameters.

The t5/core/ajax function has two parameters: url and options. Prior to Tapestry 5.4.2, the first one was difficult to get when doing AJAX requests to event handler methods. You needed to inject ComponentResources in your component class, call componentResources.createEventLink() for each event handler method, then pass all this information back to the browser through one of the JavaScriptSupport methods. For Tapestry 5.4.2 and later, your JavaScript code only needs to know the event name (also called event type) and optionally indicate a DOM element to be used as a starting point for finding the event URL.

All event data is stored in data-componenent-events attributes. For page classes, the attribute is added to the <body> element. For components, it's added to the first element rendered by the component. Given an HTML element, the search is performed in the following order until information for the given event is first found:

1. The element itself
2. The element's previous siblings, closest first (bottom-up)
3. The element's parents
4. The page's <body> element

Here's one example:

public class PublishEventDemoComponent
{
    @OnEvent("answer")
    @PublishEvent
    JSONObject answer() {
        return new JSONObject("origin", "componentAnswer");
    }
    
    @PublishEvent
    JSONObject onAction()
    {
        return new JSONObject("origin", "componentAction");
    }   
}

Notice that answer() and onAction() are ordinary event handlers, with nothing specific besides the @PublishEvent annotation.

<div id="component" xmlns:t="http://tapestry.apache.org/schema/tapestry_5_0_0.xsd">
	<p id="componentParagraph">I'm a component</p>
    <p id="result">(no result yet)</p>
</div>

The template also has nothing special. When rendered, the component's events information is placed in the outer <div id="component">.

We want to update the text of <p id="result"> with the value of the origin property of the returned JSON object when that element itself is clicked, so here's our JavaScript code, supposing we want to trigger the answer event:

require(["t5/core/ajax", "jquery"], function (ajax, $) {
    // Creating a callback to be invoked with <p id="result"> is clicked.
	$('#result').click(function() {
		ajax('answer', { 
			element: $('#result'), // This doesn't need to be the same element as the one two lines above
            // Callback called when the request is finished. 
            // response.json is the object returned by the event handler method
			success: function(response) {
				$('#result').text(response.json.origin);
			}
		});
	});
});

If you're trying to invoke a page class event handler, you can change line 5 above to element: null. You do need to explicitly set the element property, otherwise the ajax function will treat the first parameter, url, as an URL and not as an event name.