Ajax and Zones

Tapestry provides easy-to-use support for Ajax, the technique of using JavaScript to dynamically updating 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 zone parameter.

Zones

Zones are Tapestry's approach to performing partial page updates. A Zone component renders as an HTML element, typically a <div>, with the "t-zone" CSS class. (It also adds some JavaScript to the page to "wire up" a Tapestry.ZoneManager object to control updating that element.)

A Zone can be updated via an EventLink, ActionLink or Select component, or by a Form. All of these components support a zone parameter, which provides the id of the Zone's <div>. Clicking such a link will invoke an event handler method on the server as normal ... except that the return value of the event handler method is used to send a partial page response 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 is used to determine which page will render a complete response, and a redirect is sent to the client to render the new page (as a new request).

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

This return value is often just the zone's own body (as below), but it can also be an injected component or block. The value will be rendered, and that markup will be used on the client side to update the Zone's <div>.

The possible return values are:

  • A 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
  • 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.

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.

To support graceful degradation, you should detect that case in your event handler method and return a traditional response: a page, page name or page class. 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.

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.

For Tapestry 5.3 and later
For Tapestry 5.1, 5.2 and 5.3

    Note that MultiZoneUpdate is deprecated starting with Tapestry 5.3.

These examples assume that there are two zones, "userInput" and "helpPanel", somewhere in the rendered page, waiting to receive the updated content.

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:

Added in 5.2

Icon

If the Form or Link is enclosed by the Zone itself, then the zone parameter may be set to the special value ^. The carat is evaluated, on the client side, by searching up form the form or link element for the first enclosing element with the t-zone CSS class. 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 many cases, it is no longer necessary to specify the Zone's id parameter.

An Update div within a Zone div

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

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:

Tapestry.ElementEffect Function

Result

highlight()

(the default) highlight changes to an already-visible zone

show()

make the zone visible if it isn't already visible

slidedown()

scroll the content down

slideup()

slide the content back up (opposite of slidedown)

fade()

fade the content out (opposite of show)

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

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

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).

The show and update function names 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.

More Information

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

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:

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:

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.

Tapestry's default stylesheet includes entries for controlling the look of the floating popup of selections.

You may override DIV.t-autocomplete-menu UL to change the main look and feel, DIV.t-autocomplete-menu LI for a normal item in the popup list, and DIV.t-autocomplete-menu LI.selected for the element under the cursor (or selecting using the arrow keys).