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Designing Tapestry Mega-Components
Pages: 1, 2

Body Component

The majority of the DHTML support in Tapestry is provided by the Body component. The Body component replaces the <body> tag in most Tapestry pages. The Body component provides a number of services to all the Tapestry components it wraps:



  • Image preloading. The Body component generates JavaScript to preload any images that may be needed on the page. This is used by Rollover components (to provide mouse-over images). Preloading an image forces the Web browser to load the image before it is needed and keep it in memory; this allows instant visual changeovers when the source of an image is changed.

  • JavaScript containment. Any component can ask the Body component to include JavaScript; the script for all components goes into a single large <script> block just preceding the <body> tag.

  • Initialization. Any component can add JavaScript initialization to be executed from the <body> tag's onload event handler. This event handler goes, with all other JavaScript, in the single <script> block preceding the <body> tag.

The Palette component leverages all three of these services. It preloads the images for the Select, Deselect, Move-up and Move-down buttons (in both their enabled and disabled states). It includes a large amount of JavaScript for each Palette component in the form of event handling functions for the buttons, supported by additional utility functions. The Palette component uses the initialization support to assign event handler functions to the various controls within the Palette.

Tapestry Client-Side Scripting

The Tapestry framework includes a powerful mechanism for dynamically generating JavaScript from a template. The main feature of Tapestry scripting is how the input script is parameterized to produce the output JavaScript. The parameterization includes a combination of symbol replacement, conditional blocks, and even looping operations.

In effect, a Tapestry script is an object that takes as input a collection of symbols and produces, as output, the desired JavaScript. Along the way, it will create new symbols that are communicated back to the caller.

For the Palette component, the associated script takes as input the names assigned to the form and to the Palette component, some flags used to indicate how items are to be sorted (by label, by value, or manually), and the URLs for the various images used on the buttons.

Tapestry script documents have three main sections. In the first section, new symbols are defined, using the <let> directive. The second section is the <body> directive, where functions and variables are created. The final section, <initialization>, contains JavaScript code to be executed only after the overall HTML page loads.

Tapestry script documents contain additional directives to accomplish conditional blocks, looping, and inserting symbols.

The Palette script starts by creating a new symbol, baseName, which combines the name of the form and the name of the Palette component together. All other functions and objects incorporate this base name, which provide a reasonable guarantee that there will not be any name collisions (regardless of the number of forms or Palette components on the page).

<let key="baseName">
  <insert property-path="formName"/>_<insert property-path="name"/>
</let>

For example, the select button will be associated with a particular JavaScript function, executed when the user clicks the button. Two new symbols are created:

<let key="selectFunctionName">
  select_<insert property-path="baseName"/>
</let>

<let key="selectOnClickScript">
  javascript:<insert property-path="selectFunctionName"/>();
</let>

The first symbol sets the name of the JavaScript function to be executed when the select button is clicked. The second will be used as the href attribute of the HTML <a> tag for the select button, when that portion of the page's HTML is generated. Now that the name of the function is known, the function itself can be created (within the <body> section of the Tapestry script):

function <insert property-path="selectFunctionName"/>()
{
   if (<insert property-path="selectDisabled"/>)
    return;
    
    var source = <insert property-path="availablePath"/>;
    var target = <insert property-path="selectedPath"/>;
    
    palette_transfer_selections(source, target);
  
<if property-path="sortLabel">
      palette_sort_by_label(target);
</if>
<if property-path="sortValue">
      palette_sort_by_value(target);
</if>
  <insert property-path="updateFunctionName"/>();
}

Again, this function is parameterized (remember that the name of every aspect of the JavaScript must be dynamically named to prevent any naming collisions within the client Web browser). This code simply checks whether the button is disabled (due to a lack of selected options in the available list). If enabled, the selected options are transferred to the selected list. Following that, the selected list is sorted (if required). Finally, the Palette's update function is invoked; its job is to enable and disable the various buttons based on which items are selected.

At runtime, this portion of the script will generate the following JavaScript:

function select_Form0_inputColor()
{
   if (buttons_Form0_inputColor.selectDisabled)
     return;
    
   var source = document.Form0.inputColor_avail;
     var target = document.Form0.inputColor;
    
     palette_transfer_selections(source, target);
  

     palette_sort_by_label(target);


   update_Form0_inputColor();
}

This shows how the form name, Form0, and the Palette name, inputColor, have been incorporated into the names of the functions and HTML elements.

HTML Template

Like most Tapestry components, the Palette uses an HTML template. The template contains static HTML, which is passed directly back to the client Web browser. Certain portions, marked with tags (including the jwcid attribute), are placeholders for other components.

<table jwcid="table">
  <tr>
    <th>Available</th>
    <td class="controls" rowspan=2>
      <a jwcid="selectButton"><img jwcid="selectImage" alt="[Select]"/></a>
      <br>
      <a jwcid="deselectButton"><img jwcid="deselectImage" alt="[Deselect]"/></a>
    </td>
    <th>Selected</th>
  </tr>
  <tr>
    <td><select jwcid="availableSelect"/></td>
    <td><select jwcid="selectedSelect"/></td>
  </tr>
</table>

For clarity of discussion, the actual HTML template used has been simplified here. Regardless, the template shows that the component produces a table with two buttons and two <select> elements. Everything else, the names of each element, the JavaScript, the event handlers, the URLs of the images used, is provided dynamically. Defining all of this dynamic behavior is split between the Palette component's specification file, and its Java code.

For example, the specification for the select button, the button used to move items from the available list to the selected list, is built using two components:

  <component id="selectButton" type="Any">
    <static-binding name="element">a</static-binding>
    <binding name="href" property-path="symbols.selectOnClickScript"/>
  </component>

  <component id="selectImage" type="Image">
    <binding name="image" property-path="selectImage"/>
    <binding name="name" property-path="symbols.selectImageName"/>
  </component>

The first <component> element identifies the selectButton component as an Any component, a type of component that can produce any kind of HTML element. The element binding identifies that the Any is configured to produce an <a> tag. This may seem redundant of the HTML template (where it was also shown to be an <a> tag), but the HTML parser used with Tapestry component templates delibrately ignores the actual tag used -- it is only interested in the structure of the template. The fact that an <a> tag was used is a convienience for the HTML producer, who'll be able to preview the HTML in a standard editor like HomeSite.

In addition, the href attribute of the Any tag is set to the value of a symbol output from the script -- the selectOnClickScript property discussed earilier. The path, symbols.selectOnClickScript, means that the symbols property is obtained from the Palette component, and, within it, the property selectOnClickScript is extracted. The Palette class is responsible for providing access to the symbol's Map as a read-only JavaBeans property.

Wrapped inside the selectButton is the selectImage component, of type Image. This component generates an HTML <img> tag. The Palette component provides another property, selectImage, which identifies the image to be used with the button. The Palette component includes defaults for all the images it needs (for the different buttons and states), but allows any of the images to be overriden, using additional component parameters. The getSelectImage() method checks for a selectImage parameter, or provides the default image if the parameter is unspecified.

The client Web browser will see the following HTML for this portion of the Palette:

<a href="javascript:select_Form0_inputColor();"><img src="/private-assets/net/sf/tapestry/contrib/palette/select_right.gif" border="0" name="selectimage_Form0_inputColor" alt="[Select]"></a>

This same pattern -- defining names of elements and functions, implementing the functions, and linking the HTML elements to those functions -- is followed for all of the other aspects of the Palette component.

Conclusion

Tapestry demonstrates the possibilities for easily assembling complex user interfaces from reusable components. The facilities provided by the framework allow for substantial complexity, easily weaving together both client- and server-side logic with dynamically-generated HTML. Meanwhile, the open source community surrounding Tapestry continues to grow, which means that there will be a bumper crop of these kinds of sophisticated components going forward, a prospect which should delight any Java Web developer.

Howard Ship is an open source Java-based Web developers on several projects, including Tapestry.


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