JavaScript: The Definitive Guide

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Special Effects with Images

16.5 Image Replacement Example

Because image replacement is such a versatile technique we will end our discussion of the Image object with an extended example. Example 16.3 defines a ToggleButton class that uses image replacement to simulate a graphical checkbox. Because this class uses images that we provide, we can use bolder graphics than those plain-old graphics used by the standard HTML Checkbox object. Figure 16.1 shows how these toggle button graphics could appear on a web page. This is a complex, real-world example, and is worth studying carefully.

Figure 16.1: ToggleButtons implemented with image replacement

[Graphic: Figure 16-1]

Example 16.3: Implementing a ToggleButton with Image Replacement

<SCRIPT LANGUAGE="JavaScript1.1">
// This is the constructor function for our new ToggleButton class.
// Calling it creates a ToggleButton object and outputs the required
// <A> and <IMG> tags into the specified document at the current location. 
// Therefore, don't call it for the current document from an event handler.
// Arguments:
//    document: the Document object the buttons will be created in.
//    checked:  a Boolean that says whether the button is initially checked.
//    label:    an optional string that specifies text to appear after the button.
//    onclick:  an optional function to be called when the toggle button is
//              clicked. It will be passed a Boolean indicating the new
//              state of the button. You can also pass a string, which will
//              be converted to a function which is passed a Boolean argument
//              named "state".
function ToggleButton(document, checked, label, onclick)
    // first time called, document will be false. Ignore this call.
    if (document == null) return;
    // The first time we are called (and only the first time) we have
    // to do some special stuff. First, now that the prototype object
    // is created, we can set up our methods. 
    // Second, we've got to load the images that we'll be using.
    // Doing this will get the images in the cache for when we need them.
    if (!ToggleButton.prototype.over) {
        // Initialize the prototype object to create our methods.
        ToggleButton.prototype.over = _ToggleButton_over;
        ToggleButton.prototype.out = _ToggleButton_out; = _ToggleButton_click;
        // Now create an array of image objects, and assign URLs to them.
        // The URLs of the images are configurable, and are stored in an
        // array property of this constructor function itself. They will be
        // initialized below. Because of a bug in Navigator, we've got
        // to maintain references to these images, so we store the array
        // in a property of the constructor rather than using a local variable.
        ToggleButton.images = new Array(4);
        for(var i = 0; i < 4; i++) {
            ToggleButton.images[i] = new Image(ToggleButton.width,
            ToggleButton.images[i].src = ToggleButton.imagenames[i];
    // Save some of the arguments we were passed.
    this.document = document;
    this.checked = checked;
    // Remember that the mouse is not currently on top of us.
    this.highlighted = false;
    // Save the onclick argument to be called when the button is clicked.
    // If it is not already a function, attempt to convert it
    // to a function that is passed a single argument, named state.
    this.onclick = onclick;
    if (typeof this.onclick == "string")
        this.onclick = new Function("state", this.onclick);
    // Figure out what entry in the document.images[] array the images
    // for this checkbox will be stored at.
    var index = document.images.length;
    // Now output the HTML code for this checkbox. Use <A> and <IMG> tags.
    // The event handlers we output here are confusing, but crucial to the
    // operation of this class. The "_tb" property is defined below, as
    // are the over(), out(), and click() methods.
    document.write('&nbsp;<A HREF ="" ' +
      'onMouseOver="document.images[' + index + ']._tb.over();return true;" '+
      'onMouseOut="document.images[' + index + ']._tb.out()" '+
      'onClick="document.images[' + index + ']; return false;">');
    document.write('<IMG SRC="' + ToggleButton.imagenames[this.checked+0] +'"'+
                   ' WIDTH=' + ToggleButton.width +
                   ' HEIGHT=' + ToggleButton.height +
                   ' BORDER=0 HSPACE=0 VSPACE=0 ALIGN="absmiddle">');
    if (label) document.write(label);
    // Now that we've output the <IMG> tag, save a reference to the
    // Image object that it created in the ToggleButton object.
    this.image = document.images[index];
    // And also make a link in the other direction: from the Image object
    // to this ToggleButton object. Do this by defining a "_tb" property
    // in the Image object.
    this.image._tb = this;
// This becomes the over() method.
function _ToggleButton_over()
    // Change the image, and remember that we're highlighted.
    this.image.src = ToggleButton.imagenames[this.checked + 2];
    this.highlighted = true;
// This becomes the out() method.
function _ToggleButton_out()
    // Change the image, and remember that we're not highlighted.
    this.image.src = ToggleButton.imagenames[this.checked + 0];
    this.highlighted = false;
// This becomes the click() method.
function _ToggleButton_click()
    // Toggle the state of the button, change the image, and call the
    // onclick method, if it was specified for this ToggleButton.
    this.checked = !this.checked;
    this.image.src = ToggleButton.imagenames[this.checked+this.highlighted*2];
    if (this.onclick) this.onclick(this.checked);
// Initialize static class properties that describe the checkbox images. These
// are just defaults. Programs can override them by assigning new values.
// But the should only be overridden *before* any ToggleButtons are created.
ToggleButton.imagenames = new Array(4);            // create an array
ToggleButton.imagenames[0] = "togglebutton0.gif";  // the unchecked box
ToggleButton.imagenames[1] = "togglebutton1.gif";  // the box with a check mark
ToggleButton.imagenames[2] = "togglebutton2.gif";  // unchecked but highlighted
ToggleButton.imagenames[3] = "togglebutton3.gif";  // checked and highlighted
ToggleButton.width = ToggleButton.height = 25;     // size of all images
<!-- Here's how we might use the ToggleButton class. -->
Optional extras:<BR>
<SCRIPT LANGUAGE="JavaScript1.1">
// Create the buttons
var tb1 = new ToggleButton(document, true, "28.8K Modem<BR>");
var tb2 = new ToggleButton(document, false, "Laser Printer<BR>");
var tb3 = new ToggleButton(document, false, "Tape Backup Unit<BR>");
<!-- Here's how we can use the ToggleButton objects from event handlers. -->
<INPUT TYPE="button" VALUE="Report Button States"
       onClick="alert(tb1.checked + '\n' + tb2.checked + '\n' + tb3.checked)">
<INPUT TYPE="button" VALUE="Reset Buttons"
       onClick="if (tb1.checked);
                if (tb2.checked);
                if (tb3.checked);">

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