Adding Persistence to DHTML Effects

Originally published at Netscape Enterprise Developer, now archived at the Wayback Machine

Dynamic HTML (DHTML), implemented in Netscape Navigator 4.x and Microsoft Internet Explorer 4.x, gives the Web page visitor the ability to alter the position, format, or visibility of an HTML element. However, the effects that are created with DHTML are transitory in that the next time the page is refreshed, the current DHTML state is not maintained and the page opens showing the same content layout as when the page is first loaded. Any changes to the content based on DHTML are not maintained. Sometimes this isn’t a problem, but other times this is irritating to the reader. This article covers how to add persistence to a DHTML page. Examples should work with all forms of Netscape Navigator 4.x and Internet Explorer 4.x, but have only been tested with IE 4.x and Netscape Navigator 4.04 in Windows 95.With DHTML, developers can provide functionality that hides or shows whichever layer the Web page reader wants to view. Developers can also add functionality that lets readers move content. The problem is that DHTML effects are not session persistent, which means that they aren’t maintained between page reloads. Following a link to another site and returning to the page can trigger a reload, which wipes out the effect and annoys the user,. especially if he or she has spent considerable effort achieving the current DHTML state.

For example, you can use DHTML to let your reader position your page contents for maximum visibility in her browser and get the page look just right. If she decides to leave your site to check Yahoo’s news for a few minutes and then comes back, her settings will have been erased.

So, what’s the solution to these problems? It’s relatively simple — add document persistence using Netscape-style cookies.

Using cookies 
Despite the name, you won’t find Netscape-style cookies at your local bakery. Cookies are bits of information in the form of name-value pairs that are stored on the client’s machine for a set period of time or until the browser is closed. For security, cookies are created and accessed using specific syntax, are limited to 300 cookies total within the cookie database at 4K per cookie, and are limited to 20 cookies per domain (the URL where cookie was set).

Cookie syntax consists of a string with URL characters encoded, and may include an expiration date in the format:

Wdy, DD-Mon-YY HH:MM:SS GMT

Netscape has a complete writeup on cookies (see our Resource section at the end), including functions that can be copied and used to set and get cookies. I created modified versions of these functions to support my DHTML effects. As I don’t want to add to overly burdened cookie files, I don’t set the expiration date, which means the cookie does not get stored in the persistent cookie database or file. This also means that the DHTML effect only lasts for the browser session. However, this fits my needs of maintaining a persistent DHTML state in those cases where the reader moves to another site or hits the reload button.

DHTML state cookie functions
I created a JavaScript source code file called cookies.js that has two functions. One function sets the cookie by assigning the value to the document.cookie object. More than one cookie can be set in this manner, as cookie storage is not destructive — setting another cookie does not overwrite existing cookies, it only adds to the existing cookie storage for the URL. The cookie setting function is shown in the following code block:

// Set cookie name/value
//
function set_cookie(name, value) {
   document.cookie = name + "=" + escape(value);
}

Next, the function to get the cookie accesses the document.cookie object and checks for a cookie with a given name. If found, the value associated with the cookie name is returned, otherwise an empty string is returned. The code for this function is:

// Get cookie given name
//
function get_cookie(Name) {
  var search = Name + "="
  var returnvalue = "";
  if (document.cookie.length > 0) {
    offset = document.cookie.indexOf(search)
    if (offset != -1) { // if cookie exists
      offset += search.length
      // set index of beginning of value
      end = document.cookie.indexOf(";", offset);
      // set index of end of cookie value
      if (end == -1)
         end = document.cookie.length;
      returnvalue=unescape(document.cookie.substring(offset, end))
      }
   }
  return returnvalue;
}

That’s it to set and get cookie values. The next section shows how to create two Web pages with simple, cross-browser DHTML effects. Then each page is modified to include the use of cookies to maintain DHTML state.

_BREAK1 Creating the DHTML Web pages
To demonstrate how to add persistence to DHTML pages, I created two Web pages implementing simple DHTML effects. The first page layers content and then hides and shows the layers based on Web-page reader mouse clicks. The second page has a form with two fields, one for setting an element’s top position and one for setting an element’s left position. Changing the value in either field and clicking an associated button changes the position of a specific HTML element.

Adding DHTML persistence for layered content
For the first demonstration page, a style sheet was added to the top of the page that defines positioning for all DIV blocks, formatting for an H1 header nested within a DIV block, and three named style sheet classes. The complete style sheet is shown here:

<STYLE type="text/css">
        DIV { position:absolute; left: 50; top: 100; width: 600 }
        DIV H1 { font-size: 48pt; font-family: Arial }
        .layer1 { color: blue }
        .layer2 { color: red }
        .layer3 { color: green }
</STYLE>

Next, three DIV blocks are used to enclose three layers, each given the same position within the Web page. Each block also contains a header (H1), with each header given a different style-sheet style class. The HTML for these objects is:

<DIV id="layer1">
<H1 class="layer1">LAYER--BLUE</H1>
</DIV>
<DIV id="layer2" style="visibility:hidden">
<H1 class="layer2">LAYER--RED</H1>
</DIV>
<DIV id="layer3" style="visibility:hidden">
<H1 class="layer3">LAYER--GREEN</H1>
</DIV>

That’s it for the page contents. To animate the page, I created a JavaScript block that contains a global variable and two functions. The global variable maintains the number of the layer currently visible. The first function is called cycle_layer; this function determines which layer is to be hidden and which is shown next, and then calls a function that performs the DHTML effect:

<SCRIPT language="javascript1.2">
<!--

// current layer counter
current_layer = 1;

// assign document clicks to function pointer
document.onclick = cycle_layer;

// cycle through layers
function cycle_layer() {
   var next_layer;
   if (current_layer == 3)
        next_layer = 1;
   else
        next_layer = current_layer + 1;
   switch_layers(current_layer, next_layer);
   current_layer = next_layer;
}

The scripting block also assigns the function cycle_layer as the event handler function for all mouse clicks that occur within the document page. By doing this, clicking any where in the page document area that doesn’t include any other content results in a call to the function to change the layer.

The switch_layers function is the DHTML effects function, and it uses the most uncomplicated technique to handle cross-browser differences: it checks for browser type and then runs code specific to the browser. Other techniques can be used to handle cross-browser differences, but these are outside of the scope of this article. All that the function does is hide the current layer and show the next layer in the cycle, as shown here:

// hide old layer, show new
function switch_layers(oldlayer, newlayer) {
   if (navigator.appName == "Microsoft Internet Explorer") {
        document.all.item("layer" + oldlayer).style.visibility="hidden";

        document.all.item("layer" +
newlayer).style.visibility="inherit";
        }
   else {
        document.layers["layer" + oldlayer].visibility="hidden";
        document.layers["layer" + newlayer].visibility="inherit";
        }
}

Try out the first sample page. Clicking on the document background, not the text, changes the layer. Try setting the layer to the green or red layer, accessing another site using the same browser window, and returning to the sample page. When you return, the page is reset to the beginning DHTML effect, showing the blue layer.

To correct the loss of the DHTML effect, we can add persistence to the page by storing which layer is showing when the page unloads. This information is captured in a function called when the onUnLoad event fires.

To add persistence, I added the cookies Javascript source code file to the page, using an external source code reference:

<!-- add in cookie functions -->
<SCRIPT language="javascript" src="cookie.js">
</SCRIPT>

Next, I added the function to capture the DHTML state:

// onunload event handler, capture "state" of page (article)
function capture_state() {
   set_cookie("current_layer",current_layer);
}

When the page reloads, the onLoad event is fired, and a function is called from this event to “redraw” the DHTML effect. The function is called start_page, and it pulls in the cookie containing which layer should be shown:

function start_page() {
// get cookie, if any, and restore DHTML state
  current_layer = get_cookie("current_layer");
  if (current_layer == "")
        current_layer = 1;
  else
        switch_layers(1, current_layer);
}

Finally, the two event handlers, onUnLoad and onLoad, are added to the BODY HTML tag:

<BODY onload="start_page()" onunload="capture_state()">

You can try out the second sample page, which has DHTML persistence — again, change the layer, open some other site and then return to the sample page. This time, the DHTML effect persists during a page reload.

Sometimes an effect requires more than one cookie, as the next example demonstrates.

_BREAK2 Adding DHTML Persistence for Positioned Content
In the next example, I used a style sheet again to define CSS1 and CSS-P formatting for the DIV block that is going to be moved in the page:

<STYLE type="text/css">
        DIV { position:absolute; left: 50; top: 100; width: 600 }
        DIV H1 { font-size: 48pt; font-family: Arial }
        .layer1 { color: blue }
</STYLE>

Next, I created a form that has two text elements and associated buttons. One text and button element pair is used to change the DIV block’s top position, one pair is used to change the DIV block’s left position. The entire form is shown here:

<DIV id="first" style="position:absolute; left: 10; top: 10; z-index:2">
<form name="form1">
Set new Left Value: <input type="text" name="newx" value="50">
<input type="button" onclick="newleft(newx.value)" value="Set New
Left"><p>
Set new Top Value: <input type="text" name="newy" value="100">
<input type="button" onclick="newtop(newy.value)" value="Set New Top">
</form>
</DIV>

Next, the positioned DIV block is created:

<DIV id="layer1" style="z-index: 1">
<H1 class="layer1">LAYER--BLUE</H1>
</DIV>

Once the page contents have been created, you can add code to animate the DIV block based on whether the top or left position is changed. The function to change the top position is:

// change top value
function newtop(newvalue) {
   if (navigator.appName == "Microsoft Internet Explorer")
        layer1.style.pixelTop = parseInt(newvalue);
   else
        document.layer1.top = newvalue;
}

Again, the least complex technique to handle cross-browser differences is used, which is to check for the browser being used and run the appropriate code. The function to change the left position is identical to the top position function, except the CSS-P attribute “left” is changed rather than the CSS-P attribute “top”:

// change left value
function newleft(newvalue) {
   if (navigator.appName == "Microsoft Internet Explorer")
        layer1.style.pixelLeft = parseInt(newvalue);
   else
        document.layer1.left = newvalue;
}

That’s it for this page. You can try it out on the third sample page, where you can change the position of the DIV block by changing the left position, the top position, or both. Again, open another site in the browser and then return to the example page. Notice how the position of the DIV block does not persist between page reloadings.

Adding persistence to this page is very similar to adding persistence to the layered example page, except two values are maintained: the top and left values. The start_page and capture_state functions for this DHMTL effect are:

function start_page() {
// get cookie, if any, and restore DHTML state
  var tmpleft;
  tmpleft = get_cookie("currentleft");
  if (tmpleft != "") {
        currentleft = parseInt(tmpleft);
        currenttop = parseInt(get_cookie("currenttop"));
        newtop(currenttop);
        newleft(currentleft);
        if (navigator.appName == "Microsoft Internet Explorer") {
           document.forms[0].newx.value=currentleft;
           document.forms[0].newy.value=currenttop;
           }
        else {
           document.first.document.forms[0].newx.value = currentleft;
           document.first.document.forms[0].newy.value = currenttop;
           }
        }
}

function capture_state() {
        set_cookie("currentleft", currentleft);
        set_cookie("currenttop", currenttop);
}

To see how it works, try out the fourth sample page, move the DIV block, open another Web site in the browser and return to the example page. This time the DHTML effect does persist between page reloadings.

Summing up
In a nutshell, the steps to add persistence to a page are:

  1. Create the DHTML page
  2. Determine the values that must be maintained to re-create an existing effect
  3. Add the Netscape-style functions to the page
  4. Capture the onUnLoad event and call a function to capture the DHTML effect persistence values
  5. Capture the onLoad event and call a function to restore the DHTML effect from the persisted values

Using Netscape-style cookies to maintain DHTML effects won’t help with some DHTML persistence problems. For example, DHTML effects can be lost when a page is resized. In addition, if a page is resized between the time the DHTML persistence values are stored and the page is reloaded, the values may no longer work well with current page dimensions. However, for most effects and for most uses, the use of Netscape-style cookies is a terrific approach to DHTML persistence.

Netscape Navigator’s JavaScript 1.1 vs Microsoft Internet Explorer’s JScript

Originally published at Digital Cats, now archived at the Wayback Machine

Prior to Netscape implementing JavaScript in Netscape Navigator, web developers had few tools to create interactive content for their web pages. Now this scripting language gives developers the ability to do things such as check form contents, communicate with the user based on their actions, and modify the web page dynamically without the web page being re-loaded and without the use of Java, plug-ins or ActiveX controls.

Unfortunately, JavaScript was not usable by any other browser until Microsoft released Internet Explorer (IE) 3.0. With this release web developers could deliver interactive content that would at least work with the two most widely used browsers. Or could they?

On the surface, the JavaScript supported by both companies is identical. They both provide the same conditional control statements, have defined objects such as window or document,and can be used directly in HTML documents. They both support events based on user actions and support functions in a similar manner. However, this article will demonstrate that though the languages may look the same on the surface, there are differences that can trip the unwary developer.

JavaScript Objects

To understand the differences between the two implementations of JavaScript you must examine the objects both support. As an example, with Navigator 3.0 Netscape provides a new object image which is an array (defined as images that contains the images in the document currently loaded. This object allows the developer to change the images of a document without re-loading the document or using Java or other technique. With this capability the developer can write the following JavaScript code section without error:


sSelected = "http://www.some.com/some.gif"
document.images[iIndex].src = sSelected

This code will change the src property of the image that is contained in the array at the index given in the variable iIndex. If this variable contained the value 2, the 3rd image as loaded in the document (the array indices begin at 0) would be changed to the image located in the given URL. An example using images can be found here.

Running the same example with Internet Explorer will result in a alert message that states that “‘images’ is not an object”.

Other examples of objects that are defined for Netscape Navigator but not for Microsoft Internet Explorer are:

  • The Area object, which is an array of links for an image map that allows the developer to capture certain events for the image map that can be used to provide additional information to the user. With this, the developer can capture a mouseOver event to write out information about the link in the status bar.
  • The FileUpload object which provides a text like control and a button marked with “Browse” that will allow a reader to enter a file name. The JavaScript can then access the name of the file.
  • The Function object which allows the developer to define and assign a function to a variable which can then be assigned to an event.
  • The mimeTypes Array of supported MIME types.
  • The option object which is an array of the options implemented for SELECT and which allows the developer to change the text of the option at runtime
  • The applet object, which is an array of Applets in the document (read-only)
  • The plugin object, which is an array called embeds that contains the plug-ins contained in the document (read-only)
  • The plugins array, which is an array of plug-ins currently installed in the client browser

At this time there are no JavaScript or JScript objects defined for Internet Explorer that are also not defined for Netscape. However, as JScript is an implementation script for IE and Microsoft has defined their own IE scripting model, this could change in the future.

JavaScript Object Behavior and Ownership

Internet Explorer may not have additional objects but it has defined a different hierarchy and ownership for some of the objects that are used by both it and Navigator. All objects are contained within the window object in the IE scripting model, which can be viewed here, but not all objects are owned by window with the Navigator model, which can be viewed with the JavaScript Authoring Guide. The object Navigator, which is the object that stores information about the browser currently being used, is an example of an owned object by window in IE but not in Navigator.

This will not present an incompatibility problem between the two browsers as the developer will usually not preface the object with the term “window” as the following code demonstrates:


<FORM NAME="form1">
<input type=button value='press' onClick="alert(navigator.appName)">
</FORM>

The above code will work with both browsers.

The differences between the ownership can become a problem when an object is owned by different levels of objects. An example is the history object, which is owned by the window object in IE, but by the document object in Navigator. When used in the current window and document, the object will work the same as the following code will demonstrate:


<FORM NAME="form1">
<input type=button value='press' onClick="history.back()">
</FORM>

The reason the same code can work for both is that window is assumed for both IE and Netscape and document is assumed for Navigator, at least in this instance because history is also an object in its own right. In the case of a document being opened as part of a frame, the differences then become noticeable. The following code will work when the document is opened as a frame in IE, but will not work in Navigator:


<body>
<script>
function clicked() {
history.back()
}
</script>
<FORM NAME="form1">
<input type=button value='press' onClick="clicked()">
</FORM>
</body>

Clicking on the button from the code above will work for IE. The previous document in the History list will be opened, but the same code will not work for Navigator. Clicking on the button will result in neither a change of document nor an error. Prefacing the history object with parent will enable this code to work with both browsers.

JavaScript Properties

Even when IE and Navigator share a common object and a common object ownership, they can differ on the properties for an object. An example is the document object. The properties for both implementations are the same except for an URL property for the Navigator implementation and a location property for the IE implementation. However, if you examine both properties, they are identical! Both contain the URL of the document. Both are read-only. The following code will work with Navigator, but results in an empty Alert message box for IE:


<FORM NAME="form1">
<input type=button value='press' onClick="alert(document.URL)">
</FORM>

According to Microsoft documentation, the equivalent for IE would be to use document.location.href. However, though this does not result in an error, it also results in an empty alert box. The following code achieves the desired results and, happily, works in both browsers:


<FORM NAME="form1">
<input type=button value='press' onClick="alert(location.href)">
</FORM>

The above example does demonstrate another area of caution when using the JavaScript language: this language is unstable in both environments and is changing continuously. Don’t assume something will work because the documentation states it will, and don’t assume it will work the same on all operating systems. Both browsers can have different behaviors across different operating systems, sometimes because of operating system differences and sometimes because of bugs that were missed during testing on that specific OS.

JavaScript Methods and Events

A web developer can find ways of working around differences in objects and properties, but working around differences in methods may not be so easy. When we develop we expect a certain behavior to result when we call a specific function and pass to it certain parameters. With Microsoft Internet Explorer and Netscape Navigator, the best case scenario is that we can use a pre-defined object method with both browsers and have the same result. The worst case scenario is that the method works with both browsers, but the result is different.

An example of the best case scenario is to use JavaScript to validate form field contents which have changed or to use these contents to calculate a value used elsewhere. Calling a JavaScript function from the onChange event to process the changed contents as demonstrated below will work with both browsers:


<!--- Form fields
<p>Item Quantity: <INPUT TYPE="text" Name="qty">
Item Cost: <INPUT TYPE="text" Name="cost"
                onChange="NewCost()">
Total Cost: <INPUT TYPE="text" Name="total" Value=0>
…
<!--- Function
<SCRIPT LANGUAGE="javascript">
<!--- hide script from old browsers

// NewCost will
// calculate cost of qty and item
function NewCost() {
        var iCost = parseFloat(document.Item.cost.value)
        var iQty = parseInt(document.Item.qty.value)

        var iTotal = iCost * iQty
        document.Item.total.value = iTotal
        }

The above will work as expected with both IE and Navigator. When the user enters a quantity and a cost, the onChange event will fire for the cost field and a JavaScript function called NewCost() will be called. This function will call two built-in JavaScript functions, parseFloat() and parseInt(), to access and convert the form field values. These will then be used to compute a total which is placed in the total field.

So far so good. Another JavaScript function in the web page will be processed when the user presses the submit button. This pre-defined button style will normally submit the form. The developer can capture the submission and perform validation on the fields. Coding this for Navigator would look like the following:


<FORM NAME="Item"
        ACTION="some.cgi" onSubmit="return SendOrder()">
…
<!--- Function
// submit order
function SendOrder() {

        // validate data
        if (document.Item.Name.value == "") {
                alert("You must enter your name")
                return false
                }
        return true
        }

Capturing the onSubmit event will allow the developer to call a function to process the form fields. If they choose, they can perform validation in this function. If the validation fails, say the user did not provide a name, the function notifies the reader and returns false, preventing the form from being submitted. If the user did provide a name, the function would return true, and the form would be submitted.

Following the documentation that can be found at the Microsoft site, the developer would expect something like this to work for IE as well as Navigator. It does, to a point.

With IE the onSubmit event is captured and the SendOrder() function is called. If the user did not enter a name value, an alert would occur. The behavior is the same for both browsers at this point. However, if the user does provide a value, Navigator would then submit the form and a follow-up form would be displayed. This does NOT occur with IE IF you are testing the page locally, probably due to a bug missed during the testing. It does work if you run the web page documents through a web server.

However, without knowing that the difference between the two results was a matter of document location rather than document coding the web page developer could have spent considerable time trying to get the same behavior for both browsers.

Aside from the differences already noted, the browsers may process code in a funtionally identical manner and yet perform quite differently. This can be demonstrated with another popular use of JavaScript which is to open a secondary window for some purpose and to maintain communication between the two windows. This is widely used by Netscape for their tutorials.

Both browsers support a property for the window object called opener. This property can be used to contain a reference to the window that opened the secondary window.

The following code demonstrates using JavaScript to open a secondary window and to set the opener property to the original window:


<HTML>
<HEAD><TITLE> Original Window </TITLE>
<SCRIPT LANGUAGE="JavaScript">
<!--- hide script from old browsers
// OpenSecondary will open a second window
//
function OpenSecond(iType, sMessage) {
        // open window
        newWindow=window.open("second.htm","",
                "toolbar=no,directories=no,width=500,height=300")
        // modify the new windows opener value to
        // point to this window
        if (newWindow != null && newWindow.opener == null)
                newWindow.opener=window

        }
// end hiding from old browsers -->
</SCRIPT>
</HEAD>
<BODY>
<H1> Open a new Window and get information</H1>

<FORM NAME="CallWindow">
<p>Click to open Window:
<p>
<INPUT TYPE="button" NAME="OpenWin" VALUE="Click Here"
        onClick="OpenSecond()">
<p>
</FORM>
</BODY>
</HTML>

The above HTML and JavaScript creates a simple document with one button. Pressing the button opens a new window (creates a new instance of the browser) that does not have a toolbar or directories and is set to a certain width and height. The HTML source document that is opened into this new window is “second.htm”.

The HTML and JavaScript in the secondary window is shown below:


<HTML>
<HEAD><TITLE> Information</TITLE>

<SCRIPT LANGUAGE="JavaScript">
<!--- hide script from old browsers
// SetInformation
// function will get input values
// and set to calling window
function SetInformation() {
        opener.document.open()
        opener.document.write("<BODY>")
        opener.document.write("<H1> Return from Secondary</h1>")
        opener.document.write("</BODY>")
        opener.document.close()
        opener.document.bgColor="#ff0000"
        window.close()
        }
// end hiding from old browsers -->
</SCRIPT>
</HEAD>
<BODY>
<p><form>
Click on the button:<p>
<INPUT type="button" Value="Click on Me"
     onClick="SetInformation()">
</CENTER>
</FORM>
</BODY>
</HTML>

This code will display a button labeled “Click on Me”. Pressing this button will result in the document page for the original window being modified to display the words “Return from Secondary”. The JavaScript will also change the background color of the original document and then will close the secondary window. When run in Navigator, the sample behaves as expected. Not so, however, with IE.

First, when you open the secondary window in IE you will notice that the document in the original window seems to blank out for a moment. In addition, the secondary window opens at the top left hand side of the desktop with IE but opens directly over the original window with Navigator, no matter where that original window is.

When you press the button that modifies the original document, IE does modify the document with the new header, as Navigator does, and the background on the original document will change briefly. However, when the secondary window is closed the background color of the original document returns to the original color!

What is worse is that running this sample for a few times with IE will cause the browser to crash! How soon it will crash seems to suggest that resources are not being freed, or are being freed incorrectly. Whatever the cause, this behavior should make a developer feel cautious about opening up a secondary window within IE.

Solutions to working with both Internet Explorer and Navigator

JavaScript is a useful tool for creating interactive content without using Java or some other technique. As we have seen, problems arise out of the incompatibility between Navigator and IE. What are ways to avoid these problems?

  • Code for one browser only. This is not really a viable solution. While Netscape Navigator is the most popular browser in use today, Microsoft Internet Explorer is gaining quite a following. Using Navigator-only JavaScript that will limit your web page’s audience.
  • Code only to the least common denominator. By limiting the JavaScript to that which works for both Navigator and IE, developers will have access to most of the functionality they need. However, with this approach developers will need to be aware of the gotchas that have been discussed in this article. In other words, the developer has to test with both browsers; the tests will have to occur via the web server as well as run locally; the tests should be repeated several times to ensure that no adverse effects show up over time; the code should be tested with all possible environments.
  • Code for each browser within the same page. This is actually my favorite approach though it will require more effort by the developer. Both browsers will process JavaScript labeled as <SCRIPT LANGUAGE=”javascript”>, however only Navigator will process script labeled as <SCRIPT LANGUAGE=”javascript1.1″>. With this approach, the developer can place JavaScript common to both browsers in a SCRIPT tag labeled with “javascript” and Navigator specific code in a SCRIPT tag labeled with “javascript1.1”. To add in IE specific code, the developer would probably want to use VBScript code or use the tag JScript whenever applicable. At this time, JScript is implemented for scripting with events only, not as a language specification for the SCRIPT tag.

Summary

JavaScript is a very effective tool, and a relatively simple method for adding interactive content to a web page. With version 3.0 of Internet Explorer, using JavaScript is viable for both of the most popular browsers in use. However, developersneeds to be aware of the differences between Netscape Navigator and Microsoft Internet Explorer and should test thoroughly with both before posting the page to their site.

Using Dynamic HTML to create an animated menubar

Originally published in Netscape World, now archived at the Wayback Machine

Microsoft and Netscape decided to use two different techniques for Dynamic HTML. Microsoft bases its approach on exposing CSS1 (Cascading Style Sheets) attributes to script, with a little help from some interesting new built-in, lightweight, and windowless ActiveX controls. Netscape bases much of its approach on the new LAYER tag.

At first glance, the approaches seem incompatible — using one browser’s techniques will not work with the other. However, you can use both browsers’ techniques within the same page and generate the exact same result.

In this article, I create an animated menu bar using a combination of CSS1 style sheets, the PATH ActiveX control, and dynamically modifying HTML elements to work with IE 4.0. I create the exact same effect using CSS1 style sheets and the LAYER tag for Navigator 4.0. Note, though, that both IE 4.0 and Navigator 4.0 are works in progress, and this example may need to be modified, slightly, for future versions of either product.

Creating style sheetsThe first section of the example page is the CSS1 style sheet settings. Both IE 4.0 and Navigator 4.0 process these style sheets, though Navigator does not seem to like setting images using the IMG tag at this time.

This example image uses headers for the text-based portion of a menu bar. To create the effect I want, I don’t want the standard underline that occurs with hypertext links, and I do want to modify the font size and color. As I don’t want either of these changes to impact on the rest of the document, I use the cascading effect of CSS1 to apply these changes only to links contained within <H2> header tags:

h2 A { text-decoration: none; color: gold } 
h2 A:link { color: gold } 
h2 A:visited { color: gold }

Next, I create the style sheet definitions for the menu images, for use with IE 4.0 only. I use absolute positioning to place the images exactly where I want on the page, and use the visibility property to hide the images when the page is first opened:

#menu1 { position: absolute; left: 0; top: 0; 
	visibility: hidden}
#menu2 { position: absolute; left: 142; top: 0; 
	visibility: hidden}
#menu3 { position: absolute; left: 284; top: 0; 
	visibility: hidden}
#menu4 { position: absolute; left: 192; top: 0; 
	visibility: hidden}
#menu5 { position: absolute; left: 142; top: 192;
	visibility: hidden }
#menu6 { position: absolute; left: 284; top: 192; 
	visibility: hidden}

The final part of the CSS1 style sheet definition creates the text-based menu components of the menu bar. These show as soon as the page is opened, and if the Web page reader does not want to wait for the images, or has image downloading turned off, they can access the links via the text menu. The size of the area for the menu text item should be large enough to contain the complete text, otherwise odd behavior will result with Internet Explorer: It does not display the text correctly when you click on a text-based menu item. Another text-based style sheet rule is provided for a message that is displayed while the images are loading. Whenever you create content that may be invisible or not displayed for a time, provide some kind of feedback to the user about what is happening:

This is the final section of the CSS style sheet definitions.

Adding images to a Web pageThe next section in the example page is the scripting, but I will talk about that a bit later. For now, I will add the images to the Web page using two different techniques, one for IE 4.0, and one for Navigator 4.0. IE 4.0 uses the style sheet settings defined for the images, and references the appropriate definition by using the identifier of the style sheet rule. The Navigator technique is to embed the images within layers that are hidden when the page first loads. IE 4.0 ignores the LAYER tag, and Navigator ignores the image style sheet rules:

Note that a hypertext link reference is applied to the image, and the border is set to 0. As the image style sheet rules seem to be ignored by Navigator, important image information such as widthheight, and border are specified within the image tag.

Next, the text objects are added to the menu bar. This includes a text-based alternative that labels each image when it is displayed, or can be accessed directly if the image is not displayed. This also includes a message stating that the images for the menu bar are being loaded:

The “waiting…” text is placed within a LAYER tag as I need to add script to modify it later, and only layer-based elements can be modified with Navigator at this time.

The final objects to add to the page are six ActiveX PATH objects, to control the menu bar animations for IE 4.0. A path object can be used to specify both X and Y coordinates at specific tick marks and moves the visual object associated with the control along that path. Two of the parameters for the control are the X path coordinates and the Y path coordinates. Another parameter could also control the timer, but I want to synchronize my image movement, so I use a different script-based approach later:

Notice that the X, Y coordinates for each path object are different, but all of them have the same number of timing ticks, which is the first parameter given in each of the coordinate pairings. The tick timings given are in microseconds.

The PATH ActiveX controls are the last elements placed into the page. Next comes the fun part, animating the menu bar.

Adding the script to animate the menu barThe animation is started when the page loads by trapping the onLoad event for the document. This calls a function called cycle. This function is implemented in both VBScript and JavaScript, and the appropriate browser picks the appropriate implementation, with Microsoft grabbing the VBscript version, Navigator grabbing the JavaScript version.

The first script controls the menu bar for IE 4.0. This is written entirely in VBScript, though JavaScript could be used if browser differences are handled correctly. The cycle function sets the Target properties of the path objects to the style sheet rules of the menu bar images. This associates each path control with its visual object. Next, the images are set to be visible, and the “waiting..” text is hidden.

The Play method for each path control is called, beginning the animation process, though no movement occurs yet. Why? No movement occurs because no timing mechanism has been associated with each control. The timing is handled by using the setTimeout function and calling a second subroutine that handles the image animation. The complete text of the cycle subroutine is here:

I use my own timer to synchronize the images, a technique that Microsoft recommends when using more than one PATH control on the same Web page.

The MoveIEObjects subroutine calls the Tick method for each of the path controls, moving each of the objects to the next coordinate defined for the specific control. It issues another setTimeout function call to continue the animation process, after first testing to see if the last movement of the path has occurred (the last of the ticks, of which there are eight for this particular demonstration). Note that I did not necessarily have to test the tick movement because after the path has completely played, it no longer moves unless it is rewound. However, I want to make sure the timer is not still operating. The complete text of MoveIEObjects is given next:

Next, I add the JavaScript to control the animation for Navigator 4.0. The cycle function for Navigator is similar to that for IE 4.0, except that I need to create my own arrays of X and Y coordinates. These arrays, in turn, are set to hold other arrays holding the actual coordinates. After the arrays are created, the images are displayed and the timer is started, as seen next:

The final script is for the MoveObjects function for Navigator. The LAYER tag method moveTo is used to move each of the images to the new coordinate for this timer event. Note that the timer values are set to be much larger than the ones set for IE 4.0 as the PATH control does run a bit slower than using the LAYER tag approach, and I wanted both effects to be as similar as possible. MoveObjects can be seen next:

Note that resizing Navigator can misalign the images. Netscape recommends a blank document.write statement in JavaScript to compensate for this type of problem, but this does not work for this example. I will continue to investigate this and will post a fix to my Web site when one appears. In the meantime, if you resize the Web page you need to reload the example. No problem occurs when you resize IE.

That’s it for the scripting. You can test the example yourself if you are using either IE 4.0 or Netscape 4 (this script will not work on earlier browsers). View the complete source by using the appropriate “View Source” option of your browser.

 

Scenario: Internet Zoo

Page and source archived at Wayback Machine. Note, background images were cool in 1997. As for the comment, “The system needs little in the way of security and will reside on a machine totally isolated from any other system”, well, all I can say is those were the days.

The Scenario is a non-profit organization, a mythical city zoo. It represents an online web application that displays static HTML pages and provides for simple queries to a database. No online ordering is occurring, no updates are being made to a database, and no persistent information needs to be maintained between the pages.

This scenario assumes that the organization has not installed a web server. Based on this, this section, and the ones to follow, provide the steps needed to obtain a domain, set up communication protocols, prepare the computer and install and configure the web server.

The system needs little in the way of security and will reside on a machine totally isolated from any other system.

Note that the examples for this approach use Java classes that are not built into the browser, or the standard Sun class library. If you do not have these classes installed on your local machine, they will be downloaded from this server. And this server is not the speediest in the world. What I am trying to say is “Yes. Do take up knitting.”.

Source Code

The source code for the examples in this scenario can be downloaded from this page. Note, though. that the code is not supported by the author, and the author is not responsible for problems that might arise from the download. Now, on that cheerful note (and I am sure you all are breathless with anticipation about the download), grab your source: