Presentation Aspects

Browser-side control

• JavaScript
• Stylesheets
• Document Object Model
• Applets

JavaScript

• Originally proprietary to Netscape
• Incompatable version called JScript developed by MicroSoft
• Standardised as EcmaScript by the European ECMA organisation
• JavaScript is still browser dependant
• First called LiveScript, JavaScript has nothing to do with Java

JavaScript

• It is a client-side scripting language, that can respond to various user events
• It can be used for client-side validation before submitting a Form
• It cannot replace server-side validation

JavaScript Example

function validate(obj) {
   value = obj.value;
   for (n = 0; n < value.length; n++)
      if ( ! isDigit(value.charAt(n))) {
         alert("Field must be numeric");
         return;
      }
   confirm("Field is valid");
}

function isDigit(ch) {
   if (ch >= '0' && ch <= '9')
      return true;
   return false;
}

<form>

Enter number:
<input type="text" onChange="validate(this)">

</form>
                             

StyleSheets

• Can be used to eliminate text formatting markup from HTMl documents
• The major stylesheet mechanism is Cascading Style Sheets
• There are two versions of CSS: CCS 1 and CSS 2
• The major browsers still only support CSS 1
• The support is incomplete and buggy
• Stylesheets are a great idea, but the vendors are letting the developers down (again)
• Example

  <STYLE TYPE="text/css"
  <!--
    BODY {background: yellow:}
    P EM {background: green}
  -->
  </STYLE>
  

DOM

• Document Object Model treats every element of an HTML page as an object, arranged hierarchically
• Scripting languages can manipulate a page using this model
• Can be used to hide text, display new text, reformat text, etc
• Only supported properly by Internet Explorer

JavaScript

• Partly responsible for the early success of Java
• Allows a Java program to be downloaded to a browser and run in the browser
• Security restrictions do not allow applets to read/write to the disk, open arbitrary network connections, etc
• Security restrictions make applets close to useless
• Applet appearance is like an application, not like a Web page
• Java 1.2 security model can be used to overcome security restrictions in a controlled manner

  
  keystore "file:/home/jan/.keystore", "JKS";
  
  // Permissions for downloaded classes
  grant signedBy "Jan" {
      permission java.net.SocketPermission
                    "137.92.11.117:1024-", "connect,accept,
  resolve";
  };
  
  // Permissions for the Jini classes
  grant codeBase "file:/home/jan/tmpdir/jini1_1/lib/-"
                 signedBy "Jini" {
  
      // multicast request address
      permission java.net.SocketPermission
                          "224.0.1.85", "connect,accept";
      // multicast announcement address
      permission java.net.SocketPermission
                          "224.0.1.84", "connect,accept";
  };
        

• Vendor support for reliable Java 1.2 is still missing