XML Background

From Documentation


This section provides the most basic concepts of XML to work with ZK. If you are familiar with XML, you could skip this section. If you want to learn more, there are a lot of resources on Internet, such as http://www.w3schools.com/xml/xml_whatis.asp and http://www.xml.com/pub/a/98/10/guide0.html.

XML is a markup language much like HTML but with stricter and cleaner syntax. It has several characteristics worthwhile to take notes of.


The whole XML content, no matter whether if it is in a file or as a string, is called a XML document.

Character Encoding

It is, though optional, a good idea to specify the encoding in your XML so that the XML parser can interpret it correctly. Note: it must be on the first line of the XML document.

 <?xml version="1.0" encoding="UTF-8"?>

In addition to specifying the correct encoding, you have to make sure your XML editor supports it as well.


An XML element is everything from (including) the element's start tag to (including) the element's end tag.

An element can contain other elements, let it be simple text or a mixture of both. Elements can also have attributes. For example,

<window title="abc">
  <button label="click me"/>

where both window and button are elements, while title is an attribute of the window element. The button element is nested in the window element. We call the window component the parent element of button, while the button component is a child element of the window.

The document root is the topmost element (without any parent element). There is exactly one document root per XML document.

Elements Must Be Well-formed

First, each element must be closed. They are two ways to close an element as depicted below. They are equivalent.

Close by an end tag:
Close without an end tag:

Second, elements must be properly nested.

		Hello World!
		Hello World!

XML treats every tag as a node in a tree. A node without a parent node is a root component, and it is the root of a tree. In each zul file, only ONE tree is allowed.

For example, for being a whole zul file, the following is allowed, for it must have only one root component.


And for being a whole zul file, the following is not allowed, for it must have more than one root component.


You can solve the problem simply by adding a tag to enclose the whole zul file to serve as the parent node, so that the zul file has one single tree again.

	<button />
	<button />

Special Character Must Be Replaced

XML uses <element-name> to denote an element, so you have to use special characters for replacement. For example, you have to use &lt; to represent the < character.

Special Character
Replaced With
\t (TAB)
Required only if use it in a XML attribute's value
\n (Linefeed)
Required only if use it in a XML attribute's value

Alternatively, you could tell XML parser not to interpret a piece of text by using CDATA. See the following:

 void myfunc(int a, int b) {
     if (a < 0 && b > 0) {
         //do something

It is suggested to always add <![CDATA[ ]]> inside your <zscript> </zscript>. Thus you don't have to worry about the escape sequences for special characters like "&", "<". In addition, the code can also become much easier to read and for maintenance.

Attribute Values Must Be Specified and Quoted


Both the single quote (') and the double quote (") can be used, so if the value has double quotes, you could use the single quote to enclose it. For example,

<button onClick='alert("Hello, There")'/>

Of course, you can always use &quot; to denote a double quote.


A comment is used to leave a note or to temporarily disable a block of XML code. To add a comment in XML, use <!-- and --> to mark the comment body.

 <!-- this is a comment and ignored by ZK -->

Processing Instruction

A processing instruction is used to carry out the instruction to the program that processes the XML document. A processing instruction is enclosed with <? and ?>. For example,

<?page title="Foo"?>

Processing instructions may occur anywhere in an XML document. However, most ZUML processing instructions must be specified at the topmost level (the same level of the document root).

Last Update : 2022/01/12

Copyright © Potix Corporation. This article is licensed under GNU Free Documentation License.