ID Space

From Documentation

ID Space

It is common to decompose a visual presentation into several subsets or ZUML pages. For example, you may use a page to display a purchase order, and a modal dialog to enter the payment term. If all components are uniquely identifiable in the same desktop, developers have to maintain the uniqueness of all identifiers for all pages that might be created in the same desktop. This step can be tedious, if not impossible, for a sophisticated application.

The concept of ID space is hence introduced to resolve this issue. An ID space is a subset of components of a desktop. The uniqueness is guaranteed only in the scope of an ID space. Thus, developers could maintain the subset of components separately without the need to worry if there are any conflicts with other subsets.

Window (Window) is a typical component that is an ID space. All descendant components of a window (including the window itself) form an independent ID space. Thus, you could use a window as the topmost component to group components. This way developers only need to maintain the uniqueness of each subset separately.

By and large, every component can form an ID space as long as it implements IdSpace. This type of component is called the space owner of the ID space after the component is formed. Components in the same ID space are called "fellows".

When a page implements IdSpace, it becomes a space owner. In additions, the macro component and the include component (Include) can also be space owners.

Another example is idspace (Idspace). It derives from div, and is the simplest component implementing IdSpace. If you don't need any feature of window, you could use idspace instead.

You could make a standard component as a space owner by extending it to implement IdSpace. For example,

public class IdGrid extends Grid implements IdSpace {
   //no method implementation required

Tree of ID Space

If an ID space has a child ID space, the components of the child space are not part of the parent ID space. But the space owner of the child ID space will be an exception in this case. For example, if an ID space, let's say X, is a descendant of another ID space, let's say Y, then space X's owner is part of space Y. However, the descendants of X is not a part of space Y.

For example, see the following ZUML page

<?page id="P"?>
	<window id="A">
		<hbox id="B">
			<button id="D" />
		<window id="C">
			<button id="E" />
	<hbox id="F">
		<button id="G" />

will form ID spaces as follows:

Zk the id space.jpg

As depicted in the figure, there are three spaces: P, A and C. Space P includes P, A, F and G. Space A includes A, B, C and D. Space C includes C and E.

Components in the same ID spaces are called fellows. For example, A, B, C and D are fellows of the same ID space.

getFellow and getSpaceOwner

The owner of an ID space could be retrieved by Component.getSpaceOwner() and any components in an ID space could be retrieved by Component.getFellow(String), if it is assigned with an ID (Component.setId(String)).

Notice that the getFellow method can be invoked against any components in the same ID space, not just the space owner. Similarly, the getSpaceOwner method returns the same object for any components in the same ID space, no matter if it is the space owner or not. In the example above, if C calls getSpaceOwner it will get C itself, if C calls getSpaceOwnerOfParent it will get A.

Composer and Fellow Auto-wiring

With ZK Developer's Reference/MVC, you generally don't need to look up fellows manually. Rather, they could be wired automatically by using the auto-wiring feature of a composer. For example,

public class MyComposer extends SelectorComposer {
    private Textbox input; //will be wired automatically if there is a fellow named input

    public void onOK() {"You entered " + input.getValue());
    public void onCancel() {

Then, you could associate this composer to a component by specifying the apply attribute as shown below.

<window apply="MyComposer">
    <textbox id="input"/>

Once the ZUML document above is rendered, an instance of MyComposer will be instantiated, and the input member will also be initialized with the fellow named input. This process is called "auto-wiring". For more information, please refer to the Wire Components section.

Find Component Manually

There are basically two approaches to look for a component: by use of CSS-like selector and file system-like path. The CSS-like selector is more powerful and suggested if you're familiar with CSS selectors, while a filesystem-like path is recommended if you're familiar with the filesystem's path.


Component.query(String) and Component.queryAll(String) are the methods to look for a component by use of CSS selectors. For example,

comp.query("#ok"); //look for a component whose ID is ok in the same ID space
comp.query("window #ok"); //look for a window and then look for a component with ID=ok in the window
comp.queryAll("window button"); //look for a window and then look for all buttons in the window

Component.query(String) returns the first matched component, or null if not found. On the other hand, Component.queryAll(String) returns a list of all matched components.


ZK provides a utility class called Path to simplify the location of a component among ID spaces. The way of using it is similar to For example,

The formal syntax of the paths

  • The last element is a fellow (component) ID.
  • page ID should start with double slash //

For example:

//Two different ways to get the same component E
Path.getComponent("/A/C/E");//if call Path.getComponent under the same page.
new Path("/A/C", "E").getComponent(); //the same as new Path("/A/C/E").getComponent()

// B and D are fellows in the Id space of A
Path.getComponent("/A/B");  // get B
Path.getComponent("/A/D");  // get D

Different Page

If a component belongs to another page, we can retrieve it by starting with the page's ID. Notice that double slashes have to be specified in front of the page's ID.

Path.getComponent("//P/A/C/E");//for page, you have to use // as prefix

Notice that the page's ID can be assigned with the use of the page directive as follows.

<?page id="foo"?>


A component has another identifier called UUID (Universal Unique ID). It is assigned automatically when the component is attached to a page. UUID of a component is unique in the whole desktop (if it is attached).

Application developers rarely need to access it.

In general, UUID is independent of ID. UUID is assigned automatically by ZK, while ID is assigned by the application. However, if a component implements RawId, ID will become UUID if the application assigns one. Currently, only components from the XHTML component set implements RawId.

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Last Update : 2024/01/22

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