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Widget and component works hand in hand to deliver UI to user and notify the application about a user's activity

There are two kind of UI objects: components and widget. A component is a Java object running at the server, representing an UI object that an application can manipulate. A component has all the behavior that an UI object should have, except it doesn't have the visual part. For example, the following code snippet creates a window and a textbox.

Window w = new Window();
w.appendChild(new Textbox());
w.setPage(page); //assuming page is the current page

On the other hand, a widget is a JavaScript object running at the client, representing an UI object to interact with the user. To interact with the user, a widget has a visual appearance and handle events happening at the client.

Flow of changing a label

A component and a widget work hand-in-hand to deliver UI to an user and to notify the application about a user's activity, such as clicking and dragging. For example, when an application invokes Button.setLabel(String) to change the label of a button component, the Button.setLabel(String) of corresponding button widget (aka., peer widget) will be invoked at the client to change the visual appearance, as shown right. When the user clicks the button widget, the onClick event will be sent back to the server and notify the application.

Though not required, application can run at the client to control widgets directly

Thought not required, a widget is usually implemented with most functions of a component. That means developers can control them directly at the client, as shown left. It improves the responsiveness and reduces the network traffics. Of course, it also increases the development cost. For example, an application might hide or change the order of columns of a grid at the client, while the application running at the server handle the reloading of the whole content of the grid.

Widget running without component

Furthermore, a developer can create a widget at the client, and the widget will not have any peer component at the server as shown right. For example,

var w = new zk.wgt.Button();
wnd.appendChild(w); //assume wnd is another widget

Component and Page

The peer widget of a component is created automatically, when it is attached to a page. On the other hand, if a component is not attached, the client won't know its existence.

Server Client Description
Window w = new Window();
w.setTitle("Hello Window");

A Window component is instantiated but it doesn't have the peer widget. Furthermore, it will be garbage-collected if there is no reference to it


Auto invoked by ZK Client Engine

var pw = new zul.wnd.Window(uuid);
pw.setTitle('Hello World');

Attach the component to the specified page, and a peer widget will be created automatically at the client later (after processing the AU Requests).

w.setTitle("Hi ZK");

Auto invoked by ZK Client Engine

pw.setTitle('Hi ZK');

Once a component is attached to a page, any following modification will be sent to the client and invoke the corresponding method of the peer widget.

  • Notes:
    • There are two ways to attach a component to page. First, call the setPage method to make it as a root component of the page. Second, make a component as a child of another component that is attached to a page. For example, with w.appendChild(new Label("Hi"));, the label will be attached to a page if w is attached.
    • The AU request is a HTTP request, so it is a request-process-response protocol. That is, all client invocation (auto create a widget and so on) will take place after the processing is done and the response is sent back to client.

Widget and DOM

DOM, Widget and Component

A widget is an UI object at the client. Like Swing's component, creating a widget doesn't make it visible to the user. Rather, you have to attach it to the DOM tree (of the browser).

Client Widget Client DOM Description
var wp = new zul.wnd.Window();
wp.setTitle('Hello World');

Invoked by ZK Client Engine or client application


A window widget is instantiated. If it is called by ZK Client Engine (due to the invocation at the server), it has a peer component. If it is called by client application, there is no peer component.


Create one or a tree of DOM elements (depending on the implementation of a widget) and replace the specified node.

Attach a widget to the DOM tree, and the visual appearance is shown up (unless it is invisible).

wp.setTitle('Hi ZK');
Update the DOM element(s) created in the previous step A modification of the widget will modify the corresponding DOM elements

Attach a widget to the DOM tree

There are several ways to attach a widget to the DOM tree

  1. Invoke Widget.replaceHTML(Object, Desktop, Skipper) to replace an existent DOM element with the DOM element(s) of the widget (aka., the DOM content).
  2. Invoke Widget.appendChild(Widget) or Widget.insertBefore(Widget, Widget) to make a widget a child of another widget that are already attached to the DOM tree.

Version History

Last Update : 2011/8/12

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