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Override Widget's Default Behavior

There are many ways to override the default behavior of widgets or even ZK Client Engine. JavaScript is a dynamic language and you could override almost any methods you want.

Override a Widget Method

For example, suppose we want to change the CSS style of a label when its value is changed, then we might have the code as follows.

<window xmlns:w="http://www.zkoss.org/2005/zk/client">
      <label>
            <attribute w:name="setValue">
            function (value) {
                  this.$setValue(value); //call the original method
                  if (this.desktop) {
                        this._flag = !this._flag;
                        this.setStyle('background:'+(this._flag ? 'red':'green'));
                  }
            }
            </attribute>
      </label>
</window>

where

  • We specify client namespace to the setValue attribute to indicate it is the method to override
  • The content of the attribute is a complete function definition of the method, including function ()
  • You can access the widget by this in the function
  • You can access the original method by this.$xxx, where xxx is the method name being overridden. If the method doesn't exist, it is null.
  • To retrieve another widget, use this.$f('anotherWidgetId') or other methods as described in the previous section
  • You can specify EL expressions[1] in the content of the attribute, such as
w:setValue='function (value) { this.$setValue(value + "${whatever}")}';

Notice that EL expressions are evaluated at the server before sending back to the client. Thus, you could use any Java class or variables in EL expressions.


  1. EL expressions are allowed since ZK 5.0.2

Override a Default Widget Method

In previous section, we showed how to override the method of a particular widget we declared. This, however, only affects the behavior of a particular instance. If you want to modify the behavior of all instances of a widget class, you have to override the method in prototype[1].

For example,

<window xmlns:w="http://www.zkoss.org/2005/zk/client">
	<label id="labelone" value="label one"/>
	<label id="labeltwo" value="label two"/>
	<script defer="true">
		var oldSV = zul.wgt.Label.prototype.setValue;
		zul.wgt.Label.prototype.setValue = function (){
			arguments[0]="modified prototype"+arguments[0];
			oldSV.apply(this, arguments);
		}					
	</script>
	<button label="change" onClick="labelone.setValue((new Date()).toString());
    labeltwo.setValue((new Date()).toString());"/>
</window>

where we assign a new method to zul.wgt.Label.prototype.setValue. Since it is prototype, the setValue method of all instances are modified.


  1. For more information about JavaScript's prototype, please refer to Using Prototype Property in JavaScript and JavaScript prototype Property

Override a Widget Field

You can override a method or a field no matter it exists or not. For example, it is easy to pass the application-specific data to the client, such as

<label value="hello" w:myval="'${param.foo}'"/>

Notice that the content of the attribute must be a valid JavaScript snippet. To specify a string (as shown above), you have to enclose it with ' or " if you want to pass a string. It also means you can pass anything, such as new Date().

Override a Widget Method in Java

In addition to ZUML, you could override a Widget method or field by the use of Component.setWidgetOverride(String, String) at the server. For example,

label.setWidgetOverride("setValue",
    "function (value) {this.$setValue('overloaded setValue');}");

Specify Your Own Widget Class

You could specify your own implementation instead of the default widget class (at the client) as follows.

<zk xmlns:w="http://www.zkoss.org/2005/zk/client">
  ...
  <button w:use="foo.MyButton"/>
</zk>

where foo.MyButton is a widget you implement. For example,

zk.$package("foo");
zk.afterLoad("zul.wgt", function () {
  foo.MyButton = zk.$extends(zul.wgt.Button, {
    setLabel: function (label) {
      this.$supers("setLabel", arguments);
     //do whatever you want
    }
  });
});

Notice that zk.afterLoad(String, Function) is used to defer the declaration of foo.MyButton until zul.wgt has been loaded.

Load Additional JavaScript Files

You could use Script, HTML SCRIPT tag or script to load additional JavaScript files. Please refer to script for more information.

The Client-Attribute Namespace

[since 5.0.3]

The client-attribute namespace (http://www.zkoss.org/2005/zk/client/attribute; shortcut, client/attribute) is used to specify additional DOM attributes that are not generated by widgets. In other words, whatever attributes you specify with the client-attribute namespace will be generated directly to the browser's DOM tree. Whether it is meaningful, it is really up to the browser -- ZK does not handle or filter it at all.

For example, you want to listen to the onload event, and then you can do as follows[1].

<iframe src="http://www.google.com" width="100%" height="300px"
  xmlns:ca="client/attribute" ca:onload="do_whater_you_want()"/>

If the attribute contains colon or other special characters, you can use the attribute element as follows:

<div xmlns:ca="client/attribute">
  <attribute ca:name="ns:whatever">
  whatever_value_you_want
  </attribute>
</div>

The other use of the client-attribute namespace is to specify attributes that are available only to certain browsers, such as accessibility and Section 508.


  1. Fore more information, please refer to ZK Component Reference: iframe

Version History

Last Update : 2017/10/31


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