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

This way overrides a particular widget. So, it only overrides the widget’s methods you specified and does not affect other widgets on the same page. For example, suppose you want to change a label’s CSS style when its value is changed, then you can write the code as follows:

<zk 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>
</zk>

where

  • We specify client namespace to the setValue attribute to indicate 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 in the content of the attribute, such as

<label w:setValue='function (value) { this.$setValue(value + "${whatever}")}' />

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

Override a Widget in Java

In addition to ZUML, you can override a Widget’s method or field by the use of org.zkoss.zk.ui.Component#setWidgetOverride(java.lang.String, java.lang.String) at the server. For example,

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

Override a Default Widget Method in zul

In the previous section, we showed you how to override the method of a particular widget. If you want to modify the behavior of all instances of a widget class, you have to override the method in prototype (For more information about JavaScript’s prototype, please refer to Using Prototype Property in JavaScript and JavaScript prototype Property).

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() of all instances are overriden.

Override a Default Widget Method in JavaScript File

It’s easy to include an overridden js in multiple zul files. Here is a sample:

zk.afterLoad('zul.inp', function() { //specify zk widget package name
    var exWidget = {};
    zk.override(zul.inp.ComboWidget.prototype, exWidget, { //specify zk full widget name
        doClick_: function(e){
            exWidget.doClick_.apply(this, arguments); //call the original widget's overridden function
            //implement your custom logic
        },
    });

});
  • Line 1: this line will run zk.override() after the zul.inp widgets are loaded, so you don’t need to take care this overridden script’s loading order. You can include such JavaScript in any place of a zul.

Override a Widget Field

You can override a method or a field no matter it exists or not. For example, you can use this feature to pass an application-specific data to a widget, such as

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

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().

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

Load JavaScript Files for Overriding Widgets

If you put your overridden js code in a separate file, you need to include it to take effect on widgets.

Page Scope

If you just want to override widgets on some pages, you can use one of the following:

  • [[/zk_component_ref/script|

    component]]

  • HTML `

    `

  • [[ZUML Reference/ZUML/Processing Instructions/script| <?script ?>

    directive]]

Application Scope

To override all widgets in the whole application, you need to include the js file in every page with:

The Client-Attribute Namespace

You can specify additional DOM attributes that are not generated by ZK widgets with client-attribute namespace (http://www.zkoss.org/2005/zk/client/attribute. shortcut, client/attribute). 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. Fore more information, please refer to ZK Component Reference: iframe.

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

<zk xmlns:ca="client/attribute">
    HTML 5 spell check enabled:
    <textbox ca:spellcheck="true"/>
</zk>

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.