第九章
事件和事件句柄

表 9.1列出了本章的一个对象。

表 9.1 与事件相关的对象

表 9.2总览 JavaScript 事件句柄。

表 9.2 事件及对应的事件句柄。

General Information about Events

JavaScript applications in the browser are largely event-driven. Events are actions that occur usually as a result of something the user does. For example, clicking a button is an event, as is changing a text field or moving the mouse over a link. For your script to react to an event, you define event handlers, such as onChangeandonClick.

定义事件句柄

If an event applies to an HTML tag, then you can define an事件适用对象it. The name of an event handler is the name of the event, preceded by "on". For example, the事件适用对象the focus event is onFocus.

To create an事件适用对象an HTML tag, add an event handler attribute to the tag. Put JavaScript code in quotation marks as the attribute value. The general语法 is

<TAG eventHandler="JavaScript Code"> where TAG is an HTML tag and eventHandler is the name of the event handler. For example, suppose you have created a JavaScript function called compute. You can cause the browser to perform this function when the user clicks a button by assigning the function call to the button's onClick event handler:

<INPUT TYPE="button" VALUE="Calculate" onClick="compute(this.form)"> You can put any JavaScript statements inside the quotation marks following onClick. These statements are executed when the user clicks the button. If you want to include more than one statement, separate statements with a semicolon.

When you create an event handler, the corresponding JavaScript object gets a property with the name of the event handler in lower case letters. (In Navigator 4.0, you can also use the mixed case name of the事件适用对象the property name.) This property allows you to access the object's event handler. For example, in the preceding example, JavaScript creates a

JavaScript Guide

In Navigator 4.0, JavaScript includes eventobject is sent as an argument to the event handler.

Typically, the object on which the event occurs handles the event. For example, when the user clicks a button, it is often the button's event handler that handles the event. Sometimes you may want the documentobject to handle all MouseDown events no matter where they occur in the document. JavaScript's event capturing model allows you to define methods that capture and handle events before they reach their intended target.

In addition to providing the Layerobjects have these new methods:

• captureEvents
• releaseEvents
• routeEvent
• handleEvent (Not a method of the Layerobject)

For example, suppose you want to capture all click events that occur in a window. First, you need to set up the window to capture click events:

window.captureEvents(Event.CLICK); The argument to eventobject and indicates the type of event to capture. To capture multiple events, the argument is a list separated by vertical slashes (|). For example:

window.captureEvents(Event.CLICK | Event.MOUSEDOWN | Event.MOUSEUP) Next, you need to define a function that handles the event. The argument evnt is the eventobject for the event.

function clickHandler(evnt) {
   //What goes here depends on how you want to handle the event.
   //This is described below.
}

You have four options for handling the event:

• Return true. In the case of a link, the link is followed and no other event handler is checked. If the event cannot be canceled, this ends the event handling for that event.
function clickHandler(evnt) { return true; }
• Return false. In the case of a link, the link is not followed. If the event is non-cancelable, this ends the event handling for that event.
function clickHandler(evnt) { return false; }
• Call routeEvent. JavaScript looks for other event handlers for the event. If another object is attempting to capture the event (such as the document), JavaScript calls its event handler. If no other object is attempting to capture the event, JavaScript looks for an事件适用对象the event's original target (such as a button). The routeEvent method returns the value returned by the event handler. The capturing object can look at this return value and decide how to proceed.
function clickHandler(evnt) {
var retval = routeEvent(evnt);
if (retval == false) return false;
else return true;
}

Note: When routeEvent calls an event handler, the event handler is activated. If routeEvent calls an event handler whose function is to display a new page, the action takes place without returning to the capturing object.

• Call the handleEvent method of an event receiver. Any object that can register event handlers is an event receiver. This method explicitly calls the event handler of the event receiver and bypasses the capturing hierarchy. For example, if you wanted all click events to go to the first link on the page, you could use:
function clickHandler(evnt) {
window.document.links[0].handleEvent(evnt);
}

As long as the link has an onClick handler, the link handles any click event it receives.

In the following example, the window and document capture and release events:

<HTML>
<SCRIPT>

function fun1(evnt) {
   alert ("The window got an event of type: " + evnt.type +
      " and will call routeEvent.");
   window.routeEvent(evnt);
   alert ("The window returned from routeEvent.");
   return true;
}

function fun2(evnt) {
   alert ("The document got an event of type: " + evnt.type);
   return false;
}

function setWindowCapture() {
   window.captureEvents(Event.CLICK);
}

function releaseWindowCapture() {
   window.releaseEvents(Event.CLICK);
}

function setDocCapture() {
   document.captureEvents(Event.CLICK);
}

function releaseDocCapture() {
   document.releaseEvents(Event.CLICK);

}

window.onclick=fun1;
document.onclick=fun2;

</SCRIPT>
...
</HTML>