What is event bubbling and capturing?

答案1

Event bubbling and capturing are two ways of event propagation in the HTML DOM API, when an event occurs in an element inside another element, and both elements have registered a handle for that event. The event propagation mode determines in which order the elements receive the event.

With bubbling, the event is first captured and handled by the innermost element and then propagated to outer elements.

With capturing, the event is first captured by the outermost element and propagated to the inner elements.

Capturing is also called "trickling", which helps remember the propagation order:

trickle down, bubble up 向下滴水，向上冒泡

Back in the old days, Netscape advocated event capturing, while Microsoft promoted event bubbling. Both are part of the W3C Document Object Model Events standard (2000).

IE < 9 uses only event bubbling, whereas IE9+ and all major browsers support both. On the other hand, the performance of event bubbling may be slightly lower for complex DOMs.

We can use the addEventListener(type, listener, useCapture) to register event handlers for in either bubbling (default) or capturing mode. To use the capturing model pass the third argument as true.

Example

<div>
    <ul>
        <li></li>
    </ul>
</div>

In the structure above, assume that a click event occurred in the li element.

In capturing model, the event will be handled by the div first (click event handlers in the div will fire first), then in the ul, then at the last in the target element, li.

In the bubbling model, the opposite will happen: the event will be first handled by the li, then by the ul, and at last by the div element.

For more information, see

Event Order on QuirksMode
addEventListener on MDN
Events Advanced on QuirksMode

In the example below, if you click on any of the highlighted elements, you can see that the capturing phase of the event propagation flow occurs first, followed by the bubbling phase.

var logElement = document.getElementById('log');
function log(msg) {
    logElement.innerHTML += ('<p>' + msg + '</p>');
}
function capture() {
    log('capture: ' + this.firstChild.nodeValue.trim());
}
function bubble() {
    log('bubble: ' + this.firstChild.nodeValue.trim());
}
var divs = document.getElementsByTagName('div');
for (var i = 0; i < divs.length; i++) {
    divs[i].addEventListener('click', capture, true);
    divs[i].addEventListener('click', bubble, false);
}

p {
    line-height: 0;
}
div {
    display:inline-block;
    padding: 5px;
    background: #fff;
    border: 1px solid #aaa;
    cursor: pointer;
}
div:hover {
    border: 1px solid #faa;
    background: #fdd;
}

<div>1
    <div>2
        <div>3
            <div>4
                <div>5</div>
            </div>
        </div>
    </div>
</div>
<section id="log"></section>

Expand snippet

Another example at JSFiddle.

Event Flow

3. DOM Event Architecture

This section is non-normative. Refer to [DOM] for a normative description of the DOM event architecture

3.1. Event dispatch and DOM event flow

This section gives a brief overview of the event dispatch mechanism and describes how events propagate through the DOM tree.

Applications can dispatch event objects using the dispatchEvent() method, and the event object will propagate through the DOM tree as determined by the DOM event flow.

Event objects are dispatched to an event target. But before dispatch can begin, the event object’s propagation path must first be determined.

The propagation path is an ordered list of current event targets through which the event passes. This propagation path reflects the hierarchical tree structure of the document. The last item in the list is the event target, and the preceding items in the list are referred to as the target’s ancestors, with the immediately preceding item as the target’s parent.

Once the propagation path has been determined, the event object passes through one or more event phases. There are three event phases: capture phase, target phase and bubble phase. Event objects complete these phases as described below. A phase will be skipped if it is not supported, or if the event object’s propagation has been stopped. For example, if the bubbles attribute is set to false, the bubble phase will be skipped, and if stopPropagation() has been called prior to the dispatch, all phases will be skipped.

The capture phase: The event object propagates through the target’s ancestors from the Window to the target’s parent. This phase is also known as the capturing phase.
The target phase: The event object arrives at the event object’s event target. This phase is also known as the at-target phase. If the event type indicates that the event doesn’t bubble, then the event object will halt after completion of this phase.
The bubble phase: The event object propagates through the target’s ancestors in reverse order, starting with the target’s parent and ending with the Window. This phase is also known as the bubbling phase.

扩展阅读

Bubbling and capturing