How JavaScript Event Delegation Works

转自： http://davidwalsh.name/event-delegate

One of the hot methodologies in the JavaScript world is event delegation, and for good reason.  Event delegation allows you to avoid adding event listeners to specific nodes;  instead, the event listener is added to one parent.  That event listener analyzes bubbled events to find a match on child elements.  The base concept is fairly simple but many people don't understand just how event delegation works.  Let me explain the how event delegation works and provide pure JavaScript example of basic event delegation.

Let's say that we have a parent UL element with several child elements:

<ul id="parent-list">
  <li id="post-1">Item 1</li>
  <li id="post-2">Item 2</li>
  <li id="post-3">Item 3</li>
  <li id="post-4">Item 4</li>
  <li id="post-5">Item 5</li>
  <li id="post-6">Item 6</li>
</ul>

Let's also say that something needs to happen when each child element is clicked.  You could add a separate event listener to each individual LI element, but what if LI elements are frequently added and removed from the list?  Adding and removing event listeners would be a nightmare, especially if addition and removal code is in different places within your app.  The better solution is to add an event listener to the parent UL element.  But if you add the event listener to the parent, how will you know which element was clicked?

Simple:  when the event bubbles up to the UL element, you check the event object's target property to gain a reference to the actual clicked node.  Here's a very basic JavaScript snippet which illustrates event delegation:

// Get the element, add a click listener...
document.getElementById("parent-list").addEventListener("click",function(e) {
  // e.target is the clicked element!
  // If it was a list item
  if(e.target && e.target.nodeName == "LI") {
    // List item found!  Output the ID!
    console.log("List item ",e.target.id.replace("post-")," was clicked!");
  }
});

Start by adding a click event listener to the parent element.  When the event listener is triggered, check the event element to ensure it's the type of element to react to.  If it is an LI element, boom:  we have what we need!  If it's not an element that we want, the event can be ignored.  This example is pretty simple -- UL and LI is a straight-forward comparison.  Let's try something more difficult.  Let's have a parent DIV with many children but all we care about is an A tag with the "classA" CSS class:

// Get the parent DIV, add click listener...
document.getElementById("myDiv").addEventListener("click",function(e) {
  // e.target was the clicked element
  if(e.target && e.target.nodeName == "A") {
    // Get the CSS classes
    var classes = e.target.className.split(" ");
    // Search for the CSS class!
    if(classes) {
      // For every CSS class the element has...
      for(var x = 0; x < classes.length; x++) {
        // If it has the CSS class we want...
        if(classes[x] == "classA") {
          // Bingo!
          console.log("Anchor element clicked!");
          
          
          // Now do something here....
          
          
          
        }
      }
    }
    
  }
});

The example above requires not only a tag match but also a CSS class match.  While this is a bit more complex, it's still fairly simple in the grand scheme of things.  For example, if the A element had a SPAN tag in it, the SPAN tag would be the target element.  In that case, we'd need to walk up the DOM tree to find out if it contained an A.classA element we were looking for.

Since most developers use a JavaScript library for their DOM element and event handling, I recommend using the library's method of event delegation, as they capable of advanced delegation and element identification.

Hopefully this helps you visually the concept behind event delegation and convinces you of delegation's power!