JavaScript 惰性初始模式

惰性初始模式

延迟初始化 是一种允许我们延迟初始化消耗资源比较大的进程，直到需要他们的时候（才初始化）。这其中的一个例子就是jQuery的.ready()方法，它在DOM节点加载完毕之后会执行一个回调方法。

$( document ).ready( function () {

    //ajax请求不会执行，直到DOM加载完成

    var jqxhr = $.ajax({
      url: "http://domain.com/api/",
      data: "display=latest&order=ascending"
    })
    .done( function( data ) ){
        $(".status").html( "content loaded" );
        console.log( "Data output:" + data );
    });

});

jQuery.fn.ready()底层是通过byjQuery.bindReady()来实现的, 如下所示:

bindReady: function() {
  if ( readyList ) {
    return;
  }

  readyList = jQuery.Callbacks( "once memory" );

  // Catch cases where $(document).ready() is called after the
  // browser event has already occurred.
  if ( document.readyState === "complete" ) {
    // Handle it asynchronously to allow scripts the opportunity to delay ready
    return setTimeout( jQuery.ready, 1 );
  }

  // Mozilla, Opera and webkit support this event
  if ( document.addEventListener ) {
    // Use the handy event callback
    document.addEventListener( "DOMContentLoaded", DOMContentLoaded, false );

    // A fallback to window.onload, that will always work
    window.addEventListener( "load", jQuery.ready, false );

  // If IE event model is used
  } else if ( document.attachEvent ) {
    // ensure firing before onload,
    // maybe late but safe also for iframes
    document.attachEvent( "onreadystatechange", DOMContentLoaded );

    // A fallback to window.onload, that will always work
    window.attachEvent( "onload", jQuery.ready );

    // If IE and not a frame
    // continually check to see if the document is ready
    var toplevel = false;

    try {
      toplevel = window.frameElement == null;
    } catch(e) {}

    if ( document.documentElement.doScroll && toplevel ) {
      doScrollCheck();
    }
  }
},

即使不直接在jQuery核心文件中使用，有些开发者通过一些插件也可能熟悉懒加载的概念，延迟加载和揽初始化一样有效，它是一种在需要的时候（比如：当用户浏览到了页面底部的时候）才加载页面数据的技术。最近几年，这种模式已经变得非常显著并且现在可以再Twitter和Facebook的UI里面zhaoda。