惰性初始模式
最后更新于：2022-04-01 11:46:49
# 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。