jquery的ready事件的實現機制淺析

頁面初始化中，用的較多的就是$(document).ready(function(){//代碼}); 或 $(window).load(function(){//代碼});

他們的區別就是，ready是在DOM的結構加載完後就觸發，load是在頁面內包括DOM結構，css，js，圖片等都加載完成後再觸發，顯然ready更適合做爲頁面初始化使用。但有時候也不盡然。須要進一步查看其內部機制。

那麼ready的內部是如何判斷DOM的結構加載完的？而且不一樣的瀏覽器的判斷是如何的？

 

答案就在jquery代碼內，假設jquery的版本是jquery-1.11.3.js。

ready的關鍵代碼（3507~3566行），關鍵代碼用紅色標出：

jQuery.ready.promise = function( obj ) {
    if ( !readyList ) {

        readyList = jQuery.Deferred();

        // Catch cases where $(document).ready() is called after the browser event has already occurred.
        // we once tried to use readyState "interactive" here, but it caused issues like the one
        // discovered by ChrisS here: http://bugs.jquery.com/ticket/12282#comment:15
        if ( document.readyState === "complete" ) {
            // Handle it asynchronously to allow scripts the opportunity to delay ready
            setTimeout( jQuery.ready );

        // Standards-based browsers support DOMContentLoaded
        } else if ( document.addEventListener ) {
            // Use the handy event callback
            document.addEventListener( "DOMContentLoaded", completed, false );

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

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

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

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

            try {
                top = window.frameElement == null && document.documentElement;
            } catch(e) {}

            if ( top && top.doScroll ) {
                (function doScrollCheck() {
                    if ( !jQuery.isReady ) {

                        try {
                            // Use the trick by Diego Perini
                            // http://javascript.nwbox.com/IEContentLoaded/
                            top.doScroll("left");
                        } catch(e) {
                            return setTimeout( doScrollCheck, 50 );
                        }

                        // detach all dom ready events
                        detach();

                        // and execute any waiting functions
                        jQuery.ready();
                    }
                })();
            }
        }
    }
    return readyList.promise( obj );
};

上面的代碼在觸發ready時能夠分紅兩部分

1.標準瀏覽器下的觸發 

當瀏覽器是基於標準瀏覽器時，會在加載完DOM結構後觸發「DOMContentLoaded」事件，jquery內部就用此事件做爲ready的觸發源。

document.addEventListener( "DOMContentLoaded", completed, false );

 

2.IE瀏覽器下的觸發

當瀏覽器是IE瀏覽器時，由於IE瀏覽器（蛋疼並強大着）不支持「DOMContentLoaded」事件，因此只能另謀它法，

                (function doScrollCheck() {
                    if ( !jQuery.isReady ) {

                        try {
                            // Use the trick by Diego Perini
                            // http://javascript.nwbox.com/IEContentLoaded/
                            top.doScroll("left");
                        } catch(e) {
                            return setTimeout( doScrollCheck, 50 );
                        }

                        // detach all dom ready events
                        detach();

                        // and execute any waiting functions
                        jQuery.ready();
                    }
                })();

IE下的作法 就是上面代碼的紅字處，用「document.documentElement.doScroll("left")」的方法去滾動頁面，若是沒加載完就等個50毫秒後繼續滾，直到滾的動後就觸發ready。

可是，若是頁面中有frame的場合，會使用window.onload事件做爲ready的觸發源。

因此在IE下，頁面中有frame時，ready也是等到頁面內的全部內容加載完成後再觸發。