jquery的ready方法（DOM是否加載完）詳解與使用

jquery的ready方法（准備DOM觸發）還是比較復雜的，我們先看流程圖：

 

首先調用jq的raady方法，我們通過源碼可以看到，方法內部調用了jQuery.ready.promise()方法。

jQuery.fn = jQuery.prototype = {

    ........................

    ready: function( fn ) {
        // Add the callback
        jQuery.ready.promise().done( fn );

        return this;
    },

    .....................
};

 

我們再來看promise干了些什么：

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, 1 );

        // Standards-based browsers support DOMContentLoaded
        } else 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 {
            // 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 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 );
                        }

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

先判斷readyList有沒有，沒有的話給readyList賦值延遲對象，最后返回readyList.promise，很明顯promise函數只會執行一次。
給readyList賦值完后判斷document.readyState是否為complete，如果dom節點已加載完成，則調用jQuery.ready方法，否則注冊事件DOMContentLoaded與load，用來監聽dom的加載進度。
當瀏覽器有緩存時會先觸發load，為了保證第一之間最快的加載dom所以把2個事件都注冊上了。
我們來看下事件回調DOMContentLoaded做了些什么

var 
    ...................

    // The deferred used on DOM ready
    readyList,

    // The ready event handler and self cleanup method
    DOMContentLoaded = function() {
        if ( document.addEventListener ) {
            document.removeEventListener( "DOMContentLoaded", DOMContentLoaded, false );
            jQuery.ready();
        } else if ( document.readyState === "complete" ) {
            // we're here because readyState === "complete" in oldIE
            // which is good enough for us to call the dom ready!
            document.detachEvent( "onreadystatechange", DOMContentLoaded );
            jQuery.ready();
        }
    },

    ..................

DOMContentLoaded回調很簡單，先移除事件，在調用jQuery.ready()方法。所以說最終都是調用它。我們來看一下源碼：

jQuery.extend({

    .......................

    // Is the DOM ready to be used? Set to true once it occurs.
    isReady: false,

    // A counter to track how many items to wait for before
    // the ready event fires. See #6781
    readyWait: 1,

    // Hold (or release) the ready event
    holdReady: function( hold ) {
        if ( hold ) {
            jQuery.readyWait++;
        } else {
            jQuery.ready( true );
        }
    },
    // Handle when the DOM is ready
    ready: function( wait ) {

        // Abort if there are pending holds or we're already ready
        if ( wait === true ? --jQuery.readyWait : jQuery.isReady ) {
            return;
        }

        // Make sure body exists, at least, in case IE gets a little overzealous (ticket #5443).
        if ( !document.body ) {
            return setTimeout( jQuery.ready, 1 );
        }

        // Remember that the DOM is ready
        jQuery.isReady = true;

        // If a normal DOM Ready event fired, decrement, and wait if need be
        if ( wait !== true && --jQuery.readyWait > 0 ) {
            return;
        }

        // If there are functions bound, to execute
        readyList.resolveWith( document, [ jQuery ] );

        // Trigger any bound ready events
        if ( jQuery.fn.trigger ) {
            jQuery( document ).trigger("ready").off("ready");
        }
    },

    ...................
});

 ready方法先做了一些判斷，這些都是與holdReady有關，那么holdReady是干嘛的，isReady和readyWait又是干嘛的，請看下面的例子：

 

$.getScript('a.js',function(){

});
$(function(){
    alert(2);
});

a.js

alert(1);

 

因為getScript是異步的，所以代碼執行順序為先彈出alert(2) 再彈出alert(1)

那么要怎么樣同步呢，這時就需要用到holdReady了

$.holdReady(true); //先hold住，推遲執行
$.getScript('a.js',function(){
    $.holdReady(false); //當a.js加載完釋放hold
});
$(function(){
    alert(2);
});

現在代碼的執行順序為先alert(1)再alert(2)，由此可見，holdReady方法就是起一個推遲釋放作用。回頭再看ready方法就好理解了。我們再看最后2句代碼：

// If there are functions bound, to execute
readyList.resolveWith( document, [ jQuery ] );

// Trigger any bound ready events
if ( jQuery.fn.trigger ) {
    jQuery( document ).trigger("ready").off("ready");
}

第一句代碼執行了$(docment).ready方法中的回調，並且傳遞了JQ對象。第二句代碼先觸發ready事件再清除它。我們看下ready方法的使用：

$(function(scope){
    scope(this).on('click',function(){
        alert('a');
    });
});

scpoe是JQ對象，this是document，最后我們看下jq准備DOM觸發的3種寫法：

$(function(scope){
    scope(this).on('click',function(){
        alert('a');
    });
});
$(document).ready(function(scope){
    scope(this).on('click',function(){
        alert('a');
    });
});
$(document).on('ready',function(){
    $(this).on('click',function(){
        alert('a');
    });
});