jquery源码分析(三)——工具函数

jQuery.extend({
    expando: "jQuery" + ( version + Math.random() ).replace( /\D/g, "" ),//生成字符串，使用Math.random生成随机数并使用正则去掉了非数字字符。
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　//这里它作为HTMLElement或JS对象的属性名
    isReady: true,

    error: function( msg ) {
        throw new Error( msg );
    },

    noop: function() {},

    isFunction: function( obj ) {
        return jQuery.type(obj) === "function";
    },

    isArray: Array.isArray || function( obj ) {
        return jQuery.type(obj) === "array";
    },

    isWindow: function( obj ) {
        /* jshint eqeqeq: false */
        return obj != null && obj == obj.window;
    },

    isNumeric: function( obj ) {
        // parseFloat NaNs numeric-cast false positives (null|true|false|"")
        // ...but misinterprets leading-number strings, particularly hex literals ("0x...")
        // subtraction forces infinities to NaN
        return obj - parseFloat( obj ) >= 0;
    },

    isEmptyObject: function( obj ) {
        var name;
        for ( name in obj ) {
            return false;
        }
        return true;
    },

    isPlainObject: function( obj ) {
        var key;

        if ( !obj || jQuery.type(obj) !== "object" || obj.nodeType || jQuery.isWindow( obj ) ) {
            return false;
        }

        try {
            // Not own constructor property must be Object
            if ( obj.constructor &&
                !hasOwn.call(obj, "constructor") &&
                !hasOwn.call(obj.constructor.prototype, "isPrototypeOf") ) {
                return false;
            }
        } catch ( e ) {
            // IE8,9 Will throw exceptions on certain host objects #9897
            return false;
        }

        // Support: IE<9
        // Handle iteration over inherited properties before own properties.
        if ( support.ownLast ) {
            for ( key in obj ) {
                return hasOwn.call( obj, key );
            }
        }

        // Own properties are enumerated firstly, so to speed up,
        // if last one is own, then all properties are own.
        for ( key in obj ) {}

        return key === undefined || hasOwn.call( obj, key );
    },

    type: function( obj ) {
        if ( obj == null ) {
            return obj + "";
        }
        return typeof obj === "object" || typeof obj === "function" ?
            class2type[ toString.call(obj) ] || "object" :
            typeof obj;
    },

    // Evaluates a script in a global context
    // Workarounds based on findings by Jim Driscoll
    // http://weblogs.java.net/blog/driscoll/archive/2009/09/08/eval-javascript-global-context
    globalEval: function( data ) {
        if ( data && jQuery.trim( data ) ) {
            // We use execScript on Internet Explorer
            // We use an anonymous function so that context is window
            // rather than jQuery in Firefox
            ( window.execScript || function( data ) {
                window[ "eval" ].call( window, data );
            } )( data );
        }
    },

    // Convert dashed to camelCase; used by the css and data modules
    // Microsoft forgot to hump their vendor prefix (#9572)
    camelCase: function( string ) {
        return string.replace( rmsPrefix, "ms-" ).replace( rdashAlpha, fcamelCase );
    },

    nodeName: function( elem, name ) {
        return elem.nodeName && elem.nodeName.toLowerCase() === name.toLowerCase();
    },

    // args is for internal usage only
    each: function( obj, callback, args ) {
        var value,
            i = 0,
            length = obj.length,
            isArray = isArraylike( obj );

        if ( args ) {
            if ( isArray ) {
                for ( ; i < length; i++ ) {
                    value = callback.apply( obj[ i ], args );

                    if ( value === false ) {
                        break;
                    }
                }
            } else {
                for ( i in obj ) {
                    value = callback.apply( obj[ i ], args );

                    if ( value === false ) {
                        break;
                    }
                }
            }

        // A special, fast, case for the most common use of each
        } else {
            if ( isArray ) {
                for ( ; i < length; i++ ) {
                    value = callback.call( obj[ i ], i, obj[ i ] );

                    if ( value === false ) {
                        break;
                    }
                }
            } else {
                for ( i in obj ) {
                    value = callback.call( obj[ i ], i, obj[ i ] );

                    if ( value === false ) {
                        break;
                    }
                }
            }
        }

        return obj;
    },

    // Use native String.trim function wherever possible
    trim: trim && !trim.call("\uFEFF\xA0") ?
        function( text ) {
            return text == null ?
                "" :
                trim.call( text );
        } :

        // Otherwise use our own trimming functionality
        function( text ) {
            return text == null ?
                "" :
                ( text + "" ).replace( rtrim, "" );
        },

    // results is for internal usage only
    makeArray: function( arr, results ) {
        var ret = results || [];

        if ( arr != null ) {
            if ( isArraylike( Object(arr) ) ) {
                jQuery.merge( ret,
                    typeof arr === "string" ?
                    [ arr ] : arr
                );
            } else {
                push.call( ret, arr );
            }
        }

        return ret;
    },

    inArray: function( elem, arr, i ) {
        var len;

        if ( arr ) {
            if ( indexOf ) {
                return indexOf.call( arr, elem, i );
            }

            len = arr.length;
            i = i ? i < 0 ? Math.max( 0, len + i ) : i : 0;

            for ( ; i < len; i++ ) {
                // Skip accessing in sparse arrays
                if ( i in arr && arr[ i ] === elem ) {
                    return i;
                }
            }
        }

        return -1;
    },

    merge: function( first, second ) {
        var len = +second.length,
            j = 0,
            i = first.length;

        while ( j < len ) {
            first[ i++ ] = second[ j++ ];
        }

        // Support: IE<9
        // Workaround casting of .length to NaN on otherwise arraylike objects (e.g., NodeLists)
        if ( len !== len ) {
            while ( second[j] !== undefined ) {
                first[ i++ ] = second[ j++ ];
            }
        }

        first.length = i;

        return first;
    },

    grep: function( elems, callback, invert ) {
        var callbackInverse,
            matches = [],
            i = 0,
            length = elems.length,
            callbackExpect = !invert;

        // Go through the array, only saving the items
        // that pass the validator function
        for ( ; i < length; i++ ) {
            callbackInverse = !callback( elems[ i ], i );
            if ( callbackInverse !== callbackExpect ) {
                matches.push( elems[ i ] );
            }
        }

        return matches;
    },

    // arg is for internal usage only
    map: function( elems, callback, arg ) {
        var value,
            i = 0,
            length = elems.length,
            isArray = isArraylike( elems ),
            ret = [];

        // Go through the array, translating each of the items to their new values
        if ( isArray ) {
            for ( ; i < length; i++ ) {
                value = callback( elems[ i ], i, arg );

                if ( value != null ) {
                    ret.push( value );
                }
            }

        // Go through every key on the object,
        } else {
            for ( i in elems ) {
                value = callback( elems[ i ], i, arg );

                if ( value != null ) {
                    ret.push( value );
                }
            }
        }

        // Flatten any nested arrays
        return concat.apply( [], ret );
    },

    // A global GUID counter for objects
    guid: 1,

    // Bind a function to a context, optionally partially applying any
    // arguments.
    proxy: function( fn, context ) {
        var args, proxy, tmp;

        if ( typeof context === "string" ) {
            tmp = fn[ context ];
            context = fn;
            fn = tmp;
        }

        // Quick check to determine if target is callable, in the spec
        // this throws a TypeError, but we will just return undefined.
        if ( !jQuery.isFunction( fn ) ) {
            return undefined;
        }

        // Simulated bind
        args = slice.call( arguments, 2 );// 从参数列表中去掉fn,context
        proxy = function() {//使用apply修改fn执行上下文环境
            return fn.apply( context || this, args.concat( slice.call( arguments ) ) );
        };

        // Set the guid of unique handler to the same of original handler, so it can be removed
        proxy.guid = fn.guid = fn.guid || jQuery.guid++;//统一guid，使得proxy能够被移除

        return proxy;
    },

    now: function() {
        return +( new Date() );
    },

    // jQuery.support is not used in Core but other projects attach their
    // properties to it so it needs to exist.
    support: support
});

(1)、首先来了解下调用Object原型上的方法，toString方法，通过调用该方法可以返回 "[object XXX]"格式的字符串。jQuery内部就是用过这种方式实现了数据类型判断：

toString.call(obj) 返回值为

参数	返回值
true	[object Boolean]
'abcde'	[object String]
123	[object Number]
function(){}	[object Function]
new Date()	[object Date]
/g/	[object RegExp]
{}	[object object]

jQuery.each("Boolean Number String Function Array Date RegExp Object Error".split(" "), function(i, name) {
    class2type[ "[object " + name + "]" ] = name.toLowerCase();
});//统一设置通过toString来判断数据类型对应的返回值
type: function( obj ) {
        if ( obj == null ) {
            return obj + "";
        }
        return typeof obj === "object" || typeof obj === "function" ?
            class2type[ toString.call(obj) ] || "object" :
            typeof obj;
    },

　　

(2)、看下扩展了那些类型判断的方法：

isFunction、isNumeric 、isArray、isEmptyObject、

isPlainObject：obj不存在 或 非object类型 或 DOM节点 或 widnow对象，具有构造函数constructor，却不是自身的属性（即通过prototype继承的）都返回false

(3)、jQuery封装了几个常用遍历对象或数组方法：each map grep inArray  makeArray，从应用场景来分析：

1. grep  ----（筛选出满足条件的数据，返回一个数组）
2. each  ---- 对数组或对象 进行处理，不返回数据 ，但是还是可以通过在回掉函数中返回false 来终止，args用来区分jQuery内部调用该方法还是外部调用。
3. map  ----对数组或对象 进行数据处理，遍历数组或对象收集处理每个item处理后结果（对每一个属性调用callback，将返回值不为null的值，存入ret返回）。代码中针对数组和对象做了判断，但仅仅是遍历的方式不同，处理方式没有其他的区别——返回null，则忽略（无返回值的function会返回undefined）。
4. inArray  ----返回值在数组中的键值

(4)、proxy( fn, context ) ——代理方法：为fn指定上下文（即this）。修改fn使用上下文环境。这在jQuery很常用到。

实际上它是通过apply来实现上下文环境的切换的。

proxy = function() {
　　return fn.apply( context || this, args.concat( slice.call( arguments ) ) );
};

function proxy() 开始判断context类型是否为字符串。这样是处理了fn contex 参数位置互换问题。

2、前面分析这么多，突然发现把最常用的ready忘了，下面来对比下jQuery中ready与load事件。

 jQuery有3种针对文档加载的方法：

$(document).ready(function() {
    // ...代码...
})
$(function() {//上面document ready的简写
    // ...代码...
})
$(document).load(function() {
    // ...代码...
})

所以，实际也就两个针对文档加载方法：一个是ready一个是load，那么两个到底有什么区别呢？

(1)、疑问？？？ready与load谁先执行：答案是ready先执行，load后执行。

(2)、理解DOM文档加载的步骤：
要想理解为什么ready先执行，load后执行就要先了解下DOM文档加载的步骤：

(1) 解析HTML结构。
(2) 加载外部脚本和样式表文件。
(3) 解析并执行脚本代码。
(4) 构造HTML DOM模型。//ready
(5) 加载图片等外部文件。
(6) 页面加载完毕。//load

从上面的描述中应该很容易就能看出区别了吧，ready在第（4）步完成之后就执行了，但是load要在第（6）步完成之后才执行。

结论：

ready与load的区别就在于资源文件的加载，ready构建了基本的DOM结构，所以对于代码来说应该越快加载越好。在一个高速浏览的时代，没人愿意等待答案。假如一个网站页面加载超过4秒，不好意思，你1/4的用户将面临着流失，所以对于框架来说用户体验是至关重要的，我们应该越早处理DOM越好，而不需要等到图片资源都加载完才去处理框架的加载，当图片资源过多时会导致load事件就会迟迟得不到触发，严重影响用户体验。——后面有一篇专门分析  DOMContentLoaded 与onload区别 

(3)、下面我们看看jQuery是如何处理文档加载时机的问题：

// The deferred used on DOM ready
var readyList;//作为异步回调队列

jQuery.fn.ready = function( fn ) {
	// Add the callback
	jQuery.ready.promise().done( fn );//使用了状态机制

	return this;
};

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 );
		}

		// 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");
		}
	}
});

/**
 * Clean-up method for dom ready events
 */
function detach() {//清除DOM加载完成事件
	if ( document.addEventListener ) {
		document.removeEventListener( "DOMContentLoaded", completed, false );
		window.removeEventListener( "load", completed, false );

	} else {
		document.detachEvent( "onreadystatechange", completed );
		window.detachEvent( "onload", completed );
	}
}

/**
 * The ready event handler and self cleanup method
 */
function completed() {
	// readyState === "complete" is good enough for us to call the dom ready in oldIE
	if ( document.addEventListener || event.type === "load" || document.readyState === "complete" ) {
		detach();
		jQuery.ready();
	}
}

jQuery.ready.promise = function( obj ) {//定义一个状态机
	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 );
};

　　

jQuery的ready是通过promise给包装过的，这也是jQuery擅长的手法，统一了回调体系，在回调deferred中我会重点讨论下。
可见jQuery兼容的具体策略：针对高级的浏览器，我们当前很乐意用DOMContentLoaded事件了，省时省力。

那么旧的IE如何处理呢？继续看jQuery的方案：

如果浏览器存在 document.onreadystatechange 事件，当该事件触发时，如果 document.readyState=complete 的时候，可视为 DOM 树已经载入。不过，这个事件不太可靠，比如当页面中存在图片的时候，可能反而在 onload 事件之后才能触发，换言之，它只能正确地执行于页面不包含二进制资源或非常少或者被缓存时作为一个备选吧。