// 獲取當(dāng)前瀏覽器支持的transform兼容寫(xiě)法
function getTransform() {
    var transform = '',
    divStyle = document.createElement('div').style,
    // 可能涉及到的幾種兼容性寫(xiě)法，通過(guò)循環(huán)找出瀏覽器識(shí)別的那一個(gè)
    transformArr = ['transform', 'webkitTransform', 'MozTransform', 'msTransform', 'OTransform'],

    i = 0,
    len = transformArr.length;

    for(; i < len; i++)  {
        if(transformArr[i] in divStyle) {
            // 找到之后立即返回，結(jié)束函數(shù)
            return transform = transformArr[i];
        }
    }

    // 如果沒(méi)有找到，就直接返回空字符串
    return transform;
}

function getStyle(elem, property) {
    // ie通過(guò)currentStyle來(lái)獲取元素的樣式，其他瀏覽器通過(guò)getComputedStyle來(lái)獲取
    return document.defaultView.getComputedStyle ? document.defaultView.getComputedStyle(elem, false)[property] : elem.currentStyle[property];
}

function getTargetPos(elem) {
    var pos = {x: 0, y: 0};
    var transform = getTransform();
    if(transform) {
        var transformValue = getStyle(elem, transform);
        if(transformValue == 'none') {
            elem.style[transform] = 'translate(0, 0)';
            return pos;
        } else {
            var temp = transformValue.match(/-?\d+/g);
            return pos = {
                x: parseInt(temp[4].trim()),
                y: parseInt(temp[5].trim())
            }
        }
    } else {
        if(getStyle(elem, 'position') == 'static') {
            elem.style.position = 'relative';
            return pos;
        } else {
            var x = parseInt(getStyle(elem, 'left') ? getStyle(elem, 'left') : 0);
            var y = parseInt(getStyle(elem, 'top') ? getStyle(elem, 'top') : 0);
            return pos = {
                x: x,
                y: y
            }
        }
    }
}

// pos = { x: 200, y: 100 }
function setTargetPos(elem, pos) {
    var transform = getTransform();
    if(transform) {
        elem.style[transform] = 'translate('+ pos.x +'px, '+ pos.y +'px)';
    } else {
        elem.style.left = pos.x + 'px';
        elem.style.top = pos.y + 'px';
    }
    return elem;
}

// 獲取目標(biāo)元素對(duì)象
var oElem = document.getElementById('target');

// 聲明2個(gè)變量用來(lái)保存鼠標(biāo)初始位置的x，y坐標(biāo)
var startX = 0;
var startY = 0;

// 聲明2個(gè)變量用來(lái)保存目標(biāo)元素初始位置的x，y坐標(biāo)
var sourceX = 0;
var sourceY = 0;

// 獲取當(dāng)前瀏覽器支持的transform兼容寫(xiě)法
function getTransform() {}

// 獲取元素屬性
function getStyle(elem, property) {}

// 獲取元素的初始位置
function getTargetPos(elem) {}

// 設(shè)置元素的初始位置
function setTargetPos(elem, potions) {}

// 綁定在mousedown上的回調(diào)，event為傳入的事件對(duì)象
function start(event) {
    // 獲取鼠標(biāo)初始位置
    startX = event.pageX;
    startY = event.pageY;

    // 獲取元素初始位置
    var pos = getTargetPos(oElem);

    sourceX = pos.x;
    sourceY = pos.y;

    // 綁定
    document.addEventListener('mousemove', move, false);
    document.addEventListener('mouseup', end, false);
}

function move(event) {
    // 獲取鼠標(biāo)當(dāng)前位置
    var currentX = event.pageX;
    var currentY = event.pageY;

    // 計(jì)算差值
    var distanceX = currentX - startX;
    var distanceY = currentY - startY;

    // 計(jì)算并設(shè)置元素當(dāng)前位置
    setTargetPos(oElem, {
        x: (sourceX + distanceX).toFixed(),
        y: (sourceY + distanceY).toFixed()
    })
}

function end(event) {
    document.removeEventListener('mousemove', move);
    document.removeEventListener('mouseup', end);
    // do other things
}

(function() {
    ...
})();

(function() {
    // 這是一個(gè)私有屬性，不需要被實(shí)例訪(fǎng)問(wèn)
    var transform = getTransform();

    function Drag(selector) {
        // 放在構(gòu)造函數(shù)中的屬性，都是屬于每一個(gè)實(shí)例單獨(dú)擁有
        this.elem = typeof selector == 'Object' ? selector : document.getElementById(selector);
        this.startX = 0;
        this.startY = 0;
        this.sourceX = 0;
        this.sourceY = 0;

        this.init();
    }


    // 原型
    Drag.prototype = {
        constructor: Drag,

        init: function() {
            // 初始時(shí)需要做些什么事情
            this.setDrag();
        },

        // 稍作改造，僅用于獲取當(dāng)前元素的屬性，類(lèi)似于getName
        getStyle: function(property) {
            return document.defaultView.getComputedStyle ? document.defaultView.getComputedStyle(this.elem, false)[property] : this.elem.currentStyle[property];
        },

        // 用來(lái)獲取當(dāng)前元素的位置信息，注意與之前的不同之處
        getPosition: function() {
            var pos = {x: 0, y: 0};
            if(transform) {
                var transformValue = this.getStyle(transform);
                if(transformValue == 'none') {
                    this.elem.style[transform] = 'translate(0, 0)';
                } else {
                    var temp = transformValue.match(/-?\d+/g);
                    pos = {
                        x: parseInt(temp[4].trim()),
                        y: parseInt(temp[5].trim())
                    }
                }
            } else {
                if(this.getStyle('position') == 'static') {
                    this.elem.style.position = 'relative';
                } else {
                    pos = {
                        x: parseInt(this.getStyle('left') ? this.getStyle('left') : 0),
                        y: parseInt(this.getStyle('top') ? this.getStyle('top') : 0)
                    }
                }
            }

            return pos;
        },

        // 用來(lái)設(shè)置當(dāng)前元素的位置
        setPostion: function(pos) {
            if(transform) {
                this.elem.style[transform] = 'translate('+ pos.x +'px, '+ pos.y +'px)';
            } else {
                this.elem.style.left = pos.x + 'px';
                this.elem.style.top = pos.y + 'px';
            }
        },

        // 該方法用來(lái)綁定事件
        setDrag: function() {
            var self = this;
            this.elem.addEventListener('mousedown', start, false);
            function start(event) {
                self.startX = event.pageX;
                self.startY = event.pageY;

                var pos = self.getPosition();

                self.sourceX = pos.x;
                self.sourceY = pos.y;

                document.addEventListener('mousemove', move, false);
                document.addEventListener('mouseup', end, false);
            }

            function move(event) {
                var currentX = event.pageX;
                var currentY = event.pageY;

                var distanceX = currentX - self.startX;
                var distanceY = currentY - self.startY;

                self.setPostion({
                    x: (self.sourceX + distanceX).toFixed(),
                    y: (self.sourceY + distanceY).toFixed()
                })
            }

            function end(event) {
                document.removeEventListener('mousemove', move);
                document.removeEventListener('mouseup', end);
                // do other things
            }
        }
    }

    // 私有方法，僅僅用來(lái)獲取transform的兼容寫(xiě)法
    function getTransform() {
        var transform = '',
            divStyle = document.createElement('div').style,
            transformArr = ['transform', 'webkitTransform', 'MozTransform', 'msTransform', 'OTransform'],

            i = 0,
            len = transformArr.length;

        for(; i < len; i++)  {
            if(transformArr[i] in divStyle) {
                return transform = transformArr[i];
            }
        }

        return transform;
    }

    // 一種對(duì)外暴露的方式
    window.Drag = Drag;
})();

// 使用：聲明2個(gè)拖拽實(shí)例
new Drag('target');
new Drag('target2');