最简单的拖拽功能实现

简单的样式：

html,body{
    padding: 0;
    margin: 0;
    height: 100%;
    padding-top: 10%;
    box-sizing: border-box;
}
.B{
    width: 90%;
    height: 90%;
    margin: 0 auto ;
    position: relative;
    border: 1px solid red;
}
.box{
    background-color: red;
    width: 100px;
    height: 100px;
}

创建一个操作空间和拖拽目标：
<div class="B">
    <div class="box"></div>
</div>
功能实现：（需要引入JQ库）

$(".box").on({
    //PC拖拽事件
    mousedown: function (e) {
        var T,L;
        var el = $(this);
        var os = el.offset();
        var el_f = $(".B");
        var os_f = el_f.offset();
        var W = el_f.css("width").replace("px","")/1;
        var w = el.css("width").replace("px","")/1;
        var H = el_f.css("height").replace("px","")/1;
        var h = el.css("height").replace("px","")/1;
        var dx = e.pageX - os.left;
        var dy = e.pageY - os.top;

        $(document).on('mousemove.pc', function (e) {
            T = e.pageY - dy;
            L = e.pageX - dx;
            if( T < os_f.top ){
                T = os_f.top;
            }
            if( T > os_f.top+H-h ){
                T = os_f.top + H - h;
            }
            if( L < os_f.left ){
                L = os_f.left;
            }
            if( L > os_f.left+W-w ){
                L = os_f.left + W - w;
            }
            el.offset({top:T , left: L});
        });
    },
    mouseup: function () {
        $(document).off('mousemove.pc');
    },
    //手机端拖拽事件
    touchstart: function (e) {
        e.preventDefault();
        var T,L;
        var el = $(this);
        var os = el.offset();
        var el_f = $(".B");
        var os_f = el_f.offset();
        var W = el_f.css("width").replace("px","")/1;
        var w = el.css("width").replace("px","")/1;
        var H = el_f.css("height").replace("px","")/1;
        var h = el.css("height").replace("px","")/1;
        var pageX = event.touches[0].clientX;
        var pageY = event.touches[0].clientY;

        $(document).on('touchmove.phone', function (e) {
            e.preventDefault();
            L = event.touches[0].clientX;
            T = event.touches[0].clientY;

            if( T < os_f.top ){
                T = os_f.top;
            }
            if( T > os_f.top+H-h ){
                T = os_f.top + H - h;
            }
            if( L < os_f.left ){
                L = os_f.left;
            }
            if( L > os_f.left+W-w ){
                L = os_f.left + W - w;
            }
            el.offset({top:T , left: L});
        })
    },
    touchend : function (){
        $(document).off('touchmove.phone');
    }
});