javascript动画系列第二篇——磁性吸附

function limitedRange(obj,fn){
    var L0 = 0;
    var R0 = document.documentElement.clientWidth;
    var T0 = 0;
    var B0 = document.documentElement.clientHeight;
    var L = obj.offsetLeft;
    var R = obj.offsetLeft + obj.offsetWidth;
    var T = obj.offsetTop;
    var B = obj.offsetTop + obj.offsetHeight;

    if(L >= L0 && R <= R0 && T >= T0 && B <= B0){
        fn(obj);
    }
}

<div id="test" style="height: 100px;width: 100px;background:pink;position:absolute;top:0;left:0;">测试文字</div>
<script>
function drag(obj){
    obj.onmousedown = function(e){
        e = e || event;
        //获取元素距离定位父级的x轴及y轴距离
        var x0 = this.offsetLeft;
        var y0 = this.offsetTop;
        //获取此时鼠标距离视口左上角的x轴及y轴距离
        var x1 = e.clientX;
        var y1 = e.clientY;
        //鼠标按下时，获得此时的页面区域
        var L0 = 0;
        var R0 = document.documentElement.clientWidth;
        var T0 = 0;
        var B0 = document.documentElement.clientHeight;
        //鼠标按下时，获得此时的元素宽高
        var EH = obj.offsetHeight;
        var EW = obj.offsetWidth;
        document.onmousemove = function(e){
            e = e || event;
            //获取此时鼠标距离视口左上角的x轴及y轴距离
            x2 = e.clientX;
            y2 = e.clientY;    
            //计算此时元素应该距离视口左上角的x轴及y轴距离
            var X = x0 + (x2 - x1);
            var Y = y0 + (y2 - y1);
            /******范围限定*******/
            //获取鼠标移动时元素四边的瞬时值
            var L = X;
            var R = X + EW;
            var T = Y;
            var B = Y + EH;
            //在将X和Y赋值给left和top之前，进行范围限定
            //只有在范围内时，才进行相应的移动
            //如果脱离左侧范围，则left置L0
            if(L < L0){X = L0;}
            //如果脱离右侧范围，则left置为R0
            if(R > R0){X = R0 - EW;}
            //如果脱离上侧范围，则top置T0
            if(T < T0){Y = T0;}
            //如果脱离下侧范围，则top置为B0
            if(B > B0){Y = B0 - EH;}
            obj.style.left = X + ‘px‘;
            obj.style.top = Y + ‘px‘;
        }

        document.onmouseup = function(e){
            //当鼠标抬起时，拖拽结束，则将onmousemove赋值为null即可
            document.onmousemove = null;
            //释放全局捕获
            if(obj.releaseCapture){
                obj.releaseCapture();
            }
        }
        //阻止默认行为
        return false;
        //IE8-浏览器阻止默认行为
        if(obj.setCapture){
            obj.setCapture();
        }
    }    
}
drag(test);
</script>    

<div id="test" style="height: 100px;width: 100px;background:pink;position:absolute;top:0;left:0;">测试文字</div>
<script>
function drag(obj){
    obj.onmousedown = function(e){
        e = e || event;
        //获取元素距离定位父级的x轴及y轴距离
        var x0 = this.offsetLeft;
        var y0 = this.offsetTop;
        //获取此时鼠标距离视口左上角的x轴及y轴距离
        var x1 = e.clientX;
        var y1 = e.clientY;
        //鼠标按下时，获得此时的页面区域
        var L0 = 0;
        var R0 = document.documentElement.clientWidth;
        var T0 = 0;
        var B0 = document.documentElement.clientHeight;
        //鼠标按下时，获得此时的元素宽高
        var EH = obj.offsetHeight;
        var EW = obj.offsetWidth;
        document.onmousemove = function(e){
            e = e || event;
            //获取此时鼠标距离视口左上角的x轴及y轴距离
            x2 = e.clientX;
            y2 = e.clientY;    
            //计算此时元素应该距离视口左上角的x轴及y轴距离
            var X = x0 + (x2 - x1);
            var Y = y0 + (y2 - y1);

            /******磁性吸附*******/
            //获取鼠标移动时元素四边的瞬时值
            var L = X;
            var R = X + EW;
            var T = Y;
            var B = Y + EH;
            //在将X和Y赋值给left和top之前，进行范围限定
            //只有在范围内时，才进行相应的移动
            //如果到达左侧的吸附范围，则left置L0
            if(L - L0 < 50){X = L0;}
            //如果到达右侧的吸附范围，则left置为R0
            if(R0 - R < 50){X = R0 - EW;}
            //如果到达上侧的吸附范围，则top置T0
            if(T - T0 < 50){Y = T0;}
            //如果到达右侧的吸附范围，则top置为B0
            if(B0 - B < 50){Y = B0 - EH;}
            obj.style.left = X + ‘px‘;
            obj.style.top = Y + ‘px‘;
        }

        document.onmouseup = function(e){
            //当鼠标抬起时，拖拽结束，则将onmousemove赋值为null即可
            document.onmousemove = null;
            //释放全局捕获
            if(obj.releaseCapture){
                obj.releaseCapture();
            }
        }
        //阻止默认行为
        return false;
        //IE8-浏览器阻止默认行为
        if(obj.setCapture){
            obj.setCapture();
        }
    }    
}
drag(test);
</script>