运动框架必备的运动算法 留着用！

 var Tween = {
        
        // t  => 当前是第几步(总步数 = 总执行时间/每次执行时间(setInterval中的时间))

        // b  =>  开始值

        // c  =>变化量  即 终点值-开始值 

        // d  =>  经过多少次到达

        //匀速的线性运动
        Linear: function (t, b, c, d) { return c * t / d + b; }, 
        
        //变速线性运动
        //二次方加速 
        Quad: {
            
            // 速度递增 越来越快    
            easeIn: function (t, b, c, d) {
                return c * (t /= d) * t + b;
            },
            // 速度递减 越来越慢    
            easeOut: function (t, b, c, d) {
                return -c * (t /= d) * (t - 2) + b;
            },
            
            // 先加速再加速    
            easeInOut: function (t, b, c, d) {
                if ((t /= d / 2) < 1) return c / 2 * t * t + b;
                return -c / 2 * ((--t) * (t - 2) - 1) + b;
            }
        },
        
        //变速线性运动
        //三次方加速 加速幅度比二次方块 
        Cubic: {
            easeIn: function (t, b, c, d) {
                return c * (t /= d) * t * t + b;
            },
            easeOut: function (t, b, c, d) {
                return c * ((t = t / d - 1) * t * t + 1) + b;
            },
            easeInOut: function (t, b, c, d) {
                if ((t /= d / 2) < 1) return c / 2 * t * t * t + b;
                return c / 2 * ((t -= 2) * t * t + 2) + b;
            }
        },
        //变速线性运动
        //四次方加速 
        Quart: {
            easeIn: function (t, b, c, d) {
                return c * (t /= d) * t * t * t + b;
            },
            easeOut: function (t, b, c, d) {
                return -c * ((t = t / d - 1) * t * t * t - 1) + b;
            },
            easeInOut: function (t, b, c, d) {
                if ((t /= d / 2) < 1) return c / 2 * t * t * t * t + b;
                return -c / 2 * ((t -= 2) * t * t * t - 2) + b;
            }
        },
        //变速线性运动
        //五次方加速 
        Quint: {
            easeIn: function (t, b, c, d) {
                return c * (t /= d) * t * t * t * t + b;
            },
            easeOut: function (t, b, c, d) {
                return c * ((t = t / d - 1) * t * t * t * t + 1) + b;
            },
            easeInOut: function (t, b, c, d) {
                if ((t /= d / 2) < 1) return c / 2 * t * t * t * t * t + b;
                return c / 2 * ((t -= 2) * t * t * t * t + 2) + b;
            }
        },
        
        //运动更平滑
        Sine: {
            easeIn: function (t, b, c, d) {
                return -c * Math.cos(t / d * (Math.PI / 2)) + c + b;
            },
            easeOut: function (t, b, c, d) {
                return c * Math.sin(t / d * (Math.PI / 2)) + b;
            },
            easeInOut: function (t, b, c, d) {
                return -c / 2 * (Math.cos(Math.PI * t / d) - 1) + b;
            }
        },
        //加速减速更快
        Expo: {
            easeIn: function (t, b, c, d) {
                return (t == 0) ? b : c * Math.pow(2, 10 * (t / d - 1)) + b;
            },
            easeOut: function (t, b, c, d) {
                return (t == d) ? b + c : c * (-Math.pow(2, -10 * t / d) + 1) + b;
            },
            easeInOut: function (t, b, c, d) {
                if (t == 0) return b;
                if (t == d) return b + c;
                if ((t /= d / 2) < 1) return c / 2 * Math.pow(2, 10 * (t - 1)) + b;
                return c / 2 * (-Math.pow(2, -10 * --t) + 2) + b;
            }
        },
        Circ: {
            easeIn: function (t, b, c, d) {
                return -c * (Math.sqrt(1 - (t /= d) * t) - 1) + b;
            },
            easeOut: function (t, b, c, d) {
                return c * Math.sqrt(1 - (t = t / d - 1) * t) + b;
            },
            easeInOut: function (t, b, c, d) {
                if ((t /= d / 2) < 1) return -c / 2 * (Math.sqrt(1 - t * t) - 1) + b;
                return c / 2 * (Math.sqrt(1 - (t -= 2) * t) + 1) + b;
            }
        },
        // 弹性运动
        //激烈
        Elastic: {
            // 运动前弹动
            easeIn: function (t, b, c, d, a, p) {
                if (t == 0) return b; if ((t /= d) == 1) return b + c; if (!p) p = d * .3;
                if (!a || a < Math.abs(c)) { a = c; var s = p / 4; }
                else var s = p / (2 * Math.PI) * Math.asin(c / a);
                return -(a * Math.pow(2, 10 * (t -= 1)) * Math.sin((t * d - s) * (2 * Math.PI) / p)) + b;
            },
            // 运动到终点后弹动
            easeOut: function (t, b, c, d, a, p) {
                if (t == 0) return b; if ((t /= d) == 1) return b + c; if (!p) p = d * .3;
                if (!a || a < Math.abs(c)) { a = c; var s = p / 4; }
                else var s = p / (2 * Math.PI) * Math.asin(c / a);
                return (a * Math.pow(2, -10 * t) * Math.sin((t * d - s) * (2 * Math.PI) / p) + c + b);
            },
            // 前后都弹动
            easeInOut: function (t, b, c, d, a, p) {
                if (t == 0) return b; if ((t /= d / 2) == 2) return b + c; if (!p) p = d * (.3 * 1.5);
                if (!a || a < Math.abs(c)) { a = c; var s = p / 4; }
                else var s = p / (2 * Math.PI) * Math.asin(c / a);
                if (t < 1) return -.5 * (a * Math.pow(2, 10 * (t -= 1)) * Math.sin((t * d - s) * (2 * Math.PI) / p)) + b;
                return a * Math.pow(2, -10 * (t -= 1)) * Math.sin((t * d - s) * (2 * Math.PI) / p) * .5 + c + b;
            }
        },
        //弹性运动 
        //只回弹一次
        //较缓慢
        Back: {
            easeIn: function (t, b, c, d, s) {
                if (s == undefined) s = 1.70158;
                return c * (t /= d) * t * ((s + 1) * t - s) + b;
            },
            easeOut: function (t, b, c, d, s) {
                if (s == undefined) s = 1.70158;
                return c * ((t = t / d - 1) * t * ((s + 1) * t + s) + 1) + b;
            },
            easeInOut: function (t, b, c, d, s) {
                if (s == undefined) s = 1.70158;
                if ((t /= d / 2) < 1) return c / 2 * (t * t * (((s *= (1.525)) + 1) * t - s)) + b;
                return c / 2 * ((t -= 2) * t * (((s *= (1.525)) + 1) * t + s) + 2) + b;
            }
        },
        //回弹2次
        //较缓慢 Back快点
        Bounce: {
            easeIn: function (t, b, c, d) {
                return c - Tween.Bounce.easeOut(d - t, 0, c, d) + b;
            },
            easeOut: function (t, b, c, d) {
                if ((t /= d) < (1 / 2.75)) {
                    return c * (7.5625 * t * t) + b;
                } else if (t < (2 / 2.75)) {
                    return c * (7.5625 * (t -= (1.5 / 2.75)) * t + .75) + b;
                } else if (t < (2.5 / 2.75)) {
                    return c * (7.5625 * (t -= (2.25 / 2.75)) * t + .9375) + b;
                } else {
                    return c * (7.5625 * (t -= (2.625 / 2.75)) * t + .984375) + b;
                }
            },
            easeInOut: function (t, b, c, d) {
                if (t < d / 2) return Tween.Bounce.easeIn(t * 2, 0, c, d) * .5 + b;
                else return Tween.Bounce.easeOut(t * 2 - d, 0, c, d) * .5 + c * .5 + b;
            }
        }
    }

返回值为当前步数或者说时间（有误差）内，在开始状态和结束状态中间应该变化的值。

相对与不断获取位置，然后计算下一次应该运动距离，效率更高

如

obj.timer = setInterval(function(){
        
        obj.animate = true;
        
        //getStyle 通过 current或者computedStyle 获取到的元素当前状CSS的值。
        //可想而知，每秒钟获取N多次，效率肯定低下。
        
        var cur = parseInt(getStyle(obj,attr));
        //同样的不断执行parseInt
        
        cur = attr.toLowerCase()=='opacity'?parseInt(getStyle(obj,attr)*100):cur;
        
        //不断变化下一次移动的距离 递减
        var speed = (target - cur)/8;    
        
        speed = speed>0 ? Math.ceil(speed):Math.floor(speed);
        
        if(cur == target){
            
            obj.animate = false;
            clearInterval(obj.timer);
            
            if(fn){
                fn.call(obj); // 动画结束 执行回调函数
            }
                
        }else{
            
        
            
            setStyle(obj,attr,cur+speed);
        
        }
    
    },30);