长话短说，步入正题。仿照flash的动画原理，自己写了一个非常简单的js动画引擎。

首先，用过flash的同学都知道。flash有个时间线，上面布满了“帧”。其实每个帧都是一个镜头，镜头连贯起来就是一副动画效果。其实，这跟电影的原理也一样的，小时候玩过胶片的都知道，对着光可以看到一副副的镜头。人眼分辨两个画面的连贯是有时间限度的。如果想看不到两个画面变换时的闪烁大概30帧/秒左右，电影是24帧的。所以，如果能保证，动画切换能保证每秒30次，基本上，就做到了动画的流畅效果，但是这取决环境。所以具体情况，具体而定，其实关于这方面的知识我也是一知半解。就这个动画引擎而言，了解这么多也差不多足够了，有兴趣可以查找相关知识！

下面开始说说设计原理。

首先需要一个帧频，也就是多少帧每秒。如果有了总用时，就可以计算出整个动画下来有多少个“画面”（总帧数）。这种设计，显然有个不足的地方，不能保证时间正好是个整数帧。除非1ms一帧。这是一个网友提出来的，我感觉不好就没有采用。所以这个引擎是有时间误差的。有了总帧数，当动画运行到最后一帧的时候，整个动画也就播放完。如果需要重复播放，重新把当前帧归0。这种动画有一个好处，就可以直接运行到指定帧。也就是flash里的gotoAndStop和gotoAndPlay。其实整个动画设计原理都是照着flash实现的。包括一个很重要的方法：enterFrame。位置就是根据进入当前帧来计算的。还有其它一些方法：stop、play、next......等等因为目前来说，这个引擎是写非常简单和粗糙的，先不说这么详细了。下面先上代码和示例吧！

animate.js 动画核心
代码如下:
var animation = function(obj) {
this.obj = obj;
this.frames = 0;
this.timmer = undefined;
this.running = false;
this.ms = [];
}

animation.prototype = {
fps: 36,
init: function(props, duration, tween) {
//console.log('初始化');
this.curframe = 0;
this.initstate = {};
this.props = props;
this.duration = duration || 1000;
this.tween = tween || function(t, b, c, d) {
return t * c / d + b;
};
this.frames = Math.ceil(this.duration * this.fps/1000);
for (var prop in this.props) {
this.initstate[prop] = {
from: parseFloat($util.dom.getStyle(this.obj, prop)),
to: parseFloat(this.props[prop])
};
}
},
start: function() {
if (!this.running && this.hasNext()) {
//console.log('可以执行...');
this.ms.shift().call(this)
}
return this;
},
//开始播放
play: function(callback) {
//console.log('开始动画！');
var that = this;

this.running = true;

if (this.timmer) {
this.stop();
}

this.timmer = setInterval(function() {
if (that.complete()) {
that.stop();
that.running = false;
if (callback) {
callback.call(that);
}
return;
}
that.curframe++;
that.enterFrame.call(that);
},
/ this.fps);

return this;
},
// 停止动画
stop: function() {
//console.log('结束动画！');
if (this.timmer) {
clearInterval(this.timmer);
// 清除掉timmer id
this.timmer = undefined;
}

},
go: function(props, duration, tween) {
var that = this;
//console.log(tween)
this.ms.push(function() {
that.init.call(that, props, duration, tween);
that.play.call(that, that.start);
});
return this;
},
//向后一帧
next: function() {
this.stop();
this.curframe++;
this.curframe = this.curframe > this.frames ? this.frames: this.curframe;
this.enterFrame.call(this);
},
//向前一帧
prev: function() {
this.stop();
this.curframe--;
this.curframe = this.curframe < 0 ? 0 : this.curframe;
this.enterFrame.call(this);
},
//跳跃到指定帧并播放
gotoAndPlay: function(frame) {
this.stop();
this.curframe = frame;
this.play.call(this);
},
//跳到指定帧停止播放
gotoAndStop: function(frame) {
this.stop();
this.curframe = frame;
this.enterFrame.call(this);
},
//进入帧动作
enterFrame: function() {
//console.log('进入帧：' + this.curframe)
var ds;
for (var prop in this.initstate) {
//console.log('from: ' + this.initstate[prop]['from'])
ds = this.tween(this.curframe, this.initstate[prop]['from'], this.initstate[prop]['to'] - this.initstate[prop]['from'], this.frames).toFixed(2);
//console.log(prop + ':' + ds)
$util.dom.setStyle(this.obj, prop, ds)
}
},
//动画结束
complete: function() {
return this.curframe >= this.frames;
},
hasNext: function() {
return this.ms.length > 0;
}
}

下面是一个简单的工具,其中有所用到的缓动公式：

util.js

代码如下:
$util = {
/**
* 类型检测
*/
type : function(obj){
var rep = /\[object\s+(\w+)\]/i;
var str = Object.prototype.toString.call(obj).toLowerCase();
str.match(rep);
return RegExp.$1;
},
/**
* 深拷贝
*/
$unlink :function (object){
var unlinked;
switch ($type(object)){
case 'object':
unlinked = {};
for (var p in object) {
unlinked[p] = $unlink(object[p]);
}
break;
case 'array':
unlinked = [];
for (var i = 0, l = object.length; i < l; i++) {
unlinked[i] = $unlink(object[i]);
}
break;
default: return object;
}
return unlinked;
},
/**
*Dom 相关操作
*/
dom:{
$: function(id) {
return document.getElementById(id);
},
getStyle: function(obj, prop) {
var style = obj.currentStyle || window.getComputedStyle(obj, '');
if (obj.style.filter) {
return obj.style.filter.match(/\d+/g)[0];
}
return style[prop];
},
setStyle: function(obj, prop, val) {
switch (prop) {
case 'opacity':
if($util.client.browser.ie){
obj.style.filter = 'alpha(' + prop + '=' + val*100 + ')'
}else{
obj.style[prop] = val;
}
break;
default:
obj.style[prop] = val + 'px';
break;
}
},
setStyles: function(obj, props) {
for (var prop in props) {
switch (prop) {
case 'opacity':
if($util.client.browser.ie){
obj.style.filter = 'alpha(' + prop + '=' + props[prop] + ')'
}else{
obj.style[prop] = props[prop];
}
break;
default:
obj.style[prop] = props[prop] + 'px';
break;
}
}
}
},
/**
*Event 事件相关
*/
evt : {
addEvent : function(oTarget, sEventType, fnHandler) {
if (oTarget.addEventListener) {
oTarget.addEventListener(sEventType, fnHandler, false);
} else if (oTarget.attachEvent) {
oTarget.attachEvent("on" + sEventType, fnHandler);
} else {
oTarget["on" + sEventType] = fnHandler;
}
},
rmEvent : function removeEventHandler (oTarget, sEventType, fnHandler) {
if (oTarget.removeEventListener) {
oTarget.removeEventListener(sEventType, fnHandler, false);
} else if (oTarget.detachEvent) {
oTarget.detachEvent("on" + sEventType, fnHandler);
} else {
oTarget["on" + sEventType] = null;
}
}
},
/**
*Ajax 异步加载
*/
ajax : {
request:function (options) {
var xhr, res;
var url = options.url,
context = options.context,
success = options.success,
type = options.type,
datatype = options.datatype,
async = options.async,
send = options.send,
headers = options.headers;

try {
xhr = new XMLHttpRequest();
} catch(e) {
try {
xhr = new ActiveXObject('MSXML2.XMLHTTP');
} catch(e) {
xhr = new ActiveXObject('Microsoft.XMLHTTP');
}
}

xhr.onreadystatechange = function() {
if (xhr.readyState == 4 && xhr.status == 200) {
res = xhr.responseText;
success(res);
}
}
xhr.open(type, url, async);
xhr.send(send);
}
},
/**
*Array 数组相关
*/
array : {
minIndex : function(ary){
return Math.min.apply(null,ary);
},
maxitem : function(ary){
return Math.max.apply(null,ary);
}
},
/**
*Client 客户端检测
*/
client : function(){
// 浏览器渲染引擎 engines
var engine = {
ie: 0,
gecko: 0,
webkit: 0,
khtml: 0,
opera: 0,

//complete version
ver: null
};

// 浏览器
var browser = {
//browsers
ie: 0,
firefox: 0,
safari: 0,
konq: 0,
opera: 0,
chrome: 0,
//specific version
ver: null
};

// 客户端平台platform/device/OS
var system = {
win: false,
mac: false,
x11: false,

//移动设备
iphone: false,
ipod: false,
ipad: false,
ios: false,
android: false,
nokiaN: false,
winMobile: false,

//game systems
wii: false,
ps: false
};

// 检测浏览器引擎
var ua = navigator.userAgent;
if (window.opera){
engine.ver = browser.ver = window.opera.version();
engine.opera = browser.opera = parseFloat(engine.ver);
} else if (/AppleWebKit\/(\S+)/.test(ua)){
engine.ver = RegExp["$1"];
engine.webkit = parseFloat(engine.ver);

//figure out if it's Chrome or Safari
if (/Chrome\/(\S+)/.test(ua)){
browser.ver = RegExp["$1"];
browser.chrome = parseFloat(browser.ver);
} else if (/Version\/(\S+)/.test(ua)){
browser.ver = RegExp["$1"];
browser.safari = parseFloat(browser.ver);
} else {
//approximate version
var safariVersion = 1;
if (engine.webkit < 100){
safariVersion = 1;
} else if (engine.webkit < 312){
safariVersion = 1.2;
} else if (engine.webkit < 412){
safariVersion = 1.3;
} else {
safariVersion = 2;
}

browser.safari = browser.ver = safariVersion;
}
} else if (/KHTML\/(\S+)/.test(ua) || /Konqueror\/([^;]+)/.test(ua)){
engine.ver = browser.ver = RegExp["$1"];
engine.khtml = browser.konq = parseFloat(engine.ver);
} else if (/rv:([^\)]+)\) Gecko\/\d{8}/.test(ua)){
engine.ver = RegExp["$1"];
engine.gecko = parseFloat(engine.ver);

//determine if it's Firefox
if (/Firefox\/(\S+)/.test(ua)){
browser.ver = RegExp["$1"];
browser.firefox = parseFloat(browser.ver);
}
} else if (/MSIE ([^;]+)/.test(ua)){
engine.ver = browser.ver = RegExp["$1"];
engine.ie = browser.ie = parseFloat(engine.ver);
}

//detect browsers
browser.ie = engine.ie;
browser.opera = engine.opera;

//detect platform
var p = navigator.platform;
system.win = p.indexOf("Win") == 0;
system.mac = p.indexOf("Mac") == 0;
system.x11 = (p == "X11") || (p.indexOf("Linux") == 0);

//detect windows operating systems
if (system.win){
if (/Win(?:dows )?([^do]{2})\s?(\d+\.\d+)?/.test(ua)){
if (RegExp["$1"] == "NT"){
switch(RegExp["$2"]){
case "5.0":
system.win = "2000";
break;
case "5.1":
system.win = "XP";
break;
case "6.0":
system.win = "Vista";
break;
case "6.1":
system.win = "7";
break;
default:
system.win = "NT";
break;
}
} else if (RegExp["$1"] == "9x"){
system.win = "ME";
} else {
system.win = RegExp["$1"];
}
}
}

//mobile devices
system.iphone = ua.indexOf("iPhone") > -1;
system.ipod = ua.indexOf("iPod") > -1;
system.ipad = ua.indexOf("iPad") > -1;
system.nokiaN = ua.indexOf("NokiaN") > -1;

//windows mobile
if (system.win == "CE"){
system.winMobile = system.win;
} else if (system.win == "Ph"){
if(/Windows Phone OS (\d+.\d+)/.test(ua)){;
system.win = "Phone";
system.winMobile = parseFloat(RegExp["$1"]);
}
}

//determine iOS version
if (system.mac && ua.indexOf("Mobile") > -1){
if (/CPU (?:iPhone )?OS (\d+_\d+)/.test(ua)){
system.ios = parseFloat(RegExp.$1.replace("_", "."));
} else {
system.ios = 2; //can't really detect - so guess
}
}

//determine Android version
if (/Android (\d+\.\d+)/.test(ua)){
system.android = parseFloat(RegExp.$1);
}

//gaming systems
system.wii = ua.indexOf("Wii") > -1;
system.ps = /playstation/i.test(ua);

//return it
return {
engine: engine,
browser: browser,
system: system
};

}(),
/**
*Tween 缓动相关
*/
tween: {
Linear: function(t, b, c, d) {
return c * t / d + b;
},
Quad: {
easeIn: function(t, b, c, d) {
return c * (t /= d) * t + b;
},
easeOut: function(t, b, c, d) {
return - c * (t /= d) * (t - 2) + b;
},
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;
}
},
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;
}
},
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;
}
},
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;
}
},
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;
}
},
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;
}
},
Elastic: {
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;
},
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);
},
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;
}
},
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;
}
},
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;
}
}
}

}

下面是个应用：

代码如下: