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# js绘制购物车抛物线动画_javascript技巧

y1 ＝ a*x1*x1 + b*x1 + c
y2 = a*x2*x2 + b*x2 + c

a已知，代入两个已知坐标［x1, y1］[x2, y2]可以得出b、c的值，x和y的对应关系有了。

```/**
* js抛物线动画
* @param {[object]} origin [起点元素]
* @param {[object]} target [目标点元素]
* @param {[object]} element [要运动的元素]
* @param {[number]} a [抛物线弧度]
* @param {[number]} time [动画执行时间]
* @param {[function]} callback [抛物线执行完成后回调]
*/
var parabola = function(config){
var b = 0,
INTERVAL = 15,
timer = null,
x1,y1,x2,y2,originx,originy,diffx,diffy;

this.config = config || {};
// 起点
this.origin = \$(this.config.origin)||null;
// 终点
this.target = \$(this.config.target)||null;
// 运动的元素
this.element = \$(this.config.element)||null;
// 曲线弧度
this.a = this.config.a || 0.004;
// 运动时间(ms)
this.time = this.config.time || 1000;

this.init = function(){
x1 = this.origin.offset().left;
y1 = this.origin.offset().top;
x2 = this.target.offset().left;
y2 = this.target.offset().top;
originx = x1;
originy = y1;
diffx = x2-x1;
diffy = y2-y1,
speedx = diffx/this.time;

// 已知a, 根据抛物线函数 y = a*x*x + b*x + c 将抛物线起点平移到坐标原点[0, 0]，终点随之平移，那么抛物线经过原点[0, 0] 得出c = 0;
// 终点平移后得出：y2-y1 = a*(x2 - x1)*(x2 - x1) + b*(x2 - x1)
// 即 diffy = a*diffx*diffx + b*diffx;
// 可求出常数b的值
b = (diffy - this.a*diffx*diffx)/diffx;
this.element.css({
left: x1,
top: y1
})
return this;
}

// 确定动画方式
this.moveStyle = function(){
var moveStyle = 'position',
testDiv = document.createElement('div');
if('placeholder' in testDiv){
['','ms','moz','webkit'].forEach(function(pre){
var transform = pre + (pre ? 'T' : 't') + 'ransform';
if(transform in testDiv.style){
moveStyle = transform;
}
})
}
return moveStyle;
}

this.move = function(){
var start = new Date().getTime(),
moveStyle = this.moveStyle(),
_this = this;
timer = setInterval(function(){
if(new Date().getTime() - start > _this.time){
clearInterval(timer);
_this.element.css({
left: x2,
top: y2
})
typeof _this.config.callback === 'function' && _this.config.callback(_this.element);
return;
}
x = speedx * (new Date().getTime() - start);
y = _this.a*x*x + b*x;
if(moveStyle === 'position'){
_this.element.css({
left: x+originx,
top: y+originy
})
}else{
if(window.requestAnimationFrame){
window.requestAnimationFrame(_this.element[0].style[moveStyle] = 'translate('+x+'px,'+y+'px)');
}else{
_this.element[0].style[moveStyle] = 'translate('+x+'px,'+y+'px)';
}
}
},INTERVAL)
return this;
}

this.init();
}
```

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