【程序化动画の弹簧】

可以让缓动线性缓动变得Q弹。

以下是曲线参数可视化：

module vp 
{
    /** 
     * 弹簧的动画
     */
    export class SpringAnimation
    {
        /** 初始速度 */
        private initVelocity:number = 30;

        /** 当前速度 */
        private velocity: number = 30;

        /** 当前位移值 */
        private displacement:number = 0;

        /** 弹性(值越大幅度越大) */
        private spring:number = 900;
        
        /** 阻尼(值越大越快静止) */
        private damp:number = 8;

        constructor()
        {
        }

        public init(spring:number, damp:number,velocity:number,displacement:number = 0)
        {
            this.spring = spring;
            this.damp = damp;
            this.velocity = velocity;
            this.initVelocity = this.velocity;
            this.displacement = displacement;
        }

        public replay():void
        {
            this.velocity = this.initVelocity;
        }

        /** 
         * T:每帧经过的时间
         * 
         */
        public update(T:number):number
        {
            var force = -this.spring * this.displacement - this.damp * this.velocity;
            this.velocity += force * T;
            this.displacement += this.velocity * T;
            return this.displacement;
        }
    }
}