Method spawn

packages/particle/src/Particle.ts:182–191

* 获取一个空闲粒子 * Get an inactive particle

()

     * Get an inactive particle
     */
    spawn(): Particle | null {
        for (const p of this._particles) {
            if (!p.alive) {
                p.alive = true;
                this._activeCount++;
                return p;
            }
        }
        return null;
    }

    /**
     * 回收粒子