emitterCells:CAEmitterCell对象的数组,被用于把粒子投放到layer上 birthRate:可以通俗的理解为发射源的个数,默认1.0.当前每秒产生的真实粒子数为=CAEmitterLayer的birthRate*子粒子的birthRate: lifetime:The cell lifetime range is multiplied by this value when particles are created. Defaults to one. emitterPosi