为什么要定义两个一样的变量

previous是之前的没有运行的class Shape对象的状态，current是当前要准备运行的状态 。在没有运行时current = previous = s

当运行if (growing) {

current += growthFactor

} else {

current -= growthFactor

}

if (current == size || current == 0) {

growing = !growing

}

两个就不同了，设置current = previous = s是为了初始化程序

---