OOP中的物理碰撞检测，具有一个对象的两次迭代

I've come across a theoretical writing-block in trying to figure out mechanics for a game. Basically, I want to have two circles that grow until they collide, and when they collide they start to shrink, until their radius is 0, and they start growing again, collide, etc.I've come across a theoretical writing-block in