iOS：如何制作一个循环，每次都是一个接一个地运行随机动作

I am making game where random animals will emerge from every direction (up, down, left, right). There'll be an animal emerge from one side, it will move to the opposite side, and after that one's gone, another animal will emerge from another random direction. My problem is when I run it, all the animals emerge simultaneously from all directions, not one by one. It also moves in a blinking manner in which it moves a little bit, and then another one emerge from another direction moving a little bit. How do I make a it go one by one? After one animal done moving across the screen, then another will emerge.I am making game where random animals will emer