How do I create an invisible joystick that appears anywhere on the iOS screen?

Hi, I'm creating a third person game and I want to make a single joystick controller but there are two distinctive things I want this joystick to do. I want it to be invisible until the player touches the screen. Then it becomes visible under the player's finger. I also want, and this is the hard part, to have the player be able to touch anywhere on the screen and have the joystick appear. I don't want it to be set in any particular location. Is there anywhere that I might be able to find this information or can anyone help me to create it? Thanks so much! Tom