Without a way to collect input from the user, your game is nothing but a pretty graphics demo. In this chapter, we’ll look at common tasks that games often need to perform in order to get input from their players. The only way that a game can know about what the user wants to do is via the input that it collects, and the main way that the player provides that input is through the device’s built-in touchscreen.
Behind the scenes, a touchscreen is a very complex piece of technology. However, the information that it provides to your game is rather simple: you get told when a touch lands on the screen, when a touch moves, and when a touch is lifted from the screen. This might not sound like much—everyone knows you can detect taps, for example—but the touch system built into iOS is able to use this information to determine when the user is dragging, pinching, rotating, and flicking, all of which can be used to interpret the user’s will.
In addition to the touch system, iOS devices have a number of built-in sensors that detect the current state of the hardware. These include an accelerometer (which detects force and movement), a gyroscope (which detects rotation), a magnetometer (which detects magnetic fields), and a receiver for the Global Positioning System, or GPS (which can calculate where on the planet the device is).
You can combine all of this information to learn a great deal about what the user’s doing with the device, which can be used as input to your game. ...