Figure 4 18. Pressing the game controller s pushbutton in Objective-C

Drawer QR in Objective-C Figure 4 18. Pressing the game controller s pushbutton

Figure 4 18. Pressing the game controller s pushbutton
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CHAPTER 4: The Game Controller
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Figure 4 19. Rotating the game controller s knob
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This chapter provided more detail on the specific functions of the game controller and how it interfaces to the iPhone. First, I took a look at a similar iPhone accessory, the Griffin iTrip. The iTrip provides the means for connecting the audio output to a FM receiver. Using the iTrip as an example shows you what you can expect with your own accessory. Using a known, productionlevel accessory provides a fantastic baseline to use when comparing your own product development. While an accessory developer would normally create an accessory as part of a solution to a problem, in this case, for the purposes of education, I took a different approach. The rationale was to look at typical communications that happen between an iPhone and any accessory. In general, you are either going to send information to the accessory from the iPhone or the reverse. Starting with the Pong game concept, the most obvious control needed would be to set the position of the player s paddle. For this, you use a potentiometer whose electrical resistance varies as the position is changed. The processor and firmware within the controller reads the voltage from the center tap of the potientiometer and sends that to the iPhone. Though not strictly needed, you add a second user control in the form of a pushbutton to tell the application to serve the ball. Because this function operates much more infrequently than the paddle position, you use an interrupt routine to service the
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CHAPTER 4: The Game Controller
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pushbutton. The interrupt routine fires whenever the button is pressed causing whatever is currently happening to pause, and transfer control to the pushbutton routine. However, because all you need to do is inform the iPhone app that the button was pressed, only a single byte (0x10) of data is sent to the iPhone as opposed to when the knob is repositioned. In the latter case, not only does the app need to know that the event occurred (the knob turned) but it also needs to know the new position. In this case, you send two bytes; one indicates that the event occurred and the second byte contains the new position of the knob. Input and output data between the iPhone and accessory are handled by streams, in particular, NSStreams. NSStream provides events that the delegate handles indicating when data is available from the accessory to be read, when the output stream is empty and data can be sent to the accessory, as well as various error conditions.
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KEY POINTS
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Use existing accessories as reference when developing your own to provide valuable information on how a functioning system operates. Your game controller needs to send data to the iPhone to indicate the position of the player s paddle as well as an indication of when the serve button is pressed. Some functions such as serve operate infrequently and are suitable to the use of interrupts. An interrupt routine functions by pausing whatever is occurring in the main program, quickly executing the code to process the interrupt, then returning to where the main program left off. NSStreams handle the data transfer between the iPhone application and an accessory.
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Specifying a Pong Game
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So far, I ve talked about most of the tools you need to begin working on iPhone and iPod touch accessories. Now, let s get started with some actual programming and slowly bring all the pieces together.
Keeping It Simple
Although you could develop a very intricate game that you might possibly be able to put in the App store and sell, you re going to keep things very simple. The more complexity and coolness that you build into the game, the more time you ll spend writing and debugging code that doesn t pertain to the subject matter of the book. Remember, you re trying to learn about interfacing iPhone accessories, the undertapped market of the iPhone world. With over one hundred thousand games in the App store at the time of writing, and half of those being games, you can figure your chances on this Pong game being the next breakout star. So let s talk about the game. What do you want it to do In essence, you want to bounce an object that resembles a ball back and forth across the screen. The player should be able to move something that acts as a paddle to catch up with the ball when it is on his side. That brings up the notion of sides. You re going to declare that the top part of the screen the side closest to the power button is the opponent s space and the half of the screen near the bottom and the home button is the player s space. You could of course use the auto-rotate feature and play the game upside-down from those conditions, but to keep things simple, you won t. Let s talk about player and opponent. Although it seems obvious that the player is the human that taps the icon to start the game and moves the paddle, you need to state these things up front in your specification for whatever system you are building.
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