c# create barcode 2: EAAccessory Framework in Objective-C

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CHAPTER 2: EAAccessory Framework
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More on Protocols
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Now let s look at some examples of protocols that govern communications between the iPhone and external accessories. Let s start with a real-world example of a credit card reader. In Figure 2 2, you see the bi-directional link between the iPhone and your credit card reader. In normal operation, the user starts the application and selects the items that his customer is interested in purchasing. Once the total of all the items is determined, he attaches the card reader to the 30-pin dock connector of the iPhone. For the moment, let s disregard all the setup that is performed between the iPhone and the card reader so you can focus on the nominal operation of the card reader. Normally, most of the information will travel from the card reader to the iPhone, and contains all the data from one or both tracks of the magnetic stripe on the customer s card. Let s make this a little bit clearer and say that you are sending the card number, the cardholder s name, and the expiration date of the card. The app needs this information to complete the transaction. Additionally, the application can normally start diagnostics on the card reader to determine if it is functioning correctly. Finally, the card reader may contain diagnostic indicators such as LEDs that can be controlled from the iPhone to verify an operational connection.
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Figure 2 2. Typical communications flow for the Card Reader Accessory
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So, in general, commands are sent from the iPhone to the card reader accessory with data (card information) being sent in the reverse direction. Looking at this closely, you see that there are two different types of data streams. First, there is the transaction
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CHAPTER 2: EAAccessory Framework
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information that is sent during normal operation. But a second mode of operation would include the diagnostic commands sent to the accessory and the status sent back. The set of commands and status information used to communicate between the iPhone and card reader form your protocol. Specifically, each command and status chunk of information fits within a structured format that you define to be your protocol. Figure 2 3 depicts the generic format of commands and status transactions. Commands and status are sent in fixed or variable sized data packets or groups of bytes. Generally, the packets are ordered by decreasing importance of information. For example, the command that you are telling the accessory to execute, arguably the most important part of the transfer, is the first part of the packet. It would therefore reside at byte position 0 in an n-byte length packet.
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Figure 2 3. Generic command and status byte structure
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In the case of the card reader, there exists only a small set of commands. So you permit each command to be one byte in length. At eight bits per byte this gives you 256 possible commands; plenty more than the handful you need. So what is the additional information Let s consider the command to turn on an LED. In actuality, you need to be able to turn the LED on or off. You could have a command for each or you could have a single command with a parameter to say whether you want the LED to turn on or off. Similarly, if you had multiple LEDs on the accessory, you might have an additional information field that tells you which LED to command. So your LED command would contain three fields. First, the LED command itself, then the LED to operate upon, and, finally, whether you are turning it on or off. Figure 2 4 depicts the case where you have three LEDs in your accessory.
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CHAPTER 2: EAAccessory Framework
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Figure 2 4. The LED command packet structure
For example, to turn off LED #2, you send the sequence 0x010202 from the iPhone to the accessory. Status information works basically the same, but in the opposite direction. Instead of a command byte, you might have a status byte that indicates one of several conditions. For the results of a start diagnostics command, you might have 0x01 to mean that the tests completed successfully, 0x02 to mean that the tests completed but something failed, or 0x03 to mean that the tests hung up and did not complete. Additional information might include the duration of the diagnostic in milliseconds, the test that failed, or the last test that completed before the diagnostic routine hung. What does all this have to do with protocols The set of structures that you define to handle the communications between the iPhone and the accessory in both directions forms your protocol. So you might have some number of command structures like Figure 2 4 and another similar set for status as well as possibly other information that you transfer. However many of these sets of transfer structures you create, when placed all together, they form the External Accessory protocol. You might be asking, What s the big deal I always define some structure to my messaging. The big deal in this case is that the protocol has to be named and defined in the properties list of your project. In addition, Apple requires a specific naming convention, reverse-DNS, using your company s Internet address or a name that you are authorized to use. For example, if your company s name was fsda357 and you owned the domain fsda357.com, you might name your first protocol com.fsda357.p1, where p1 indicates this is your first protocol. You could just as easily call it com.fsda357.protocol1, or com. fsda357.protocolthx1138, or com. fsda357.ihatenamingprotocols. The key point to naming the protocol is that it must be unique and typically Apple likes to do that using reverse-DNS. Check out the preferences files on your Mac if you don t believe it.
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