Introducing Pandora Radio s Technology in Objective-C

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Introducing Pandora Radio s Technology
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Pandora Radio is renowned for having a great UI. But a polished UI alone won t make an application great. The mobile environment presents incredible challenges for providing Internet audio. And thus the hidden gem of the Pandora Radio application is its ability to deliver seamless, high-quality audio to your phone wherever you are, regardless of connection type even over cellular networks such as EDGE. In order to provide a terrific audio experience, though, you must first know the tools that are required. It s also useful to practice good habits in your programming.
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Grasping the Basics of Audio Development
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Writing software that plays audio is hard, especially as you get closer to the hardware layer. Apple s Core Audio API does a lot to abstract out the hardware specifics, but it s close enough to the hardware to make coding to the API a challenge. In addition, debugging broken audio is hard: how do you debug silence The audio output alone doesn t provide many clues as to what s going wrong. Since Core Audio can be so challenging and debugging so hard, you ll do yourself a favor if you get some audio any audio! playing in the simplest possible way before moving on to more advanced functionality. That way, if you make changes along the way that break your audio, you always have a working example to revert to, which can help you figure out what went awry.
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CHAPTER 6: Serious Streaming Audio the Pandora Radio Way
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Also, it s very important that you always check the return codes from audio API calls. Even if you don t do any specific error handling, you should at least log any errors. Having these log entries helps a ton when debugging mysterious audio problems.
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Start with a simple, working implementation, and check all error codes.
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This chapter s sample application will demonstrate these principles: we re going to create the simplest possible working implementation first, and at the end of this chapter, we ll discuss how to improve it. Also, as you browse the source code, you ll see that it carefully checks the status codes of each Core Audio API call like this:
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if (VERIFY_STATUS(status)) { ...
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We may not always check the return value of VERIFY_STATUS, but we almost always call VERIFY_STATUS when we use a Core Audio API. To see why this helps, let s trace VERIFY_ STATUS back to its definition:
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#define VERIFY_STATUS(status) \ AudioPlayerVerifyStatus(status, __FILE__, __LINE__)
Notice the __FILE__ and __LINE__ macros. These allow us to capture the file name and line number at which the macro is called, which is useful when debugging. Next, we'll trace this a step further to the AudioPlayerVerifyStatus definition:
BOOL AudioPlayerVerifyStatus(OSStatus status, char *file, int line) { if (status == noErr) { // Logging successes is prolific but useful for debugging. // We'll turn it off by default, but if you encounter a problem, you // can uncomment this to trace the path of execution. //NSLog(@"success at %s:%i", file, line); } else { char *s = (char *)&status; NSLog(@"error number: %i error code: %c%c%c%c at %s:%i", status, s[3], s[2], s[1], s[0], file, line); } return status == noErr; }
You see that we log the error code and its location whenever an error occurs. If, during the course of building and debugging your application, you encounter problems with audio, the logs will give you an immediate indication of what went wrong and where in your Core
CHAPTER 6: Serious Streaming Audio the Pandora Radio Way
Audio calls, even if you don t explicitly handle error cases. This technique can be a valuable time saver.
Managing Complexity
As your application grows and you add features and fix bugs, your audio code will quickly become complex. Complexity is inevitable and only grows with time, so be sure to leave time for refactoring and code simplification as you add features. Don t worry about performance issues until you encounter problems. The iPhone is surprisingly robust, and you may never encounter the performance problems you expect. For example, it s common for applications to perform audio functions in a separate audio thread to keep the application robust and responsive. But with the Pandora Radio application, we ve avoided threading and found negligible impact on application performance. The simplification gained by not having to deal with threading complexity has been a great time saver, which has allowed us to add great new features more quickly than we otherwise would have. We may have to put audio in a separate thread eventually, but in the meantime, we gain more by keeping the code simple. (It s also worth remembering that, since the iPhone has only one single-core ARM processor, threading is not as beneficial as in desktop environments.)
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