barcode maker vb.net 3: Audio Streaming: An Exploration into Core Audio in Objective-C

Generation QR Code in Objective-C 3: Audio Streaming: An Exploration into Core Audio

CHAPTER 3: Audio Streaming: An Exploration into Core Audio
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Figure 3-2. Audio Queue Services playback cycle
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Apple provides a sample application that illustrates this approach, which is called SpeakHere.
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A WORD ON PROGRAMMING LANGUAGES
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In the course of discussions with other iPhone developers, I ve met a surprising number of people who don t simply prefer to stick to Objective-C for iPhone development but who regard pure C APIs with something approaching fear and dread. If that s not you, fantastic, but it s an attitude that I ve encountered much more than I would have expected. For some people, this feeling goes so far that they d prefer not to do something if using C is the only way. I m not sure what the source of this feeling is, but if you re going to use Core Audio, you ll need to recognize one very important detail: Core Audio is entirely a C-based API. No Objective-C is involved. Of course, Objective-C is a proper superset of C, so you already know C at least as well as you know Objective-C. You will, of course, have to use C-style arrays instead of NSArray, and you ll encounter malloc/free-style memory management. Not to mention potentially using pointers in new and unfamiliar ways. The callback function shown in Figure 3-2 is a C function that you would implement. But you can and should use Objective-C for most other aspects of your application, and Core Audio code will integrate well. If this sounds dangerous, error prone, or just distasteful, relax. You can do this. There is no monster hiding under this bed.
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If you ve done any significant amount of iPhone development work, you ve probably run across cases where the iPhone Simulator differs from working on a real iPhone or iPod touch. Often it s something obvious like the lack of a camera or the fact that the simulator s Core Location data always shows the current location to be at Apple s headquarters. With Core Audio, it s less obvious but no less important. Developing Core Audio code on the iPhone Simulator is a recipe for frustration and confusion, because although code often fails to work as expected, it doesn t fail in a manner that s immediately obvious as being a simulator issue. You may find, for example, that a key Core Audio function call never returns, for no readily apparent reason, and that while
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CHAPTER 3: Audio Streaming: An Exploration into Core Audio
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it s busy doing who-knows-what it also draws 90 percent or more of your Mac s CPU. Application bugs are often your own fault, so it s natural to try to analyze your code to see where you went wrong. But then you try the same code on a phone, and it just works. I ve filed bugs with Apple about this, but as of this writing, they re unresolved in any released version of the iPhone SDK. With any luck the situation will improve in a future release. Until then, if you choose to work in the simulator and you find that your Core Audio code is not working quite as expected, try working on a real device first before spending too much time trying to find the bug. Listing 3-8 shows a class declaration for a class that uses Audio Queue Services to play audio files. A number of instance variables are necessary to track audio format and maintain the audio queue. Property declarations are a matter of style, but some kind of setter and getter methods will be useful later.
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Listing 3-8. Class Declaration for a Class Using Audio Queue Services @interface SimpleAQPlayViewController : UIViewController { AudioStreamBasicDescription mDataFormat; AudioQueueRef mQueue; AudioQueueBufferRef mBuffers[kNumberBuffers]; AudioFileID mAudioFile; UInt32 bufferByteSize; SInt64 mCurrentPacket; UInt32 mNumPacketsToRead; AudioStreamPacketDescription *mPacketDescs; BOOL mIsRunning; } @property @property @property @property @property @property (readwrite) (readwrite) (readwrite) (readwrite) (readwrite) (readwrite) UInt32 AudioFileID AudioStreamPacketDescription SInt64 AudioQueueRef BOOL mNumPacketsToRead; mAudioFile; *mPacketDescs; mCurrentPacket; mQueue; mIsRunning;
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- (IBAction)play:(id)sender;
The play method to start audio playback is designed to be the target of a button or other user interface item. Listing 3-9 shows its implementation. There are a lot of steps, but it s not as complicated as it might look. The following are the key details to be aware of: Core Audio has its own API for opening, reading, and closing files. This starts with AudioFileOpenURL. The third argument to this call is a hint about the file type. Usually, AudioFileOpenURL can work out the file type for itself, so you pass 0 to indicate you re not giving it any hints. If the code was intended to use a specific file type, you could pass a format-specific value here, such as kAudioFileMP3Type or kAudioFileAIFFType. Other Core Audio file management calls will appear later.
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