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

Make 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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Listing 3-14. Initializing the Audio Streaming Object - (id)initWithURL:(NSURL *)audioUrl { if (self = [super init]) { url = [audioUrl retain]; playQueueDataRecs = (PlayQueueData_t *)malloc(sizeof(PlayQueueData_t) * kNumAQBufs); } return self; }
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Listing 3-15 shows the starting point for audio playback. This is a very short method, especially in comparison to the one used earlier in the Audio Queue Services example. The reason is that with streaming audio you don t have an audio file at first, so you can t start looking at its audio format or other characteristics yet. Instead, you just create the AudioFileStreamID myAudioStream and then start the network connection. The second and third arguments to AudioFileStreamOpen() are the property listener and audio data callback functions, which will be called as soon as the stream has enough data to start making sense of the incoming data. The fourth argument to AudioFileStreamOpen() is a hint about the audio format, which you saw earlier in the Audio Queue Services code. In this case I m passing 0, which implies that the stream should attempt to determine the format.
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Listing 3-15. Starting the Stream - (void)play { // Create audio stream using callback functions. // Third argument is an optional hint to file type. AudioFileStreamOpen(self, propertyListenerCallback, audioDataCallback, 0, &myAudioStream); // Create the network connection NSURLRequest *networkRequest = [NSURLRequest requestWithURL:self.url]; networkConnection = [[NSURLConnection alloc] initWithRequest:networkRequest delegate:self]; }
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Note that I ve used the trick of passing self again here, so that the callback functions will be able to make method calls on the streaming object. The NSURLConnection created in Listing 3-15 will connect and begin receiving data as soon as it has been created. Since I ve set self as the connection s delegate, the connection will supply this data in the connection:didReceiveData: method on the streaming object. Recall that this method may be called many times, often with as little as 1KB to 2KB of data. Listing 3-16 shows this method. Whenever this happens, I pass the data to the parsing function. The first argument, myAudioStream, is the stream object I created earlier. The parsing function will use this argument to find the callbacks that I registered for it.
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CHAPTER 3: Audio Streaming: An Exploration into Core Audio
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Listing 3-16. Receiving Data from the Network and Parsing It // NSURLConnection delegate method - (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data { // Pass incoming bytes to audio stream parser. AudioFileStreamParseBytes(myAudioStream, [data length], [data bytes], 0); }
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When enough data has been received, it will be possible to start determining properties of the audio. The property callback will be called for each one. The property information continues to be available after the callback, though, so although the property listener will be called as soon as the data format is known, it s still possible to look up this property on the audio stream later. It s not necessary to store this information in your own variables. As a result, the property listener function will do nothing until the stream is ready to produce audio packets, at which point it can look up all the information you need. This property indicates that all metadata has been received and that playback can begin. Listing 3-17 shows the property listener callback.
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Listing 3-17. Property Listener Callback for Audio File Stream Services void propertyListenerCallback (void *inClientData, AudioFileStreamID inAudioFileStream, AudioFileStreamPropertyID inPropertyID, UInt32 *ioFlags) { SimpleStreamer *self = (SimpleStreamer *)inClientData; OSStatus err = noErr; UInt32 propertySize; if (inPropertyID == kAudioFileStreamProperty_ReadyToProducePackets) { // The stream is ready to produce audio packets // Get the audio format AudioStreamBasicDescription dataFormat; propertySize = sizeof(dataFormat); err = AudioFileStreamGetProperty(inAudioFileStream, kAudioFileStreamProperty_DataFormat, &propertySize, &dataFormat); // Create the play queue AudioQueueRef playQueue; err = AudioQueueNewOutput(&dataFormat, postPlayCallback, self, NULL, kCFRunLoopCommonModes, 0, &playQueue); [self setPlayQueue:playQueue]; // Set up audio buffer structures for (int i=0; i<kNumAQBufs; i++) { self.playQueueDataRecs[i].queuedCondition = [[NSCondition alloc] init]; err = AudioQueueAllocateBuffer(playQueue, kAQBufSize, &(self.playQueueDataRecs[i].buffer)); } self.currentBufferIndex = 0; // Lock the initial buffer, which is where we'll start writing data. NSCondition *queuedCondition = (NSCondition *)self.playQueueDataRecs[0].queuedCondition;
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