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

Making 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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The minimal set of delegate methods covers those that receive data from the connection and those that are called when the connection has finished or failed. Listing 3-6 shows a simple implementation of these methods. See the NSURLConnection class documentation for information on others.
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Listing 3-6. Delegate Methods for NSURLConnection // NSURLConnection delegate method - (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data { // This method may be called repeatedly [receivedData appendData:data]; } // NSURLConnection delegate method - (void)connectionDidFinishLoading:(NSURLConnection *)connection { // This method will be called once, if no errors occurred. //All data has been received and the connection is closed. // Call any methods that process the received data (e.g. XML parsing) here. [receivedData release]; receivedData = nil; [myConnection release]; myConnection = nil; } // NSURLConnection delegate method - (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error { // This method is called once, if an error occurred. // The error argument contains information about the error. [receivedData release]; receivedData = nil; [myConnection release]; myConnection = nil; }
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In addition, Listing 3-7 shows an additional delegate method that can be used to retrieve the HTTP status code, if the URL scheme was HTTP. The response argument is declared as an NSURLResponse object, but if the protocol is HTTP, it will actually be a subclass, NSHTTPURLResponse, which contains the HTTP status. You might prefer to allocate the NSMutableData object in this method instead of when starting the connection and do so only if the status code indicates HTTP success (200). An empty NSMutableData object takes up so little space that it s more a matter of style than anything else.
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Listing 3-7. Getting the HTTP Status // NSURLConnection delegate method - (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response { [receivedData setLength:0]; // See if it s an NSHTTPURLResponse and typecast it if it is. if ([response isKindOfClass:[NSHTTPURLResponse class]]) { NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
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CHAPTER 3: Audio Streaming: An Exploration into Core Audio
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NSLog(@"HTTP Status: %d", [httpResponse statusCode]);
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Doing It the Cowboy Way with Core Audio
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Using MPMoviePlayer or one of the other APIs I had tried would have been the easy way. But as the legendary Ranger Doug would say, It wouldn t be the cowboy way. Clearly, it was time for Core Audio. All along it had been lurking in the SDK, an API so powerful I dreaded I might hurt myself trying to use it. This was going to be a learning experience, and I hoped it wouldn t be the kind where you learn not to do something again after inciting a disaster. Not really knowing where to begin, I opened up Xcode s documentation viewer and typed Core Audio into the search field. The results were kind of overwhelming at first. I tried to start with the simple stuff, but before I had finished the first page of Core Audio Overview, it was already talking about setting up a Core Audio based recording studio in a diagram containing about 15 different blocks, none of which I had the first clue about. After spending some time exploring the documentation, skipping stuff I didn t understand or that seemed not relevant to my project, I decided that the most likely option was something called Audio Queue Services. The documentation for this API described a scheme for playing audio files via Core Audio, so I started following along, implementing my own code to parallel the documented scheme.
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Getting Halfway There: Audio Queue Services
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Reading though the documentation and the sample code, I gradually realized that the Audio Queue Services approach was designed around reading audio from a file, as with previous methods I d tried. I pressed on anyway. The sample code didn t seem to grab the entire audio file at once instead, it used a collection of buffers that would be successively filled with part of an audio file and then played. By cycling through the buffers, the entire file would gradually be played. That sounded like a promising approach, since I expected that once I knew what I was doing, I could modify the code to take data from the network instead of from a file. The general approach is a repeating cycle in which you read a chunk of audio data from a file into a buffer and add it to an audio queue. The audio queue plays the buffer and then calls a callback function, which repeats the process until no more file data remains. Figure 3-2 illustrates this cycle.
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