vb.net print barcode labels 6: Serious Streaming Audio the Pandora Radio Way in Objective-C

Painting Data Matrix 2d barcode in Objective-C 6: Serious Streaming Audio the Pandora Radio Way

CHAPTER 6: Serious Streaming Audio the Pandora Radio Way
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you run out of data. You instead have to infer based on the size of the packets you ve queued and the audioQueue:isDoneWithBuffer: callbacks you ve received. You also have to be careful to manage the pause/play state of AudioQueue with respect to both network conditions and the user s actions. If the user pauses during a network interruption, you want to stay paused when the data starts flowing again.
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Dropped Connections
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While Wi-Fi connections can be fairly reliable, you can count on cellular network connections disconnecting or timing out on a regular basis. For this reason, you may want to implement some form of connection healing that resumes dropped connections. Of course, you don t want to restart a download from the beginning of the file. So, the first thing you ll need is a server that supports delivery of a range of bytes from a file. Most modern web servers do this out of the box using the HTTP Range header. Once you have the ability to download a partial file, the rest is straightforward code modification that can be limited to the AudioRequest class (this is one of the benefits of a clean code factoring). Let AudioRequest keep track of how many bytes have been downloaded, and when a connection drops, it can issue a new NSURLConnection request for the offset location where you left off. Figure 6-6 demonstrates how a file may be split into several requests due to dropped connections.
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request #1 offset: 0 bytes: 328,587
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request #2 offset: 328,587 bytes: 872,974
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request #3 offset: 1,201,561 bytes: 438,275
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file length: 1,639,836
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Figure 6-6. Splitting a file into multiple requests
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The only remaining issue is the length of time-outs on NSURLConnection. This is a tricky issue: If your time-outs are too long, dropped connections will result in lengthy gaps in audio playback. If your time-outs are too aggressive, you risk making things worse for your servers when they are distressed. For Pandora Radio, we ve invested in a reliable server infrastructure, which allows us to have aggressive time-outs. Our (nonscientific) experimentation also shows that when cellular network connections become significantly delayed, they are unlikely to ever recover. For these reasons, our time-outs tend to be pretty aggressive. Your mileage may vary.
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CHAPTER 6: Serious Streaming Audio the Pandora Radio Way
Minimizing Gaps Between Songs
Initiating a new network connections and loading enough data to begin playing a song can take a while, often 10 seconds or more on a cellular connection. When listening to a series of songs in a playlist, this can result in substantial gaps between songs, which is a less-thanstellar user experience. To solve this problem, you can begin downloading audio for the upcoming song as the current song nears completion. You ll want to be careful, though, never to have two network connections open at the same time, or you effectively halve your bandwidth capacity for each song. Also beware: for processor-intensive audio encodings such as MP3 and AAC, the iPhone allows only one AudioQueue to exist at any time. So when preloading the next song, you must take care to make sure that next AudioQueue isn t created until after the current one is destroyed.
Resuming a Song
Since the iPhone is such an incredible, Swiss Army knife of a device, and since your application isn t allowed to run in the background, it s not surprising that your users will want to exit your application from time to time, if only briefly. Maybe a phone call or an important e-mail will come in. Whatever the reason, your user will feel comfort and ease if leaving your application won t result in missing the rest of an enjoyable song. For this reason, we ve invested much effort into supporting the ability to resume interrupted songs in Pandora Radio. This feature requires a three-pronged attack: knowing how many bytes have played in your song thus far, persisting that information when the application terminates (perhaps by using NSUserDefaults), and resuming the interrupted song on startup by initiating a partial download starting at the offset where you left off. It s not necessary (and probably not a good idea) to resume songs that have been interrupted for extremely long periods of time. When you use an application after two days absence, you may not remember what song you were listening to before. But if you leave the application to take a phone call and come back 10 minutes later, resuming interrupted songs is a great feature.
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