8: How iTap Tackles the Challenges of Networking in Objective-C

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CHAPTER 8: How iTap Tackles the Challenges of Networking
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functions called setup and shutdown, which we use in Listing 8-13 to create and destroy the one instance of this class.
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Listing 8-13. Ceasing to Transmit and Receive Datagrams While the Device Is Locked - (void)applicationWillResignActive:(UIApplication *)application { [NetworkDiscovery shutdown]; } - (void)applicationDidBecomeActive:(UIApplication *)application { [NetworkDiscovery setup]; }
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The Networking Subsystem of iTap
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We will now give you a tour through the code of one of iTap s core components: the networking subsystem. After reading the previous sections, you are well adept at the inner workings of the iPhone networking APIs. Having discussed most of the core networking-related function of iTap, we will now focus on the bigger picture. We ll explain some of the design decisions we faced while implementing iTap and show you how we integrated the networking component into the rest of the application. Our code samples will again closely follow the downloadable version of the sample code.
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To use Bonjour or Not to Use Bonjour
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One of the first decisions we faced while designing the networking subsystem of iTap was whether to use Bonjour for autodiscovery or to implement our own protocol for that. Here is what Bonjour has to offer to the programmer: The ability to publish services identified by a type and a name on connected networks The ability to browse for services published by others Notifications if new services are added or removed by others The same APIs work on both Mac OS X and iPhone OS This functionality is available via both an Objective-C API consisting of the classes NSNetService and NSNetServiceBrowser and a C-based API called CFNetService, which is part of Core Foundation. The service browsing and publishing parts of Bonjour are based on an extension of the DNS protocol used to translate names to IP addresses on the Internet called DNS-SD. A third API, also available on both iPhone OS and Mac OS X, provides raw access to DNS-SD. NOTE: Since Apple provides extensive documentation and sample code covering both APIs, we will not provide additional code samples here.
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CHAPTER 8: How iTap Tackles the Challenges of Networking
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Since supporting both Mac OS X and Windows was one of the goals of iTap right from the start, let s take a look at the state of Bonjour on Windows: Although Bonjour is an integral part of Mac OS X and iPhone OS, it isn t on Windows. To use Bonjour in a Windows application, you either need to require your users to download and install Bonjour for Windows themselves or include that step in the installation process of your application. This would preclude any download-and-run version of the iTap receiver for Windows. Both higher-level APIs to Bonjour are too deeply tied to other core frameworks on Mac OS X and iPhone OS to be usable on Windows. The only API remaining is the raw DNS-SD one. This is what the iTap receiver would need to use if iTap were based on Bonjour. In the end, we thought that although Bonjour might have some technical merits, from a user s point of view rolling our own solution was clearly beneficial. Forcing our Windows users to install a whole new system component just to use our receiver application just didn t seem right. Besides, each additional component used is an additional component to support. Since we anticipated that supporting iTap in all kinds of different networking environment would not be an easy task, we were reluctant to add yet another possible source of problems.
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To keep the code of a larger application as easy to understand and extend as possible, you will usually strive to separate the application into separate components. Ideally, these components are largely self-contained and able to perform their task with as little knowledge about other parts of the application as possible. Trying to adhere to this ideal proves to be difficult in practice, though. For example, take a look at our little sample application called Discover. This application contains two components: the NetworkDiscovery class introduced earlier and a UITableView plus its view controller NetworkDiscoveryPeerTable. It s the responsibility of NetworkDiscovery to monitor WiFi availability and to detect other instances of Discover running on the same network. The table view and its associated view controller are responsible for visualizing this information. Given this separation of responsibilities, you need a way for the NetworkDiscovery class to inform NetworkDiscoveryPeerTable about changes to the list of peers or to WiFi availability. You could of course let NetworkDiscovery store a reference to the instance of NetworkDiscoveryPeerTable somewhere and simply send that instance some message to signal an event. But doing so would mean abandoning the modularity of the application that you seek to achieve. For example, imagine the application contained some button that you wanted to be visible only if at least one other device is detected. Since that button would presumably not be managed by our table view controller, you d
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