barcode maker vb.net 9: Fake It Til You Make It: Tips and Tricks for Improving Interface Responsiveness in Objective-C

Encoding Quick Response Code in Objective-C 9: Fake It Til You Make It: Tips and Tricks for Improving Interface Responsiveness

CHAPTER 9: Fake It Til You Make It: Tips and Tricks for Improving Interface Responsiveness
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WRITING TO THE IPHONE S NAND FLASH MEMORY
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With the iPhone s NAND flash memory, writing is expensive both in terms of speed and in terms of hardware lifetime. It will eventually wear out with use. Apple recommends that you write to disk only when necessary. Since our application checks to see whether the data is stale, it is unlikely to download stock data more than once or twice per day, so you can reasonably store it to disk when it arrives. If your data were more often malleable, you might consider storing it only when the application closes or if you ran out of memory. In gogoDocs, we only download and write the updated feed if its last changed date is later than that of our cached information. This keeps the application from making unnecessary writes to the flash memory. Apple supplies a convenience method in your application s delegate where you can save data before the app closes:
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Using Plists to Persist Data
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Dumping NSDictionary objects to plists has proven to be a simple way of persisting small amounts data that I use often. It s especially useful when you have control over the server because you can have the server send you a plist that you can persist more or less directly. In an open source library called TouchEngine for communicating with Google App Engine that I m working on with Noah Gift, a great Python programmer, we chose to use plists as our communication medium, and we automatically cache any plist that we get on the iPhone from the Google servers. We are automatically loading the cached data from the plists before we fetch new data. TouchEngine is available from Google Code at http://code.google.com/p/touchengine/. In AAPLot, you are already using an array of NSDictionary objects to store your data within the APYahooDataPuller, so it is trivial to persist them because an NSDictionary or an NSArray can be written to disk as a property list as long as it contains only property list objects (instances of NSData, NSDate, NSNumber, NSString, NSArray, or NSDictionary). The NSDecimalNumbers are subclasses of NSNumber, so you can store those with one caveat: they re going to get converted to floating point first, which will reduce their precision. For demonstration purposes, I ll just round them when reading them back in. The precision you lose might cause a graph line to move by a pixel, which isn t a big deal for this application. Let s add some caching methods to APYahooDataPuller. First you ll add a method called plistRep that returns a dictionary representation of the APYahooDataPuller s data. Then you ll add a method that writes that dictionary to a file, calling the built-in NSDictionary writeToFile:atomically: method. You should also take this opportunity to further modify APYahooDataPuller to better model your new strategy. Since you are caching the startDate and endDate values to disk and will need them for comparison later, you will want to add a few instance variables to track the dates you want from the server and also the symbol you re looking for, which may be different from those you re loading from the cache, and you ll also want to change the designated initializer accordingly. You should change the behavior with respect to notifying the delegate. Since you are caching financial data, it s possible that the target startDate, endDate, and symbol will match that which is already cached. If that is the case, you
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CHAPTER 9: Fake It Til You Make It: Tips and Tricks for Improving Interface Responsiveness
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won t need to reload the graph, and you should probably not even notify your delegate. You ll change the interface with the delegate so that you notify only when the financial data changes as a result of a fetch. Listing 9-2 shows the code for inserting the instance variables of the APYahooDataPuller object into a dictionary and then writing that dictionary to a plist on disk, which you can load the next time the user runs the application.
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Listing 9-2. Inserting Instance Variable Values into an NSDictionary Object and Writing It to a Plist on Disk - (NSDictionary *)plistRep { NSMutableDictionary *rep = [NSMutableDictionary dictionaryWithCapacity:7]; [rep setObject:[self symbol] forKey:@"symbol"]; [rep setObject:[self startDate] forKey:@"startDate"]; [rep setObject:[self endDate] forKey:@"endDate"]; [rep setObject:[self overallHigh] forKey:@"overallHigh"]; [rep setObject:[self overallLow] forKey:@"overallLow"]; [rep setObject:[self financialData] forKey:@"financalData"]; return [NSDictionary dictionaryWithDictionary:rep]; } - (BOOL)writeToFile:(NSString *)path atomically:(BOOL)flag; { NSLog(@"writeToFile:%@", path); BOOL success = [[self plistRep] writeToFile:path atomically:flag]; return success; }
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