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Generation Data Matrix in Objective-C Getting Down to Business

Getting Down to Business
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The application will read in and construct a dictionary of about 200,000 common words. Whenever letters are typed in, it will search for the words that can be spelled using those letters. There are numerous ways of solving this kind of problem, but this application opts for the simple approach: a brute force search of all 200,000 words. This could be time consuming, so the search shouldn t occur on the main UI thread of the application. Otherwise, the application will appear to freeze whenever a new letter is typed.
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The solution is to perform the search on a background thread. The search thread will send messages to the main thread whenever it finds a word. This could be accomplished with threads and semaphores, but it s easier to let NSOperationQueue do the heavy lifting for us. Back in the Xcode project, select the Classes group and add a new Objective-C class file named WordFinder.m, along with its companion WordFinder.h header file. Make the WordFinder a subclass of NSOperation, as shown in Listing 4-2. Listing 4-2. WordFinder Interface @class ScrapWordsController; @interface WordFinder : NSOperation { ScrapWordsController *controller; NSArray *letterSet; } + (NSArray*)words; - (id)initWithLetters:(NSString*)letters controller:(ScrapWordsController*)windowController; - (void)main; @end When the user types in some letters, the view object will update the data model by sending a setLetters: message to the controller. The implementation of setLetters:, shown in Listing 4-3, creates a new WordFinder operation and queues it up to execute. When the WordFinder thread runs, it sends removeWords and foundWord: messages back to the ScrapWordsController on the main thread. All the main application thread has to do is start the operation, then sit idly and wait for the results to come pouring in. The application s interface never blocks and is never unresponsive. The removeWords and foundWords: methods must modify the data model through the array controller object. The array controller is responsible for keeping the data model, the view, and itself in synchronization. Listing 4-3. ScrapWordsController Implementation @implementation ScrapWordsController @synthesize wordsController; - (id) init { self = [super init]; if (self != nil) { words = [NSMutableArray new]; finderQueue = [NSOperationQueue new]; } return self; } - (NSString*)letters { return (letters); // reference to controller // set of letters to search
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CHAPTER 4 CREATING AN XCODE PROJECT
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} - (void)setLetters:(NSString*)newLetters { if (newLetters==nil) newLetters = @""; if (![letters isEqualToString:newLetters]) { letters = newLetters; [finderQueue cancelAllOperations]; WordFinder *finder = [[WordFinder alloc] initWithLetters:newLetters controller:self]; [finderQueue addOperation:finder]; } } - (void)removeWords { NSRange all = NSMakeRange(0,[words count]); NSIndexSet *everyItemIndex = [NSIndexSet indexSetWithIndexesInRange:all]; [wordsController removeObjectsAtArrangedObjectIndexes:everyItemIndex]; } - (void)foundWord:(NSString*)word { if ([words count]==0 || ![[words lastObject] isEqualTo:word]) [wordsController addObject:word]; } @end A skeleton of the WordFinder implementation is shown in Listing 4-4. In brief, it contains a class method to construct and return a singleton array of all possible words. This method is synchronized in case it is invoked from multiple WordFinder threads. The main method is invoked to perform the operation. It simply tests each word against the letters in the set. If successful, it sends a message to the main thread using [controller performSelectorOnMainThread:@selector(foundWord:) withObject:candidate waitUntilDone:YES]. See 6 for more on sending messages to objects. Listing 4-4. WordFinder Implementation Skeleton static NSArray *DictionaryWords; list @implementation WordFinder + (NSArray*)words { @synchronized(self) { if (DictionaryWords==nil) { NSMutableArray *words = [NSMutableArray new]; ... DictionaryWords = [NSArray arrayWithArray:words]; } // singleton copy of English word
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CHAPTER 4 CREATING AN XCODE PROJECT
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} return DictionaryWords; } - (void)main { // Get the list of possible words NSArray* possibleWords = [WordFinder words]; // First, signal to the controller that a new word search has started [controller performSelectorOnMainThread:@selector(removeWords) withObject:nil waitUntilDone:YES]; // Brute force search of every word in the dictionary... for ( NSString *candidate in possibleWords ) { ... } } @end The finished application is shown in Figure 4-12. When letters are typed, a search thread is spawned that finds the possible words.
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