vb.net barcode printing 17: Future Paths in Java

Making Data Matrix 2d barcode in Java 17: Future Paths

CHAPTER 17: Future Paths
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[super dealloc]; }
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Now, what about the notifier itself, in our case the network-reading object that s going to broadcast its status That s taken care of as simply as this:
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if (some condition is met) { [[NSNotificationCenter defaultCenter] postNotificationName:DATA_RECEIVED object:self]; }
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The idea is that the notifier can fling out a notification like that, and the notification center takes care of the actual delivery. The notifier can also pass along additional information, in the form of a dictionary which can be retrieved by an observer, like this:
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// in the notifier NSDictionary *info = [NSDictionary dictionaryWithObject:someData forKey:@"data"]; [[NSNotificationCenter defaultCenter] postNotificationName:DATA_RECEIVED object:self userInfo:info]; // in the observer NSLog(@"received data %@", [[notification userInfo] objectForKey:@"data"]);
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Blocks
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In 16, we introduced you to blocks, an addition to C that Apple has come up with and included in Snow Leopard. We brought up blocks in the context of the concurrency features provided by Grand Central Dispatch, where blocks fit in really well, but there are many more uses for blocks. In Snow Leopard, Apple extended several Cocoa classes, adding dozens of new methods that take blocks as parameters. Let s take a look at some of them.
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Enumeration
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Let s start with something simple: enumeration. You re probably familiar with the standard C-based ways of stepping through a list, and perhaps the use of NSEnumerator and even the new fast enumeration ( for in loops) that s been available since the release of Leopard. Now, blocks provide us with yet another way to do the same thing:
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NSArray *array = [NSArray arrayWithObjects:@"one", "two", @"three"]; // C-style enumeration int i; for (i = 0; i < [array count]; i++) { NSLog(@"C enumeration accessing object: %@", [array objectAtIndex:i]); } // NSEnumerator, the "classic" Cocoa way to enumerate
CHAPTER 17: Future Paths
NSEnumerator *aEnum = [array objectEnumerator]; id obj1; while ((obj1 = [aEnum nextObject])) { NSLog(@"NSEnumerator accessing object: %@", obj1); } // "Fast enumeration", released as part of Leopard id obj2; for (obj2 in array) { NSLog(@"Fast enumeration accessing object: %@", obj2); } // "Block enumeration", new in Snow Leopard [array enumerateObjectsUsingBlock:^(id obj3, NSUInteger i, BOOL *stop) { NSLog(@"Block enumeration accessing object: %@", obj3); }];
The block we pass in to enumerateObjectsUsingBlock: takes three arguments, and returns nothing. That s the block signature declared by the method, and that s what we have to follow. The three arguments sent into our block are an object from the array, an integer containing that object s index in the array, and a BOOL passed by reference that lets us halt the enumeration by setting its value to YES. Looking at it that way, it may not be obvious at first why the block version is any better than the others, but the fact is that it really combines the best of all the other ways of enumerating. For one thing, it gives you the index of the current object, which is really handy if you want to do something like print out a numbered list of the items in an array. We can t tell you how many times we ve dropped down to C-style iteration just for easy access to each object s index value! Also, there s a variant of this method that lets you specify options defining how the enumeration runs, such as make it run concurrently:
[array enumerateObjectsWithOptions:NSEnumerationConcurrent usingBlock:^(id obj3, NSUInteger i, BOOL *stop) { NSLog(@"Block enumeration accessing object: %@", obj3); }];
That method actually uses GCD to spread the work around to all available processor cores, which will make your app run more quickly and better utilize system resources. And you get it for free! Similar enumeration methods exist for the NSSet class, but without the index parameter (because the objects in a set are, by definition, unordered). NSDictionary has also gotten some good block action, with new methods such as enumerateKeysAndObjectsUsingBlock: (and its options-taking variant that allows for concurrency), letting you specify a block that gets the key and value together. This is much better than previous ways of enumerating the contents of a dictionary, which typically involved stepping through all the keys, and looking up the value for each key.
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