.net barcode generator suite 14: Game Center in Objective-C

Painting Data Matrix ECC200 in Objective-C 14: Game Center

CHAPTER 14: Game Center
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-(void) setLastError:(NSError*)error { [lastError release]; lastError = [error copy]; if (lastError != nil) NSLog(@"GameKitHelper ERROR: %@", [[lastError userInfo] description]); }
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If you receive an error and would like to know more about it, you can refer to Apple s Game Kit Constants Reference, which describes the error constants defined in the GameKit/GKError.h header file. You can find the Constants Reference here: http://developer.apple.com/library/ios/#documentation/GameKit/Reference/GameKit _ConstantsRef/Reference/reference.html. After the local player has successfully signed in, you can access his friend list, leaderboards, and achievements. But before I get to that, let s sidestep for a moment and review the important aspects of block objects.
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Block Objects
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The inline method shown in Listing 14 5 is called a block object. You might have heard of closures from other languages, or anonymous functions, which are essentially the same concept. Block objects (known less accurately as blocks) are the way Objective-C declares a method that can be implemented within another method and even assigned to variables or passed as parameters. Refer to Apple s Blocks Programming Topics documentation if you are interested in the details of block objects: http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/Blocks/A rticles/00_Introduction.html. I ll cut out the actual block object from Listing 14 5 to discuss it separately:
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^(NSError* error) { [self setLastError:error]; }
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It looks like a method, except it has no name and it begins with a caret symbol (^). The NSError pointer is the only variable passed to it, but there can be multiple variables delimited by commas, as in this example:
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^(NSArray* scores, NSError* error) { [self setLastError:error]; [delegate onScoresReceived:scores]; }
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If that reminds you of a C method s parameters, you are correct. If you will, you can consider a block to be a C method whose name is ^ and can be passed to one of the many Game Kit methods taking block objects as parameters.
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CHAPTER 14: Game Center
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There are two technicalities I d like to point out. First, local variables can be accessed in a block. But they can t normally be modified, unless they are prefixed with the __block keyword. Consider this code snippet:
__block bool success = NO; [localPlayer authenticateWithCompletionHandler:^(NSError* error) { success = (error == nil); lastError = error; }];
With blocks, it is only legal to modify a local variable declared outside the block s scope if the variable is declared with the __block keyword. In this case, the success variable is declared locally outside the block, but is modified within the block, so it must be prefixed with the __block keyword. On the other hand, the lastError variable is a member variable of the class. Member variables can be modified within blocks without the use of the __block keyword. Also, in the case of Game Kit, you ll be frequently passing block objects to Game Kit methods, but the block objects won t be run until a later time. You are probably used to code being executed in sequence, but in Game Kit programming it is not! The block passed to a Game Kit method is only called when the call completes a round trip to and from the Game Center server. That takes time because data needs to be transmitted to the Game Center servers and processed, and then a result needs to be returned to the device. Only then does the block object gets executed. Let s take an example. You may find yourself tempted to write something like this:
__block bool success = NO; [localPlayer authenticateWithCompletionHandler:^(NSError* error) { success = (error == nil); }]; if (success) NSLog(@ Local player logged in! ); else NSLog(@ Local player NOT logged in! );
However, this example will always report that the player is not logged in. Why Well, the execution path is such that the authenticateWithCompletionHandler will take your block as a parameter and store it while it sends a request to the server and waits for the response to come back. However, the execution continues right away after the authenticateWithCompletionHandler method, and that s where the success variable decides which log statement to print. The problem is, the success variable is still set to NO because the block hasn t been executed yet. Several milliseconds later, the server responds to the authentication and that triggers the completion handler the block object to be run. If it returns without error, the success variable is set to YES. But alas, your logging code has already been run, so the assignment has no effect.
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