barcode maker vb.net Figure 7-15. Selecting Managed Object Class as an available template in Objective-C

Drawing QR Code in Objective-C Figure 7-15. Selecting Managed Object Class as an available template

Figure 7-15. Selecting Managed Object Class as an available template
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You ll notice a new Message class has been created for you. All this work has been done with very little effort. There shouldn t be anything new to you in the generated class except for Message s base class (NSManagedObject) and perhaps the @dynamic directive. All the @dynamic directive does is promise the compiler (or Core Data in this case) that you ll supply the implementation for the properties at compile time. NSManagedObject is simply a generic class that Core Data molds as a model for you. Now you ll dive back into emailInvocationOperation:. You re about to inhale the fresh aura that Core Data emits. In the next several lines, you re going to avoid the hassle of going back and forth between objects and SQL. That is a process that can grow tedious and error-prone. This is handled much more elegantly with the advent of Core Data. First, you re checking to see whether you have a messageID. The messageID helps you identify a unique message in a pool of messages. If you have experience with databases, this is a fairly similar approach to using an autoincrement field. The code for getNewMessageID looks like the following:
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CHAPTER 7: Smart In-Application E-mail with Core Data and Three20
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- (NSString *)getNewMessageID { NSString *newId = @"1"; NSEntityDescription *entity = [NSEntityDescription entityForName:@"Message" inManagedObjectContext:managedObjectContext]; NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init]; [fetchRequest setEntity:entity]; NSString *predicateString = @"messageID = max(messageID)"; NSPredicate *predicate = [NSPredicate predicateWithFormat:predicateString]; [fetchRequest setPredicate:predicate]; NSError *error = nil; NSArray *allMessages = [managedObjectContext executeFetchRequest:fetchRequest error:&error]; if ([allMessages count] > 0) { Message *message = [allMessages objectAtIndex:0]; NSString *messageID = [message valueForKey:@"messageID"]; NSNumberFormatter *numberFormatter = [[[NSNumberFormatter alloc] init] autorelease]; NSNumber *num = [numberFormatter numberFromString:messageID]; newId = [NSString stringWithFormat:@"%d", [num intValue] +1]; } [fetchRequest release]; return newId; }
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This method contains some new classes, so let s go over them briefly. First you use NSEntityDescription to describe what objects you re particularly interested in working with. In this case, it s the only Core Data class you have, so you set the entity name Message. The NSFetchRequest is where you retrieve our data from storage. It s also where you can set a predicate using NSPredicate. If you re a database person, it might help you to think of the NSPredicate class as a wrapper around the SQL keyword WHERE. It is used to look for a specific domain of objects that are available from a much larger pool of objects. I highly recommend looking up the documentation for this class and getting better acquainted with it. It s fairly simple to use; as you can see in the predicateString variable, you set a format of messageID = max(messageID). What this does is retrieve one instance of message (if you have one) with the highest messageID value. You simply take that value, increment it by one, and return it. This will give the caller the capability to set the value for the next Message object. After you fetch the new ID, you useNSEntityDescription to inject a new Message object into your managed object context. The entity you re creating is named in insertNewObjectForEntityForName:. You then use a common pattern of Key Value Coding (KVC) to properly initialize your message object. Finally, you save the state of the context. Note that I m not doing any real error handling here besides leaving a template for where you would normally do it. If this were a production application, you most certainly would want to fill in this code here in the event of an error.
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CHAPTER 7: Smart In-Application E-mail with Core Data and Three20
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You ve gotten this far, but you have yet to see the workhorse of what s responsible for handling the gamut of paramount details behind the scenes. The SKPSMTPMessage is an open source library that is a simple but well-working wrapper that sends messages via SMTP on the iPhone and Mac. CAUTION: Applications that tend to replicate Apple s functionality such as Mail.app stand the risk of being rejected from the App Store. We use this simply as a demo for the book. The instantiation of the SKPSMTPMessage class is straightforward. You should be able to understand its usage by reading the source in DataManager.m. I made some minor modifications to SKPSMTPMessage in order to make it work with this app. First, I do the following:
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smtpMsg.messageID = messageID;
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I added the following instance variable to SKPSMTPMessage.h:
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NSString *messageID;
The reason for this is simple after the message delegate receives a messageSent: or messageFailed: callback that is defined in the SKPSMTPMessageDelegate, I need to know which message it is. You could have an infinite amount of messages queued (although your ISP might end up knocking on your door if you should ever try to reach this egregious quantity ), and whenever one fails or succeeds, you need to update your database.
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