Create the Class Files

The first step is to create the files for the new class. If you had a managed object model as a project resource, you could use the New File assistant to create a managed object class from an entity in the model. In this case, however, you do not, so create the files as you would for any other Objective-C class.

You declare properties for the managed object’s attributes. Core Data automatically implements accessors for managed object properties at runtime, so typically you don’t have to implement them yourself. To indicate to the compiler that the accessor methods will be made available through another mechanism, declare the properties as dynamic.

In the awakeFromInsert method (see Implement the Initializer), you set a value for a Run object’s date property when it is first created. To avoid recording an undo operation for the new date, you use a primitive accessor instead of the usual property accessor. Primitive accessors set do not emit key-value observing (KVO) change notifications that cause the change to be recorded as a separate undo event. You declare primitive accessors using a property with the name primitive<PropertyName>, so for the date property the primitive property is primitiveDate.

To create the basic implementation of the Run class

1. In Xcode, add a new Objective-C class file (.h and .m files) for the Run class.

2. Complete the interface for the Run class.

   In the Run.h file, set the class’s superclass to NSManagedObject, and declare properties for date and processID. Import the Core Data framework header file. The interface should look like this:

   #import <CoreData/CoreData.h>

   @interface Run : NSManagedObject

   @property (strong) NSDate *date;

   @property (assign) NSInteger processID;

   @end

3. Primitive properties should only be used by the class that implements them, so should not be declared publicly. You can “hide” the declaration of the primitiveDate in a class extension in the the Run.m file.

   Before the @implementation block in the Run.m file, add the following:

   @interface Run ()

   @property (strong) NSDate *primitiveDate;

   @end

4. Indicate that the properties are dynamic.

   In the @implementation block in the Run.m file, add the following:

   @dynamic date, primitiveDate, processID;

Dealing With nil Values

If you represent an attribute using a scalar value, you should specify what happens if the value is set to nil using key-value coding. You do this with the setNilValueForKey: method. In this case, simply set the process ID to 0.

To deal with nil values

• Implement an appropriate setNilValueForKey: method.

  If the key is “processID” then set processID to 0.

  - (void)setNilValueForKey:(NSString *)key {

  if ([key isEqualToString:@"processID"]) {

  self.processID = 0;

  }

  else {

  [super setNilValueForKey:key];

  }

  }

Implement the Initializer

NSManagedObject provides a special method—awakeFromInsert—that is invoked only when a new managed object is first created (strictly, when it is inserted into the managed object context) and not when it is subsequently fetched from a persistent store. You can use it here to record the date and time when a new record is created (the value won’t then be updated when an object is fetched).

To implement the initializer

• Implement an awakeFromInsert method that sets the receiver’s date to the current date and time.

  - (void) awakeFromInsert {

  [super awakeFromInsert];

  self.primitiveDate = [NSDate date];

  }

The Run Class

A complete listing of the declaration and implementation of the Run class is shown in Listing 5-2.

Listing 5-1  Complete listing of theRun.h file

#import <CoreData/CoreData.h>

@interface Run : NSManagedObject

@property (strong) NSDate *date;

@property (assign) NSInteger processID;

@end

Listing 5-2  Complete listing of the Run.m file

#import "Run.h"

@interface Run ()

@property (strong) NSDate *primitiveDate;

@end

@implementation Run

@dynamic date, primitiveDate, processID;

- (void) awakeFromInsert

{

    [super awakeFromInsert];

    self.primitiveDate = [NSDate date];

}

- (void)setNilValueForKey:(NSString *)key

{

    if ([key isEqualToString:@"processID"]) {

        self.processID = 0;

    }

    else {

        [super setNilValueForKey:key];

    }

}

@end

The main() Function

The main function is shown in Listing 5-3.

Listing 5-3  Listing of the main function

int main (int argc, const char * argv[]) {

    @autoreleasepool {

        NSManagedObjectModel *mom = managedObjectModel();

        NSLog(@"mom: %@", mom);

        if (applicationLogDirectory() == nil) {

            NSLog(@"Could not find application logs directory\nExiting...");

            exit(1);

        }

        NSManagedObjectContext *moc = managedObjectContext();

        NSEntityDescription *runEntity = [[mom entitiesByName] objectForKey:@"Run"];

        Run *run = [[Run alloc] initWithEntity:runEntity insertIntoManagedObjectContext:moc];

        NSProcessInfo *processInfo = [NSProcessInfo processInfo];

        run.processID = [processInfo processIdentifier];

        NSError *error;

        if (![moc save: &error]) {

            NSLog(@"Error while saving\n%@",

                ([error localizedDescription] != nil) ? [error localizedDescription] : @"Unknown Error");

            exit(1);

        }

        // Implementation will continue...

    }

    return 0;

}