I've always wondered if I insert a managed object, then later fetch it
repeatedly from the same managed object context, do I get the same object every
time? (Assume that there is only one object which matches the given
predicate.) This may be important if I want to, say, set an instance variable
value.
So I just wrote a little experiment, using an in-memory store, and found that
the answer is yes, even if I run a run loop and fetch on a timer.
Is this expected, or is this an implementation detail which Apple may change at
any time?
Thanks,
Jerry
CONSOLE OUTPUT:
Inserted: Foo 0x1e0e960 name=Murphy ivar=Squawk
First Fetched: Foo 0x1e0e960 name=Murphy ivar=Squawk
Second Fetched: Foo 0x1e0e960 name=Murphy ivar=Squawk
Delay Fetched: Foo 0x1e0e960 name=Murphy ivar=Squawk
Delay Fetched: Foo 0x1e0e960 name=Murphy ivar=Squawk
Delay Fetched: Foo 0x1e0e960 name=Murphy ivar=Squawk
...
CODE (A Cocoa Tool Project):
#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>
@interface Foo : NSManagedObject {
NSString* m_ivar ;
}
@property (copy) NSString* ivar ;
@end
@implementation Foo
@synthesize ivar = m_ivar ;
- (void)dealloc {
[m_ivar release] ;
[super dealloc] ;
}
- (NSString*)description {
return [NSString stringWithFormat:
@"Foo %p name=%@ ivar=%@",
self,
[self valueForKey:@"name"],
[self ivar]] ;
}
@end
// Note: This function returns a retained, not autoreleased, instance.
NSManagedObjectModel *getStaticManagedObjectModel() {
static NSManagedObjectModel *mom = nil;
if (mom != nil) {
return mom;
}
mom = [[NSManagedObjectModel alloc] init];
NSEntityDescription *fooEntity = [[NSEntityDescription alloc] init];
[fooEntity setName:@"Foo"];
[fooEntity setManagedObjectClassName:@"Foo"];
[mom setEntities:[NSArray arrayWithObject:fooEntity]];
NSMutableArray* properties = [[NSMutableArray alloc] init] ;
NSAttributeDescription *attributeDescription;
// Add an attribute. (Copy this section to add more attributes.)
attributeDescription = [[NSAttributeDescription alloc] init];
[attributeDescription setName:@"name"];
[attributeDescription setAttributeType:NSStringAttributeType];
[attributeDescription setOptional:YES];
[properties addObject:attributeDescription] ;
[attributeDescription release] ;
[fooEntity setProperties:properties];
[properties release] ;
[fooEntity release] ;
return mom;
}
// Note: This function returns a retained, not autoreleased, instance.
NSManagedObjectContext *getStaticManagedObjectContext() {
static NSManagedObjectContext *moc = nil;
if (moc != nil) {
return moc;
}
moc = [[NSManagedObjectContext alloc] init];
NSPersistentStoreCoordinator *coordinator =
[[NSPersistentStoreCoordinator alloc]
initWithManagedObjectModel:getStaticManagedObjectModel()];
[moc setPersistentStoreCoordinator: coordinator];
[coordinator release] ;
NSError *error;
NSPersistentStore *newStore ;
newStore = [coordinator addPersistentStoreWithType:NSInMemoryStoreType
configuration:nil
URL:nil
options:nil
error:&error];
if (newStore == nil) {
NSLog(@"Store Configuration Failure\n%@",
([error localizedDescription] != nil) ?
[error localizedDescription] : @"Unknown Error");
}
return moc;
}
@interface DelayedFetcher : NSObject {
}
@end
@implementation DelayedFetcher
+ (void)fetchFoo:(NSTimer*)timer {
NSFetchRequest* fetchRequest = [[NSFetchRequest alloc] init];
NSManagedObjectContext *moc = getStaticManagedObjectContext() ;
[fetchRequest setEntity:[NSEntityDescription entityForName:@"Foo"
inManagedObjectContext:moc]];
NSArray* fetches = [moc executeFetchRequest:fetchRequest
error:NULL] ;
[fetchRequest release] ;
Foo* delayFetchedFoo = [fetches objectAtIndex:0] ;
NSLog(@" Delay Fetched: %@", delayFetchedFoo) ;
}
@end
int main (int argc, const char * argv[])
{
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
// Create Core Data stack
NSManagedObjectModel *mom = getStaticManagedObjectModel();
NSManagedObjectContext *moc = getStaticManagedObjectContext();
// Insert a Foo and name it "Murphy"
Foo* insertedFoo = [NSEntityDescription
insertNewObjectForEntityForName:@"Foo"
inManagedObjectContext:moc] ;
[insertedFoo setValue:@"Murphy"
forKey:@"name"] ;
[insertedFoo setIvar:@"Squawk"] ;
[moc processPendingChanges] ;
[moc save:NULL] ;
NSFetchRequest* fetchRequest ;
NSArray* fetches ;
// Fetch the Foo
fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setEntity:[NSEntityDescription entityForName:@"Foo"
inManagedObjectContext:moc]];
fetches = [moc executeFetchRequest:fetchRequest
error:NULL] ;
[fetchRequest release] ;
Foo* firstFetchedFoo = [fetches objectAtIndex:0] ;
// Fetch the Foo again
fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setEntity:[NSEntityDescription entityForName:@"Foo"
inManagedObjectContext:moc]];
fetches = [moc executeFetchRequest:fetchRequest
error:NULL] ;
[fetchRequest release] ;
Foo* secondFetchedFoo = [fetches objectAtIndex:0] ;
// See what we got
NSLog(@" Inserted: %@", insertedFoo) ;
NSLog(@" First Fetched: %@", firstFetchedFoo) ;
NSLog(@"Second Fetched: %@", secondFetchedFoo) ;
// Fetch again and again
NSRunLoop* runLoop = [NSRunLoop currentRunLoop] ;
[NSTimer scheduledTimerWithTimeInterval:1.0
target:[DelayedFetcher class]
selector:@selector(fetchFoo:)
userInfo:nil
repeats:YES] ;
[runLoop run] ;
[moc release] ;
[mom release] ;
[pool release] ;
return 0;
}_______________________________________________
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