Am 11.02.2010 um 04:07 schrieb Jerry Krinock:
>
> On 2010 Feb 10, at 18:05, Jens Alfke wrote:
>
>>> 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?
>>
>> Yes, basically. There is only going to be one in-memory object at a time
>> that represents the same managed object.
>
> It certainly seems to be sensible, but I just wish someone could find such
> documentation. I can't.
Every NSManagedObjectContext holds its own but unique copy of your object.
> In the last few minutes here, I improved my demo to test an sqlite store as
> well as an in-memory store, and also I setStalenessInterval to 0 and threw in
> some -refreshObject:mergeChanges:NO and more saves. The only difference I
> found was that, while the in-memory store fetches the same object that you
> inserted, the sqlite store does not, although subsequent fetches return the
> same object.
This is because the object in the sql-store needs a primary key. As long as it
was not saved it has a temporary key only. When saved it gets a persistent key.
After that it is identifiable by this and therefore you get alwas the same
object back.
I would have thought that your inserted object gets the persistent key set and
will therefore be the same object as the fetched ones. But I did not check your
code for a misconfiguration.
> Can anyone guarantee this?
> I would imagine that, even though it's not documented, Apple must realize
> that if they were ever to change this behavior, it would probably break alot
> of apps which have been relying on it without the designers realizing this.
> Would Apple ever ever do anything like that?
No, you will always get the same object. Because this was why CoreData and EOF
where made for. Exactly this.
atze
> Jerry
>
>
> REVISED CONSOLE OUTPUT:
>
> Using sqlite store at /Users/jk/Desktop/FooTest.sqlite
> Inserted: Foo 0x30022c0 name=Murphy ivar=BeenInserted
> First Fetched: Foo 0x3006120 name=Murphy ivar=BeenFetched
> Second Fetched: Foo 0x3006120 name=Murphy ivar=BeenFetched
> Delay Fetched: Foo 0x3006120 name=Murphy ivar=BeenFetched saved=1
> Delay Fetched: Foo 0x3006120 name=Murphy ivar=BeenFetched saved=1
>
> Using in-memory store
> Inserted: Foo 0x300c1a0 name=Murphy ivar=BeenFetched
> First Fetched: Foo 0x300c1a0 name=Murphy ivar=BeenFetched
> Second Fetched: Foo 0x300c1a0 name=Murphy ivar=BeenFetched
> Delay Fetched: Foo 0x300c1a0 name=Murphy ivar=BeenFetched saved=1
> Delay Fetched: Foo 0x300c1a0 name=Murphy ivar=BeenFetched saved=1
>
>
> REVISED DEMO 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 ;
> #if 0
> NSLog(@"Using in-memory store") ;
> newStore = [coordinator addPersistentStoreWithType:NSInMemoryStoreType
> configuration:nil
> URL:nil
> options:nil
> error:&error];
> #else
> #warning USING_SQLITE_STORE
> NSString* path = NSHomeDirectory() ;
> path = [path stringByAppendingPathComponent:@"Desktop"] ;
> path = [path stringByAppendingPathComponent:@"FooTest.sqlite"] ;
> NSLog(@"Using sqlite store at %@", path) ;
> NSURL* url = [NSURL fileURLWithPath:path] ;
> newStore = [coordinator addPersistentStoreWithType:NSSQLiteStoreType
> configuration:nil
> URL:url
> options:nil
> error:&error];
> #endif
>
> 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] ;
> [moc refreshObject:delayFetchedFoo
> mergeChanges:NO] ;
> #if 1
> BOOL didSave = [moc save:NULL] ;
> #else
> BOOL didSave = NO ;
> #endif
> NSLog(@" Delay Fetched: %@ saved=%d", delayFetchedFoo, didSave) ;
> }
>
> @end
>
>
> int main (int argc, const char * argv[])
> {
> NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
>
> // Create Core Data stack
> NSManagedObjectModel *mom = getStaticManagedObjectModel();
> NSManagedObjectContext *moc = getStaticManagedObjectContext();
>
> [moc setStalenessInterval:0.0] ;
>
> // Insert a Foo and name it "Murphy"
> Foo* insertedFoo = [NSEntityDescription
> insertNewObjectForEntityForName:@"Foo"
>
> inManagedObjectContext:moc] ;
> [insertedFoo setValue:@"Murphy"
> forKey:@"name"] ;
>
> // Set an ivar
> [insertedFoo setIvar:@"BeenInserted"] ;
>
> [moc processPendingChanges] ;
> [moc save:NULL] ;
>
> NSFetchRequest* fetchRequest ;
> NSArray* fetches ;
>
> [moc refreshObject:insertedFoo
> mergeChanges:NO] ;
>
> // 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] ;
>
> [firstFetchedFoo setIvar:@"BeenFetched"] ;
>
> // 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) ;
>
> // Create a run loop
> 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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