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.
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.
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?
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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