My NSPersistentDocument subclass contains a singular managed object
which I'd like to make an observer immediately after the document is
loaded. So I invoked my little fetcher method to get it during -init,
after invoking super, but was surprised to find that the fetch
returned empty at this point.
I found the same behavior in Apple's DepartmentAndEmployees [1] sample
project by adding these two methods to the MyDocument implementation:
- (void)logDepartment {
// -department "fetches" the singular 'department' [2]
NSLog(@" 'Department': %p", [self department]) ;
}
- (id)init {
self = [super init] ;
NSLog(@"Fetch during init:") ;
[self logDepartment] ;
NSLog(@"Fetched after delay") ;
[self performSelector:@selector(logDepartment)
withObject:nil
afterDelay:0] ;
return self ;
}
Here's the console log I get upon opening an existing document file:
17:06:02.259 Fetch during init:
17:06:02.268 'Department': 0x0
17:06:02.269 Fetched after delay
17:06:02.388 'Department': 0x1dc730
Besides the fact that it's a kludge, the
performSelector:withObject:afterDelay occurs too late for my
purposes. So, I've got one fetch that's too early and one that's too
late. I looked in the debugger's call stack but don't see any other
intermediate method I could legally hook in to.
There are other ways I could set my observer, but I'm quite surprised
that executing a fetch in an initialized NSPersistentDocument doesn't
return a qualified object which is definitely in there, and
understanding this might help me with other issues. Note that
executeFetchRequest:error: is happy to return the incorrect answer of
an empty array -- it does not set an NSError.
Can someone please explain at what point, during loading, an
NSPersistentDocument becomes ready to fetch and return correct results?
Thanks as always,
Jerry Krinock
Note: In MyDocument.m, there is an implementation of -
initWithType:error: which inserts a 'Department', but that method only
runs for new documents and is not involved here. I'm trying to fetch
a 'Department' from the store/file.
[1] http://developer.apple.com/samplecode/DepartmentAndEmployees/index.html
[2] The following method is "right out of the box" of MyDocument.m,
reproduced here for convenience.
- (NSManagedObject *)department
{
// Apparently, 'department' is cached as an ivar for efficiency.
if (department != nil)
{
return department;
}
// The remainder of this method, executing a fetch, only executes
// the first time this method is invoked on a document.
NSManagedObjectContext *moc = [self managedObjectContext];
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSError *fetchError = nil;
NSArray *fetchResults;
@try
{
NSEntityDescription *entity = [NSEntityDescription
entityForName:@"Department"
inManagedObjectContext:moc];
[fetchRequest setEntity:entity];
fetchResults = [moc executeFetchRequest:fetchRequest
error:&fetchError];
} @finally
{
[fetchRequest release];
}
if ((fetchResults != nil) && ([fetchResults count] == 1) &&
(fetchError == nil))
{
[self setDepartment:[fetchResults objectAtIndex:0]];
return department;
}
if (fetchError != nil)
{
[self presentError:fetchError];
}
else {
// should present custom error message...
}
return nil;
}
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