As a followup to my own question, it seems that the context is not
created until awakeFromNib.
If I wait until awakeFromNib, I can access it via:
[[NSApp delegate] managedObjectContext];
OR
[[[NSApplication sharedApplication] delegate] managedObjectContext];
and those return the same value. Clearly you don't want to alloc/init
a new instance of the delegate class.
Regarding the singleton issue, I realized you need to grab the
singleton from the persistent store and count it. Thus,
NSError *error;
NSManagedObjectContext *moc = [[NSApp delegate] managedObjectContext];
NSUInteger numLibrarians = [EntityUtilities countObjectsOfType:
@"Librarian" inContext: moc];
if (numLibrarians == 0) {
librarian = [NSEntityDescription insertNewObjectForEntityForName:
@"Librarian"
inManagedObjectContext: moc]; // moc becomes nil here !!!
etc.
enables me to determine how many librarians I have (once I set up such
a utility).
Thanks for looking.
On Sep 9, 2008, at 3:11 PM, Daniel Child wrote:
Hi All,
I am trying to implement a singleton class which is a subclass of
NSManagedObject, but it is not behaving as expected. The librarian
singleton is supposed to keep a list of the data sources, which are
added periodically. I'm trying to cache the Librarian as an instance
variable of my App Controller so that I can access it easily. The
app controller tries to create/retrieve the instance during its own
init as follows:
<in AppController>
- (id)init
{
if (self = [super init])
{
// a bunch of variables initialized here
theLibrarian = [Librarian getLibrarianInstance];
}
return self;
}
What's totally weird is that my managed object context disappears
midway through the following lines of code.
<in Librarian.m>
static Librarian *librarian = nil;
@implementation Librarian
@dynamic dataSources;
@dynamic numSources;
+ (Librarian *) getLibrarianInstance {
NSError *error;
NSManagedObjectContext *moc = [[NSApp delegate]
managedObjectContext]; // moc exists at this point
if (librarian == nil) {
librarian = [NSEntityDescription insertNewObjectForEntityForName:
@"Librarian"
inManagedObjectContext: moc]; // moc
becomes nil here !!!
[moc insertObject: librarian];
if (![moc save: &error]) {
[[NSApplication sharedApplication] presentError:error];
}
}
// [moc insertObject: librarian];
return librarian;
}
I have two basic questions. Where is the best place to initialize a
singleton for a Core Data application?
And secondly, what is the best way to access the managed object
context. In my init, if I call
moc = [[NSApp delegate] managedObjectContext]; // value is nil
moc = [[[NSApplication sharedApplication] delegate]
managedObjectContext]; // value is nil
coreDataDelegate = [[CoreDataLibrarian_AppDelegate alloc] init];
moc = [coreDataDelegate managedObjectContext]; // value exists
Surely you aren't supposed to alloc/init the AppDelegate class
everywhere in your app just to get the context. What is the best way
for accessing it throughout your code? (I notice most examples
write //assume context exists.....)
Thanks in advance.
Daniel
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