来自NSSet的NSArray - 我必须自己排序吗？

Question

I've got data in an NSSet, and I need to get it into an NSArray.

Do I need to sort it myself (again, this came from Core Data) or can I get it out in a sorted order?

Answer 1

You can specify a sort when you retrieve data with an NSFetchRequest by setting the sortDescriptors property to an array of NSSortDescriptor s. But if you already have it in an NSSet and don't want to make another fetch request, you can use:

[[theSet allObjects] sortedArrayUsingDescriptors:sortDescriptors];

It'll create an interim NSArray when you call allObjects and then another sorted array afterwards, so it's not 100% efficient, but the overhead should be negligible for reasonably-sized data sets (and certainly less than the cost of sorting).

Edit

Actually, I was wrong - NSSet has the sortedArrayUsingDescriptors: method too. So you can just call [theSet sortedArrayUsingDescriptors:descriptors] and do it all in one go.

Answer 2

You've got to sort it yourself, but it's not very hard...

NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Sprocket" inManagedObjectContext:managedObjectContext];
[request setEntity:entity];

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"sortVariable" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];   
[request setSortDescriptors:sortDescriptors];

NSError *error;
NSMutableArray *mutableFetchResults = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy];    
self.sprocketArray = mutableFetchResults;

[sortDescriptors release];
[sortDescriptor release];
[mutableFetchResults release];
[request release];