Using an object as key for NSDictionary

Question

I have a bunch of Lessons and the class works great. There is a view controller that works with these lessons. This controller needs to know the upload status of a lesson so we have a NSDictionary with the Lesson as key and an NSNumber having the percent of upload status.

This is a problem because after you insert a Lesson you want to later do a lookup on that same Lesson (perhaps in cellForRowAtIndexPath:) to get the progress. This does not work because keys are copied in NSDictionary.

Is it good form to save and fetch keys with something like this:

NSNumber *key = [NSNumber numberWithUnsignedInt:[obj hash]];
[dict setObject:@"... upload progress" forKey:key];

Or there a better approach?

Answer 1

I have used this technique many times in the past, and have had great success by wrapping the key-objects into an NSValue:

NSValue *myKey = [NSValue valueWithNonretainedObject:anInstance];
id anItem =[myDict objectForKey:myKey];

https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/nsvalue_Class/Reference/Reference.html#//apple_ref/occ/clm/NSValue/valueWithNonretainedObject:

(forgive the formatting; I'm on iPhone. I'll format later :-)