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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?
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The keys in a dictionary can be a reference type, i.e., objects. When an object is used as the key, the virtual methods "GetHashCode()" & "Equals()" can change how the dictionary search for the entries depending on if they are overridden, and how they are overridden.
An object representing a dynamic collection of key-value pairs, for use instead of a Dictionary variable in cases that require reference semantics.
An NSDictionary will retain it's objects, and copy it's keys.
Use -mutableCopy . NSDictionary *d; NSMutableDictionary *m = [d mutableCopy]; Note that -mutableCopy returns id ( Any in Swift) so you will want to assign / cast to the right type. It creates a shallow copy of the original dictionary.
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 :-)
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