CODEWITHSUNDEEP
objective-cios4types
arlomedia
arlomedia

Reputation: 9071

need help converting an MPMediaItemPropertyPersistentID to a string and back again

I'm getting the ID of a media item from the MPMediaPickerController. According to the documentation, this value is an NSNumber object containing a uint64_t (unsigned long long). I would like to convert it to an NSString for saving in my data model. However, when I convert it to a string its value changes, and when I convert it back to a number it changes again! Obviously I'm not understanding something about these data types. Can you help?

MPMediaItem *mediaPicked;

// set mediaPicked to an MPMedia item using MPMediaPickerController...

NSLog(@"id as number: %qu", [mediaPicked valueForProperty:MPMediaItemPropertyPersistentID]); // outputs 566042331449280
NSLog(@"id as string: %@", [[mediaPicked valueForProperty:MPMediaItemPropertyPersistentID] stringValue]); // outputs 16204893883745507648
NSLog(@"id as number: %qu", [[[mediaPicked valueForProperty:MPMediaItemPropertyPersistentID] stringValue] longLongValue]); // outputs 9223372036854775807

If I try to play the media item before this conversion, it always works. But if I try to play the media item after this conversion, only about half of the media items I've tried work. So some ID values survive the conversion and some don't.

Upvotes: 2

Views: 3027

Answers (1)

matt
matt

Reputation: 535945

You are starting by saying this:

NSLog(@"id as number: %qu", [mediaPicked valueForProperty:MPMediaItemPropertyPersistentID]); // outputs 566042331449280

But that's wrong. %qu means "this thing is an unsigned long long". But this thing is not an unsigned long long. It's an object! It's an NSNumber wrapped around an unsigned long long. You are lying to NSLog, so you're getting garbage output in your very first statement.

Now, try this on your own machine:

uint64_t x = 16204893883745507648ULL;
NSLog(@"%qu", x);
NSNumber* n = [NSNumber numberWithUnsignedLongLong:x];
NSLog(@"%@", n);
NSLog(@"%@", [n stringValue]);

All of those NSLog statements give the same result - because they are all correct formulations, unlike the one you started with. So, those NSLog statements show you the kind of thing you ought to be saying.

Now, you might think: Oh, great, so I can get from an NSNumber to an NSString with stringValue after all. Yes, but you can't get back again. We cannot get from [n stringValue] to a correct NSNumber by using longlongValue, because a long long is not an unsigned long long. There is no unsignedLonglongValue. So you can't get there from here.

So what's the right thing to do? Don't convert at all! You've got an NSNumber, it's valid, just keep it and use it. An NSNumber is a value you can store in your model. (For example, it can go into a dictionary as a value or as a key, it can be a value in user defaults, and so on.)

Upvotes: 6

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