Question

I am confused by the memory management in this scenario.
In my app user makes periodic input inside UITextField tf and the typed strings (NSString*) are stored as elements of a MSMutableArray *arr through addObject. The stored collection is displayed inside a UITableView. My app can go into bkgr and is periodically awakened by push notifications. As I understand it, the data stored in arr can be lost while my app is non-active and, to preserve it, I need to do archive/restore.
My archive/restore are using

NSUserDefaults*prefs;  
[prefs setObjectForKey:x forKey:key] 

to archive and

[prefs objectForKey:key]

to restore every item of arr.

Question1: I think that to prevent the memory leak I need to do [arr release]
Do I also need to do a release on every object which I have added to arr or, since I did not allocate the NSString for tf, it will be done for me automatically?

Question2: in restore I start with something like arr=[[NSMutableArray alloc] initWithObjects:nil]; before I can read and add archived items back to arr. I think that [prefs objectForKey:key] is released as soon as I leave the scope in which it was read – thus I need something like retain to keep it in arr. Would this schema work in the next archive/restore cycle due to another app deep sleep?
Is there a cleaner way of achieving the same?
Thanks.
Victor

Answer 1

Adding objects to an NSArray causes the NSArray to retain each object.

So in a case where you are instantiating objects, then adding them to an array, those objects do not need to be further retained:

// saving strings inside an array, then array to the NSUserDefaults
NSString *string1 = @"My String 1";
NSString *string2 = @"My String 1";
NSMutableArray *arr = [[NSMutableArray alloc] initWithCapacity:10];
[arr addObject:string1];
[arr addObject:string2];

NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];;  
[prefs setObject:arr forKey:@"MyArray"];

[arr release];

Then to restore the entire array from prefs:

NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults]; 
NSArray *array = [prefs objectForKey:@"MyArray"];

Alternately, to save strings under separate keys, it would be something like this:

[prefs setObject:[arr objectAtIndex:0] forKey:@"MyFirstStringKey"];
[prefs setObject:[arr objectAtIndex:1] forKey:@"MySecondStringKey"];

For the restore, you will also just add the items to the array, no retain required:

// assuming this time several keys added to an array
// also note using autoreleased version of array - much easier
NSMutableArray *arr = [NSMutableArray arrayWithCapacity:10];
NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];;  
[arr addObject:[prefs objectForKey:@"MyFirstStringKey"]];
[arr addObject:[prefs objectForKey:@"MySecondStringKey"]];

// then assign arr or use it otherwise

Also easier still is to use a non-mutable array and instantiate the array with the list of objects you want to have on the array:

NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];;  
NSArray *arr = [NSArray arrayWithObjects:[prefs objectForKey:@"MyFirstStringKey"], [prefs objectForKey:@"MySecondStringKey"], nil];