使用nsuserdefaults中的数据时，应用程序崩溃

Question

I have a class "MainClass". That class implements NSCoding protocol. In that class I have an array with objects of another class "Object", which also implements NSCoding protocol. When I archive an object of "MainClass" in NSUserDefaults and then unarchive it and try to get the object of class "Object" from array and use it properties then I get message:

[NSConcreteMutableData property]: unrecognized selector sent to instance.

What do I do wrong? How to fix it?

Edit: This is code I use with NSUserDefaults:

- (Settings *) readData
{
     NSUserDefaults *currentDefaults = [NSUserDefaults standardUserDefaults];
     NSData *dataRepresentingSettings = [currentDefaults objectForKey:@"data"];

     if (dataRepresentingSettings != nil) {
             return (MainClass *)[NSKeyedUnarchiver unarchiveObjectWithData:dataRepresentingSettings];
     }

     return nil;
}

- (void) saveData
{
     [[NSUserDefaults standardUserDefaults] setObject:[NSKeyedArchiver archivedDataWithRootObject:self] forKey:@"data"];
     [[NSUserDefaults standardUserDefaults] synchronize];
}

Answer 1

There's two options :

(1) You're not retaining the Object that you got from the user defualts. When you get an object from NSUserDefaults it's autoreleased - if you want to keep it, you have to retain it.

(2) You've forgotten to unarchive it. You used NSKeyedArchiver to archive your Object but didn't unarchive it :)

you might have done something like

Object *object = [defaults objectForKey:@"object"];

instead of

NSData *data = [defaults objectForKey:@"object"];
Object *object = [NSKeyedUnarchiver unarchiveObjectWithData:data];

NB Don't forget that object in my example is still autoreleased; if you want to keep it, you have to retain it ;)