Why does NSUserDefaults fail to save NSMutableDictionary? - iphone

Question

I’m trying to save a NSMutableDictionary with NSUserDefaults. I read many posts on the topic in stackoverflow... I also found one option that worked; however unfortunately it worked only once and then it started to save (null) only.
Does anybody have a hint?
Thanks
Code to save:
[[NSUserDefaults standardUserDefaults] setObject:[NSKeyedArchiver archivedDataWithRootObject:dictionary] forKey:#"Key"];
[[NSUserDefaults standardUserDefaults] synchronize];
Code to load:
NSMutableDictionary *dictionary = [[NSMutableDictionary alloc]init];
NSData *data = [[NSUserDefaults standardUserDefaults]objectForKey:#"Key"];
dictionary = [NSKeyedUnarchiver unarchiveObjectWithData:data];
Code to add Objects to the NSMutableDictionary:
[dictionary setObject:[NSNumber numberWithInt:0] forKey:#"Key 1"];
[dictionary setObject:[NSNumber numberWithInt:1] forKey:#"Key 2"];
[dictionary setObject:[NSNumber numberWithInt:2] forKey:#"Key 3"];
Code to NSLog() values:
for (NSString * key in [dictionary allKeys]) {
NSLog(#"key: %#, value: %i", key, [[dictionary objectForKey:key]integerValue]);
}
And also the keys are (null):
NSLog(#"%#"[dictionary allKeys]);

Answer 1

From Apple's documentation for NSUserDefaults objectForKey:
The returned object is immutable, even if the value you originally set was mutable.
The line:
dictionary = [NSKeyedUnarchiver unarchiveObjectWithData:data];
discards the previously created NSMutableDictionary and returns a NSDictionary.
Change the loading to:
NSData *data = [[NSUserDefaults standardUserDefaults]objectForKey:#"Key"];
dictionary = [NSKeyedUnarchiver unarchiveObjectWithData:data];
Complete example, there is also no need to use NSKeyedArchiver in this example:
NSDictionary *firstDictionary = #{#"Key 4":#4};
[[NSUserDefaults standardUserDefaults] setObject:firstDictionary forKey:#"Key"];
NSMutableDictionary *dictionary = [[[NSUserDefaults standardUserDefaults] objectForKey:#"Key"] mutableCopy];
dictionary[#"Key 1"] = #0;
dictionary[#"Key 2"] = #1;
dictionary[#"Key 3"] = #2;
for (NSString * key in [dictionary allKeys]) {
NSLog(#"key: %#, value: %#", key, [dictionary objectForKey:key]);
}
NSLog output:
key: Key 2, value: 1
key: Key 1, value: 0
key: Key 4, value: 4
key: Key 3, value: 2