Working with NSUserDefaults when saving own datatype

Question

i had read this topic How to save My Data Type in NSUserDefault? and get from there this useful part of code:

MyObject *myObject = [[MyObject alloc] init];

NSData *myObjectData  = [NSData dataWithBytes:(void *)&myObject length:sizeof(myObject)];

[[NSUserDefaults standardUserDefaults] setObject:myObjectData forKey:@"kMyObjectData"];

for saving data and this for reading

 NSData *getData = [[NSData alloc] initWithData:[[NSUserDefaults standardUserDefaults] objectForKey:@"kMyObjectData"]];

MyObject *getObject;

[getData getBytes:&getObject];

its works very good when i save data in one ViewController and read it in other. but when i whant to use it in the same class:

 - (IBAction)linkedInLog:(UIButton *)sender
{
    NSUserDefaults *myDefaults = [[NSUserDefaults standardUserDefaults] objectForKey:@"linkedinfo"];
    NSData *getData = [[NSData alloc] initWithData:myDefaults];
    LinkedContainer *getObject;
    [getData getBytes:&getObject];
    if (!myDefaults) {
        mLogInView = [[linkedInLoginView alloc]initWithNibName:@"linkedInLogInView" bundle:nil];
            [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(loginViewDidFinish:)
                                                     name:@"loginViewDidFinish"
                                                   object:mLogInView];
        [self.navigationController pushViewController:mLogInView animated:YES];
        if ((FBSession.activeSession.isOpen)&&(mLinkedInIsLogegOn)) {
            mMergeButton.hidden = NO;
        }
    }
    else{
        mLinkedInIsLogegOn= YES;
        mLinkedInInfo.mConsumer = getObject.mConsumer;
        mLinkedInInfo.mToken = getObject.mToken;
    }
}

something going wrong. in @selector:loginViewDidFinish i am saving my data to NSUserDefaults:

    -(void) loginViewDidFinish:(NSNotification*)notification
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
    mLinkedInInfo.mConsumer = mLogInView.consumer;
    mLinkedInInfo.mToken = mLogInView.accessToken;
    NSData *myObjectData  = [NSData dataWithBytes:(void *)&mLinkedInInfo length:sizeof(mLinkedInInfo)];
    NSUserDefaults *lSave = [NSUserDefaults standardUserDefaults];
    [lSave setObject:myObjectData forKey:@"linkedinfo"];
    [lSave synchronize];
    if (mLinkedInInfo.mToken) {
        mLinkedInIsLogegOn = YES;
    }   
}

the program always crashes when it comes to else part. If somebody knows what I am doing wrong please help me)

error message: Thread 1 : EXC_BAD_ACCESS(code=2,address 0x8) when compiling getObject.Consumer

I explained it in my answer. It's smashing the stack. This approach you've picked up from http://stackoverflow.com/questions/11788408/how-to-save-my-data-type-in-nsuserdefaul is an **awful, horrible** approach, and I've commented as such on that question. See my answer for details. — ipmcc, Aug 29 '13 at 12:37

ipmcc · Accepted Answer · 2013-08-29T15:32:37.387

In the vast majority of cases, this is not going to be a meaningful way to serialize your object into an NSData:

MyObject *myObject = [[MyObject alloc] init];

NSData *myObjectData  = [NSData dataWithBytes:(void *)&myObject length:sizeof(myObject)];

[[NSUserDefaults standardUserDefaults] setObject:myObjectData forKey:@"kMyObjectData"];

The canonical way to do this would be for MyObject to adopt the NSCoding protocol. Based on the code you posted here, an adoption of NSCoding might look like this:

- (id)initWithCoder:(NSCoder *)coder
{
    if (self = [super init])
    {
        mConsumer = [coder decodeObjectForKey: @"consumer"];
        mToken = [coder decodeObjectForKey: @"token"];
    }
    return self;
}

- (void)encodeWithCoder:(NSCoder *)coder 
{
    [coder encodeObject:mConsumer forKey: @"consumer"];
    [coder encodeObject:mToken forKey:@"token"];
}

Once you had done that work, you would convert MyObject to and from NSData like this:

NSData* data = [NSKeyedArchiver archivedDataWithRootObject: myObject];
MyObject* myObject = (MyObject*)[NSKeyedUnarchiver unarchiveObjectWithData: data];

The code you have here is totally going to smash the stack and crash (because this line [getData getBytes:&getObject]; will cause the NSData to write bytes to the address of getObject, which is locally declared on the stack. Hence stack smashing.) Starting from your code, a working implementation might look something like this:

- (IBAction)linkedInLog:(UIButton *)sender
{
    NSData* dataFromDefaults = [[NSUserDefaults standardUserDefaults] objectForKey:@"linkedinfo"];
    LinkedContainer* getObject = (LinkedContainer*)[NSKeyedUnarchiver unarchiveObjectWithData: dataFromDefaults];
    if (!dataFromDefaults) {
        mLogInView = [[linkedInLoginView alloc]initWithNibName:@"linkedInLogInView" bundle:nil];
        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(loginViewDidFinish:)
                                                     name:@"loginViewDidFinish"
                                                   object:mLogInView];
        [self.navigationController pushViewController:mLogInView animated:YES];
        if ((FBSession.activeSession.isOpen)&&(mLinkedInIsLogegOn)) {
            mMergeButton.hidden = NO;
        }
    }
    else{
        mLinkedInIsLogegOn= YES;
        mLinkedInInfo.mConsumer = getObject.mConsumer;
        mLinkedInInfo.mToken = getObject.mToken;
    }
}

-(void) loginViewDidFinish:(NSNotification*)notification
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
    mLinkedInInfo.mConsumer = mLogInView.consumer;
    mLinkedInInfo.mToken = mLogInView.accessToken;
    NSData* objectData = [NSKeyedArchiver archivedDataWithRootObject: mLinkedInInfo];
    [[NSUserDefaults standardUserDefaults] setObject: objectData forKey: @"linkedinfo"];
    [[NSUserDefaults standardUserDefaults] synchronize];
    if (mLinkedInInfo.mToken) {
        mLinkedInIsLogegOn = YES;
    }
}

Thank you it was very useful for me) everything working perfectly) — xaoc1993, Aug 29 '13 at 15:17
The OP (@xaoc1993) suggested [this edit](http://stackoverflow.com/review/suggested-edits/2820610) which was rejected by me. I did not realize it was the OP making the suggestion. ipmcc please update if it is correct. — chue x, Aug 29 '13 at 15:31

score 3 · Answer 2 · answered Aug 29 '13 at 12:37

I agree with ipmcc 's answer, another viable option would be to add methods to your object to convert it to an NSDictionary. You could add methods to -initWithDictionary as well and should make instantiation very easy. Pull from dictionary in NSUserDefaults to use, convert to dictionary to save.

Here is an example of those 2 methods with generic data:

- (id)initWithDictionary:(NSDictionary *)dict
{
    self = [super init];

    // This check serves to make sure that a non-NSDictionary object
    // passed into the model class doesn't break the parsing.
    if(self && [dict isKindOfClass:[NSDictionary class]]) {
        NSObject *receivedFences = [dict objectForKey:@"fences"];
        NSMutableArray *parsedFences = [NSMutableArray array];
        if ([receivedFences isKindOfClass:[NSArray class]]) {
            for (NSDictionary *item in (NSArray *)receivedFences) {
                if ([item isKindOfClass:[NSDictionary class]]) {
                    [parsedFences addObject:[Fences modelObjectWithDictionary:item]];
                }
            }
        }
    }
    // More checks for specific objects here

    return self;

}

- (NSDictionary *)dictionaryRepresentation
{
    NSMutableDictionary *mutableDict = [NSMutableDictionary dictionary];
    NSMutableArray *tempArrayForFences = [NSMutableArray array];
    for (NSObject *subArrayObject in self.fences) {
        if([subArrayObject respondsToSelector:@selector(dictionaryRepresentation)]) {
            // This class is a model object
            [tempArrayForFences addObject:[subArrayObject performSelector:@selector(dictionaryRepresentation)]];
        } else {
            // Generic object
            [tempArrayForFences addObject:subArrayObject];
        }
    }

    [mutableDict setValue:[NSArray arrayWithArray:tempArrayForFences] forKey:@"fences"];

    return [NSDictionary dictionaryWithDictionary:mutableDict];
}

This is basically boilerplate code that is generated by a program I use called JSON Accelerator. It will read a JSON string returned by an API and generate object code for you. Not really a new concept, but makes created classes for API's very easy. And this bit of code works great for creating dictionaries to be saved to NSUserDefaults. Hope this helps.

Put differently, if you don't want to implement `NSCoding` yourself, convert your object to a type that already does. — ipmcc, Aug 29 '13 at 12:39
unfortunately i don't know how to implement NSCoding, now i'm reading about it and would try to implement it myself. — xaoc1993, Aug 29 '13 at 12:51
I provided an example of an NSCoding implementation based on your code in my answer. — ipmcc, Aug 29 '13 at 13:02
i know but the type of mConsumer and mToken cannot be decoded too. so i had read something about it. so i should add NSCoding delegate to my class, write encodeWithCoder and initWithCoder methods and it should work&? — xaoc1993, Aug 29 '13 at 14:12

Working with NSUserDefaults when saving own datatype

2 Answers2