Objective C: Serializing/Archiving issues

I'm an Android dev who's recently started IOSing. In all my of my Android projects, I have an IO class that does the following:

public static Object load(String fname , Context cont)
{
    FileInputStream fis = cont.openFileInput(fname);
    ObjectInputStream ois = new ObjectInputStream(fis);
    Object loadedObject = ois.readObject();
    ois.close();
    fis.close();
    Father.print("Loaded from file "+fname+" successfully");
    return loadedObject;
}   

and the equivalent for save. It works great, I can load and save instances from anything with it.

In Objective C, I'm using NSKeyedArchiver/NSKeyedUnarchiver. Archiving seems to work, but unarchiving causes this:

*** -[NSKeyedUnarchiver initForReadingWithData:]: data is empty; did you forget to send -finishEncoding to the NSKeyedArchiver?

I'm using the following static methods:

+(BOOL)saveObject: (id<NSCoding>)obj forKey:(NSString*)key;
{
        NSKeyedArchiver *archiver;
        NSMutableData *bytes = [NSMutableData data];
  开发者_开发知识库      archiver = [[NSKeyedArchiver alloc] initForWritingWithMutableData:bytes];
        [archiver encodeObject:obj forKey:key];
        [archiver finishEncoding];
        NSLog(@"Successfully saved object <%@> to %@", obj, key);
        return TRUE;
}

and

+(id)loadForKey:(NSString*)key
{
    NSMutableData *bytes = [NSMutableData data];
    NSKeyedUnarchiver *unrar = [[NSKeyedUnarchiver alloc] initForReadingWithData:bytes];
    id result = [unrar decodeObjectForKey:key];
    [unrar finishDecoding];
    NSLog(@"Successfully loaded object <%@>", result);

    return result;
}

I'm calling them as such

[IVIO saveObject:thing forKey:@"thekey"];
Thing *miracle = [IVIO loadForKey:@"thekey"];

What's going on?

Thank you

In your +loadForKey:

NSMutableData *bytes = [NSMutableData data];
NSKeyedUnarchiver *unrar = [[NSKeyedUnarchiver alloc] initForReadingWithData:bytes];

you're trying to read from an empty data object. Creating a mutable data object using [NSMutableData data] just gives you an object that contains nothing. You'll need to READ the data from somewhere. Likewise, in your +saveObject:forKey: you successfully archive the object, but you don't do anything with the resulting data object.

To fix, you'll want to write the data into a file or URL in your save method, and read it from a file or URL in your load method.