Passing a NSArray between classes

Question

So, my second question based off of this application I'm teaching myself Objective C with. I have a Data Source class, which for now looks mostly like:

- (id) init
{
    if (self = [super init]){
    listNames =  [[NSArray alloc] initWithObjects: @"Grocery", @"Wedding", @"History class",@"CS Class",@"Robotics",@"Nuclear Sciences",
                  @"Video",@"Library",@"Funeral", nil];
    NSLog(@"%@",listNames);
    }
    return self;
}

The .h looks as follows:

@interface MainViewDataSource : NSObject {
    NSArray *listNames;
}

@property (nonatomic, retain) NSArray *listNames;
-(NSArray *)getListNames;

So that's where I make it. Now the problem is that when I try to get the array listNames, it returns nothing.

The following piece:

NSArray* data = [listData listNames];

Is supposed to put the information in listNames in data, but... isn't. Since I'm rather used to JAva, I'm betting this is an Objective C quirk that I don't know how to fix. Which is why I'd be here asking for help. What's the proper way to pass around NSArrays like this?

Answer 1

You have to create the implementation of the getter. The compiler can do that for you by using @synthesize:

@synthesize listNames;

You don't have to declare the getter method.

Answer 2

Are you running -init on MainViewDataSource?

MainViewDataSource *_source = [[MainViewDataSource alloc] init];
NSLog(@"listNames --> %@", [_source listNames]);
...
[_source release];

Also make sure your -dealloc method includes a release for listNames or you will have a memory leak.

Answer 3

Without seeing all of your code, I think you may need to retain the array in your other class. The idea is if you are using another classes object, you need to retain it so that it doesn't get released with MainViewDataSource object. Try...

NSArray* data = [[listData listNames] retain];

And see if you can then get your array to print.

Answer 4

I would guess you have not initialized listData. You can alloc and init it like this:

MainViewDataSource* listData = [[MainViewDataSource alloc] init];

then you can get a pointer to the array like this:

NSArray* data = [listData listNames]; 

OR

NSArray* data = listData.listNames;

Answer 5

Accessors... Accessors... Accessors...

Use them ALWAYS. No exceptions (until you learn them).

when you do:

listNames =  [[NSArray alloc] initWithObjects: @"Grocery", @"Wedding", @"History class",@"CS Class",@"Robotics",@"Nuclear Sciences", @"Video",@"Library",@"Funeral", nil];

you really want to be doing:

[self setListNames:[NSArray arrayWithObjects: @"Grocery", @"Wedding", @"History class",@"CS Class",@"Robotics",@"Nuclear Sciences", @"Video",@"Library",@"Funeral", nil];

or

self.listNames = [NSArray arrayWithObjects: @"Grocery", @"Wedding", @"History class",@"CS Class",@"Robotics",@"Nuclear Sciences", @"Video",@"Library",@"Funeral", nil];

by setting the listNames directly (no reference to self), you are not using the accessors and will open a whole world of hurt as you try to get you leaks cleared up. To get to your list, simply use the accessor:

[self listNames];

or

self.listNames;

Again, avoid the temptation to NOT use the accessors.