When should I release my array?

Question

I am parsing some JSON from the internet and then adding them to an array which is the datasource for my UITableView. I am not sure when I should be releasing my array?

.h: items

@property(nonatomic,retain)NSMutableArray*  items;

.m: connectionDidFinishLoading

// fetch succeeded    
    NSString* json_string = [[NSString alloc] initWithData:retrievedData encoding:NSUTF8StringEncoding];

    //Check ST status
    int status =  [[[[json_string objectFromJSONString] valueForKey:@"response"] valueForKey:@"status"]intValue];
    //NSLog(@"Status: %d", status);

    items = [[NSMutableArray alloc] init];
    NSDictionary* messages = [[NSDictionary alloc] init]; 

    switch (status) {
        case 200:
            messages = [[[json_string objectFromJSONString] valueForKey:@"messages"] valueForKey:@"message"];

            for (NSDictionary *message in messages)
            {
                [items addObject:message];
            }
            [self.tableView reloadData];
        break;

        default:
        break;
    }

Answer 1

One, you might want to declare items as an instance of NSMutableArray if you intend to call addObject: on it.

Two, declare it as a property so that if you end up getting it multiple times the older value will be released when you do.

self.items = [NSMutableArray array];

And the correct point of releasing it would be dealloc.

Answer 2

Probably you don't want to release it immediately if you:

• use didSelectRowAtIndexPath: method for detail views and pass this data to them
• define custom UITableViewCell styles in cellForRowAtIndexPath: method
• use this data elsewhere

Best practice is declare an instance variable and synthesize it in .m, use in appropriate operations and release in dealloc method.

One possible release point that you could use is where you refresh your data that shown on table.

Example:

I get dictionaries in an array from an API in my app and use something like that.

MyTableViewController.h

@interface MyTableViewController {
    NSMutableArray *items;
}

@property (nonatomic, retain) NSMutableArray *items;

@end

MyTableViewController.m

@implementation MyTableViewController

@synthesize items;

- (void)dealloc
{
    [items release];
    [super dealloc];
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return [items count];
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {

    static NSString *cellIdentifier = @"FilesCellIdentifier";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier] autorelease];
    }

    cell.textLabel.text = [[items objectAtIndex:indexPath.row] valueForKey:@"name"];
    cell.imageView.image = [UIImage imageNamed:[[NSString alloc] initWithFormat:@"filetype_%@.png", [[items objectAtIndex:indexPath.row] valueForKey:@"type"]]];

    return cell;
}

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
        MyDetailViewController *detailViewController = [[MyDetailViewController alloc] initWithNibName:@"MyDetailViewController" bundle:[NSBundle mainBundle]];
        detailViewController.item = [items objectAtIndex:indexPath.row];
        [self.navigationController pushViewController:detailViewController animated:YES];
        [detailViewController release];
        detailViewController = nil;
    }
}

- (void)getItems
{
    [items release];
    items = [[NSMutableArray alloc] init];

    //Do some requests here

    for (NSDictionary *dict in results)
    {
        [items insertObject:dict atIndex:0];
    }

    [self.tableView reloadData];
    [self stopLoading];
}

@end