ios - Memory management using ARC on iOS

Question

Just have a quickly question (more of a curiosity thing) based on a problem I just solved (I will post the answer to my problem in the post, which can be found here: My former question

The thing is that I have this UITableView which contains custom cell objects. Every time you enter this view, I generate new cells for the UITableView like this:

    if (cell == nil)
{
    [[NSBundle mainBundle] loadNibNamed:@"UploadCellView" owner:self options:nil];

    cell = customCell;
}

Which happens in the standard method:

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

Now the problem is that my custom cell objects listens for NSNotifications about upload objects happening in the background, so they can update its model data to their labels and progress bars etc. It happens like this (this is a method from the custom cell objects):

-(void) uploadProgress: (NSNotification*)notification
{
NSDictionary *userInfo = [notification userInfo];

NSNumber *uploadID = [userInfo valueForKey:@"uploadID"];

if (uploadID.integerValue == uploadActivity.uploadID)
{
    UIProgressView *theProgressBar = (UIProgressView*)[self viewWithTag:progressBarTag];

    [theProgressBar setProgress:(uploadActivity.percentageDone / 100) animated:YES];

    UILabel *statusText = (UILabel*)[self viewWithTag:percentageTag];

    [statusText setText:[NSString stringWithFormat:@"Uploader - %.f%% (%.01fMB ud af %.01fMB)", uploadActivity.percentageDone, uploadActivity.totalMBUploaded, uploadActivity.totalMBToUpload]];
}
}

When an upload finish they simply do this:

-(void) uploadFinished: (NSNotification*)notification
{
NSDictionary *userInfo = [notification userInfo];

NSNumber *uploadID = [userInfo valueForKey:@"uploadID"];

if (uploadID.integerValue == uploadActivity.uploadID)
{        
    [self setUploadComplete];

    [[ApplicationActivities getSharedActivities] markUploadAsFinished:uploadActivity];

    NSLog(@"BEGINNING RELOAD");
    [parentTable reloadData];
    NSLog(@"ENDING RELOAD");
}
}

Now the problem is when they call their owning tableview. When the view which the tableview is contained within dismisses, the old custom cell objects are still alive in the background getting NSNotfications. And when that upload is then done, the old custom cell objects from the former table views still tries to call that parentTable property which was set at that time, now resulting in calling random junk memory.

The way I solved this was to keep an array of all cell objects getting created in the table and then make them stop listening when the view is dismissed like this:

-(void) viewWillDisappear:(BOOL)animated
{
    for (UploadCell *aCell in lol)
    {
        [aCell stopListening];
    }

    [self.navigationController popViewControllerAnimated:YES];
}

But this seems like a bit of a hack. How would I go about making sure that the custom cell objects are deleted when the view is dismissed? Because when the view is intialized again, new cells are simply made anyways, so I have no use for the old ones.

The custom view cells have a strong property pointer to the tableview they get associated with, but I thought the ARC would make sure that TableView pointer would not get invalidated then? Obviously it is somehow. Maybe because of the containing view being deleted when popped?

Answer 1

Sounds like the cells have a retain property pointing back to your UITableViewDataSource class.

They should instead have an assign property, then they will be released properly when the table view is released (which it currently cannot be if your cells are retaining it).

Also, the cells should shut down notifications when they are dropped out of the tableview, by overriding the cells didMoveToSuperview method:

- (void)didMoveToSuperview
{
    [super didMoveToSuperview];
    if ( [self superview] == nil )
    {
        [self unsubscribeFromYourNotifications];
    }
}

That is so if they scroll off screen they will not be wasting resources updating things.

Answer 2

Have you considered a separate update model that keeps a map between uploadIDs and cells that listens for the notification? That way, the cells aren't responsible for updating the table themselves, the update model would do it. When the table goes away, you can shut down the update model.