iOS tableView cell caching issues not loading every cell

Question

I'm having a bit of difficulty understanding what is going wrong and how to fix this.

-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = @"Cell";

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if(cell == nil)
{
    cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];


    NSUInteger row = [indexPath row];

    cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
    cell.selectionStyle = UITableViewCellSelectionStyleBlue;

    UILabel *where = [[UILabel alloc] initWithFrame:CGRectMake(88.0, 0.0, 155.0, 22.0)];
    where.text = [delegate.destinationArray1 objectAtIndex:row];
    where.font = [UIFont fontWithName:@"Helvetica" size:12.0];
    where.textColor = [UIColor blueColor];
    [cell.contentView addSubview:where];
    return cell;
}

This doesn't work properly but this does:

-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = @"Cell";

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if(cell == nil)
{
    cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
}

NSUInteger row = [indexPath row];

    cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
    cell.selectionStyle = UITableViewCellSelectionStyleBlue;

    UILabel *where = [[UILabel alloc] initWithFrame:CGRectMake(88.0, 0.0, 155.0, 22.0)];
    where.text = [delegate.destinationArray1 objectAtIndex:row];
    where.font = [UIFont fontWithName:@"Helvetica" size:12.0];
    where.textColor = [UIColor blueColor];
    [cell.contentView addSubview:where];
    return cell;
}

They both get populated by "delegate.destinationArray1" but when all the code is inside the curly braces of

if(cell == nil)

the list gets unordered and repeats cells and misses some out. I can't use the latter way as it creates a MASSIVE memory leak when scrolling.

Any ideas?

Answer 1

I did the exact same thing when I started using UITableViews. The reason for the memory leak is that in the second implementation (the one that works) you are actually creating every cell, every single time. Let me try to explain a bit more.

You never want to set content of a cell between the (cell == nil). The reason for this is the reuseIdentifier. If the table needs to display a new cell it will grab one and see if it has already been alloced/inited. If it has it will just use it. If that is the case the content will already be set in the cell you grabbed and you are not telling it to set it any differently.

between the (cell == nil) only create and establish the view. Not the content. All content should be set after. So then no matter what cell it grabs it can always set the content. So this is what you want:

-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
     static NSString *CellIdentifier = @"Cell";

     UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
     if(!cell) // or (cell == nil)
     {
          cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
          cell.selectionStyle = UITableViewCellSelectionStyleBlue;
          UILabel *where = [[UILabel alloc] initWithFrame:CGRectMake(88.0, 0.0, 155.0, 22.0)];
          where.font = [UIFont fontWithName:@"Helvetica" size:12.0];
          where.textColor = [UIColor blueColor];
          where.tag = 1;
          [cell addSubview:where];
     }

     NSUInteger row = indexPath.row;

     UILabel *where = [cell viewWithTag:1];
     where.text = [delegate.destinationArray1 objectAtIndex:row];

     return cell;
}

I just coded this in StackoverFlow so sorry if there are any small syntax errors.