cellForRowAtIndexPath memory management

Question

So, here\'s the code for my cellForRowAtIndexPath:

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


        

Answer 1

In order to be more efficient with regards to memory usage and to get smoother scrolling performance, you should create a custom UITableViewCell subclass which contains a custom UIView as subview of the UITableViewCell's contentView. You should draw on this custom view using -drawRect:. Having a big view hierarchy is very memory intensive, as opposed to having just 1 view on which all content gets drawn on directly.

Apple's TableViewSuite examples actually contains an example of this direct drawing approach. Download the project here:

http://developer.apple.com/library/ios/#samplecode/TableViewSuite/Introduction/Intro.html

Check out the fifth example to get an idea on how to implement high performance, low memory custom tableview cell drawing in your own project.

Answer 2

in one word, leak reason is additional reference by addSubview. so ensure addSubview once or remove it before adding new one.

Answer 3

Why not build your cell in IB and subclass UITableViewCell to tailor it to your needs (3 UILabels and a UIImageView)? I have designs with complex tableview cells and this system works well for me. Keep the code much cleaner and easier to manage. If you need a working example, I can send it to you.

Answer 4

Try setting a static string and use that in the methods below:

    static NSString *identifier = @"identifier";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:identifier];
if (cell == nil) {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:identifier]autorelease];

Answer 5

You're leaking a lot of labels there. Every time you alloc/init a new label you need to release it. Currently you are re-using the same label variable several times by assigning it a new label object without releasing the old one.

Instead of putting your [label release] at the end, put [label release] after every [cell addSubview:label];

Your next problem is you've misunderstood how the table cell recycling works. UITableCell objects are reused over and over again within a table, that why you do this bit:

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"identifier"];

if (cell == nil) {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:@"identifier"]autorelease];
}

That's basically saying "check if there's a cell available, and if not create a new one". When a cell is re-used it keeps all the old stuff you've added to it. So when you add those labels and image view to the cell, they stay in that cell the next time it is used, and the next time.

But because you add the labels again each time the cell is re-used, it builds up multiple copies of the labels, so the second time the cell is displayed it has twice as many labels, and the next time 3 times as many, and so on. You can't see the duplicates because they're in exactly the same place, but they're there using up memory and slowing your table down.

You need to move the code where you append new views to the cell inside the "if (cell == nil) { ... }" statement, so the labels are only added once when the cell is created.

Of course that means that every cell will have the same text and image, which you don't want, so you need to split out the logic where you set the text and images and put it after the if statement. That way you only create the labels once but you set the text each time they are displayed in a different table row. That's a bit tricky because you don't have the references to the labels any more, but you can do it by setting unique tags on the labels so you can find them again.

Here's a cleaned-up version with all the leaks removed:

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

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"identifier"];

    NSInteger artistTag = 1;
    NSInteger albumTag = 2;
    NSInteger dateTag = 3;
    NSInteger imageTag = 4;

    if (cell == nil) {

        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:@"identifier"]autorelease];

        // ARTIST
        CGRect frame = CGRectMake(59, 11, 244, 13);
        UILabel *label = [[UILabel alloc]initWithFrame:frame];
        label.font = [UIFont boldSystemFontOfSize:13];
        label.textColor = [UIColor blackColor];
        label.tag = artistTag;
        [cell addSubview:label];
        [label release];

        // ALBUM (more like description...
        frame = CGRectMake(60, 30, 244, 11);
        label = [[UILabel alloc]initWithFrame:frame];
        label.font = [UIFont boldSystemFontOfSize:11];
        label.textColor = [UIColor darkGrayColor];
        label.tag = albumTag;
        [cell addSubview:label];
        [label release];

        // DATE
        frame = CGRectMake(59, 49, 244, 10);
        label = [[UILabel alloc]initWithFrame:frame];
        label.font = [UIFont fontWithName:@"Helvetica" size:10.0];
        label.textColor = [UIColor darkGrayColor];
        label.textAlignment = UITextAlignmentRight;
        label.tag = dateTag;
        [cell addSubview:label];
        [label release];

        // IMAGE
        UIImageView *imageView = [[UIImageView alloc]init];
        imageView.frame = CGRectMake(8,9,44,44);
        imageView.tag = imageTag;
        [imageView.layer setMasksToBounds:YES];
        [imageView.layer setCornerRadius:3.0];
        [[cell contentView] addSubview:imageView];
        [imageView release];
    }

    NSInteger artistIndex = 1; // 1
    NSInteger albumIndex = 3;  // 3
    NSInteger dateIndex = 6;   // 6
    NSInteger imageIndex = 8;  // 5

    ((UILabel *)[cell viewWithTag:artistTag]).text = [[musicList.list objectAtIndex:indexPath.row] objectAtIndex:artistIndex];

    ((UILabel *)[cell viewWithTag:albumTag]).text = [[musicList.list objectAtIndex:indexPath.row] objectAtIndex:albumIndex];

    ((UILabel *)[cell viewWithTag:dateTag]).text = [[musicList.list objectAtIndex:indexPath.row] objectAtIndex:dateIndex];

    ((UIImageView *)[cell viewWithTag:imageTag]).image = [[musicList.list objectAtIndex:indexPath.row] objectAtIndex:imageIndex];


    return cell;
}