How can I speed up a UITableView?

Question

I have a UITableView with about 400 cells in 200 sections and it's a little sluggish in responding to user interaction (scrolling, selecting cells.) I've made sure the methods for retrieving cells and header views do the bare minimum as it's running, and I don't think I'm doing anything out of the ordinary to make it slow. The cells and headers just have a background image and text. Has anyone else had this kind of problem, and do you know any way to make it run a little faster?

Edit: I'm offering a bounty because I'd love to get some useful feedback on this. I don't think the answer lies in a problem in my code. Instead I'm looking for strategies to re-engineer the UITableView so that it runs faster. I'm totally open to adding new code and I look forward to hearing what you guys have to say.

Sluggishness is observed on both the simulator and my device, an iPhone 4. Here are my implementations of viewForHeaderInSection and cellForRowAtIndexPath, which are the only UITableViewDelegate methods implemented nontrivially. I am reusing cells and header views.

- (UIView*)tableView:(UITableView *)tableView viewForHeaderInSection:(NSInteger) section
{
    HaikuHeaderView* view= [m_sectionViews objectAtIndex:section];
    NSMutableArray* array= [m_haikuSearch objectAtIndex:section];
    Haiku* haiku= [array objectAtIndex:0];

    [view.poetLabel setText:[haiku nameForDisplay]];

    return view;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"Cell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithFrame:CGRectZero reuseIdentifier:CellIdentifier] autorelease];

        cell.backgroundView= [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"cell gradient2.png"]];

        // (Set up a bunch of label attributes in the cell...)
    }

    NSMutableArray* array= [m_haikuSearch objectAtIndex:indexPath.section];
    Haiku* haiku = [array objectAtIndex:indexPath.row];
    cell.textLabel.text = [haiku.m_lines objectAtIndex:0];

    return cell;
}

Answer 1

Even if your cell is actually that simple (background image and label) there are some things to consider

Image caching This is the obvious thing - if you are using the same image everywhere, load it once into the UIImage and reuse it. Even if the system will cache it on its own, directly using the already loaded one should never hurt.

Fast calculation Another rather obvious thing - make calculating height and content as fast as possible. Don't do synchronous fetches (network calls, disk reads etc.).

Alpha channel in image What's expensive when drawing is transparency. As your cell background has nothing behind it, make sure that you save your image without alpha channel. This saves a lot of processing.

Transparent label The same holds true for the label on top of your background view, unfortunately making it opaque might ruin the looks of your cell - but it depends on the image.

Custom cell In general, subclassing UITableViewCell and implementing drawRect: yourself is faster than building the subview hierarchy. You might make your image a class variable that all instances use. In drawRect: you'd draw the image and the text on top of it.

Check compositing The simulator has a tool to highlight the parts that are render-expensive because of transparency (green is ok, red is alpha-blending). It can be found in the debug menu: "Color Blended Layers"

Answer 2

The best thing you can do if you're looking to speed up your code is to profile it. There are two reasons for this:

1. You can read about some things that'll improve table performance in general, like using fixed-height cells and re-using cells, and it'll probably help to implement those things (looks like you've already done that). But when it comes to speeding up your code, you really need to know where you app is spending most of its time. It might be that there are a few methods that take a very long time, or a method that's relatively quick but gets called a lot more often than you'd expect.

2. It's impossible to know whether the changes you make in an effort to speed things up truly make a difference unless you have some numbers to measure against. If you can show that your code was spending 80% of its time in one routine and you cut that down to 35%, you know you're making progress.

So, break out Instruments and start measuring. If you can, it's a good idea to measure while you're doing each of the different activities that you want to speed up... do one profiling session while scrolling, one while selecting as many different cells as you can in a fixed period, etc. Don't forget to save the results so you can compare later.

Answer 3

Just note these points..

1. Are you reusing the cells..Which is a good practice to do..
2. Make sure you are not doing any expensive calculations in cellForRowAtIndexPath callback, or in a function called from CellForRowAtIndexPath..
3. You said there is a background image. Another reason that you must reuse your cell.

Some good info about cell reuse is here..

EDIT : Found this page very late..

This SO question thread might help you...especially the accepted answer...

Answer 4

1. use a shared image instance for the background (you alloc/init/release one for every time a new cell is created). When your table view is big , this means that the background X cells in memory takes much more memory than it should.

   instead of
   cell.backgroundView= [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"cell gradient2.png"]];
   just use :
   cell.backgroundView= [SomeHelperClass sharedBackgroundUIImageResource];

2. If that doesn't help , use CG instead of labels and other subviews (a screenshot will help here.. to know what we're talking about).

Answer 5

Does the table view's delegate implement:

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath

If so you may wish to consider setting your UITableViewCell's rowHeight property instead.

Answer 6

Are you using a lot of subviews?

If so, a good technique is to, instead of adding a lot of labels and images, draw them using CoreGraphics.

To do this, you'd have to subclass UITableViewCell and implement the -(void)drawRect:(CGRect)rect method.

Answer 7

Two suggestions: One is to use -initWithStyle:reuseIdentifier: for your table view cells instead of -initWithFrame:. The other is to comment out setting the cell.backgroundView to an image with a gradient and see if that's the culprit. Every time I've had poor performance in a table view it's been because of an image.