How do I cache something for a tableview?

Question

I have a tableview with large images that fill the cells and the row heights are set based on the image size. Unfortunately, the table jerks badly when scrolling to the next cell.

I've been told that my tableview will scroll more smoothly if I cache the row heights and the images before they are loaded into the table. All my data are stored in a plist.

How do I go about caching something? What does the code look like and where does it go?

Thanks!

Here's my code for loading the images:

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

    static NSString *detailTableViewCellIdentifier = @"Cell";

    DetailTableViewCell *cell = (DetailTableViewCell *) 
        [tableView dequeueReusableCellWithIdentifier:detailTableViewCellIdentifier];

    NSArray *nib = [[NSBundle mainBundle] loadNibNamed:@"DetailTableViewCell" owner:self options:nil];
    for(id currentObject in nib)
    {
        cell = (DetailTableViewCell *)currentObject;
    }
    AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
    NSString *Path = [[NSBundle mainBundle] bundlePath];
    NSString *MainImagePath = [Path stringByAppendingPathComponent:([[appDelegate.sectionsDelegateDict objectAtIndex:indexPath.section] objectForKey:@"MainImage"])];

    cell.mainImage.image = [UIImage imageWithContentsOfFile:MainImagePath];

    return cell;
}

I'm also using the following for calculating the row height:

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath{
    AppDelegate *appDelegate = (DrillDownAppAppDelegate *)[[UIApplication sharedApplication] delegate];
    NSString *Path = [[NSBundle mainBundle] bundlePath];
    NSString *MainImagePath = [Path stringByAppendingPathComponent:([[appDelegate.sectionsDelegateDict objectAtIndex:indexPath.section] objectForKey:@"MainImage"])];
    UIImage *imageForHeight = [UIImage  imageWithContentsOfFile:MainImagePath]; 
    imageHeight = CGImageGetHeight(imageForHeight.CGImage);  
    return imageHeight;
}

EDIT: Here is the final code below.

#define PHOTO_TAG 1
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
 static NSString *CellIdentifier = @"Photo";

 UIImageView *photo;
 UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];

 AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
 UIImage *theImage = [UIImage imageNamed:[[appDelegate.sectionsDelegateDict objectAtIndex:indexPath.section] objectForKey:@"MainImage"]];

 imageHeight = CGImageGetHeight(theImage.CGImage);
 imageWidth = CGImageGetWidth(theImage.CGImage);

if (cell == nil) {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
    photo = [[[UIImageView alloc] initWithFrame:CGRectMake(0, 0, imageWidth, imageHeight)] autorelease];
    photo.tag = PHOTO_TAG;
    [cell addSubview:photo];
   } else {
    photo = (UIImageView *) [cell viewWithTag:PHOTO_TAG];
    [photo setFrame:CGRectMake(0, 0, imageWidth, imageHeight)];
   }

 photo.image = theImage;
 return cell;
 }

Answer 1

Caching is not a panacea for tableview performance. Caching is only valuable if there is something expensive to calculate, and you can avoid calculating it. If, on the other hand, you simply have too many views in your UITableViewCell, then caching will do nothing for you. If your row heights are all the same, then there's nothing to cache. If you use +[UIImage imageNamed:], then the system is already caching your images for you.

The most common first-order problem with UITableViewCells is putting too many subviews in them. How have you constructed your cell? Have you spent time studying the Table View Programming Guide, particularly A Closer Look at Table-View Cells? Understanding this document will save you much grief later.

EDIT: (Based on code above)

First, you're fetching a reusable cell, and then immediately throwing it away, reading a NIB and iterating over all the top level objects looking for a cell (one that looks almost exactly like the one you just threw away). Then you work out a string, which you use to open a file and read the contents. You do this every time UITableView wants a new cell, which is a lot. And you do it over and over again for the same rows.

Then, when UITableView wants to know the height, you read the image off of disk again. And you do that every time UITableView asks (and it may ask many times for the same row, though it does try to optimize this).

You should start by reading the UITableView Programming Guide I link above. That's hopefully going to help a lot. When you've done that, here are the things you should be thinking about:

• You indicated that there is nothing but a single image view in this cell. Do you really need a NIB for that? If you do stick with a NIB (and there are reasons to use them in some case), then read the Programming Guide about how to implement a NIB-base cell. You should be using IBOutlet, not trying to iterate over the top-level objects.

• +[UIImage imageNamed:] will automatically find files in your Resources directory without you having to work out the bundle's path. It will also cache those images for you automatically.

• The point of -dequeueReusableCellWithIdentifier: is to fetch a cell that UITableView is no longer using and that you can reconfigure rather than you making a new one. You're calling it, but you immediately throw it away. You should check if it returned nil, and only load it out of the NIB if it did. Otherwise, you just need to change the image. Again, read the Programming Guide; it has many, many examples of this. Just make sure that you really try to understand what -dequeueReusableCellWithIdentifier: is doing, and don't treat it as just something you type at this point in the program.

Answer 2

If you do need to cache the heights, I did something like this (caching heights for a cell displaying an "article" object - article maybe one of several subclasses):

+ (CGFloat) heightForArticle: (Article*) article atWidth: (CGFloat) width {

    static NSCache* heightCache = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        heightCache = [NSCache new];
    });

    NSAssert(heightCache, @"Height cache must exist");

    NSString* key = @"unique"; //Create a unique key here
    NSNumber* cachedValue = [heightCache objectForKey: key];
    if( cachedValue ) 
        return [cachedValue floatValue];
    else {
        CGFloat height = 40;//Perform presumably large height calculation here

        [heightCache setObject: [NSNumber numberWithFloat: height] forKey: key];

        return height;
    }
}