How can I improve the performance for obtaining indexed sections for UITableView

Question

I am creating indexed sections for a table view to display song titles from the iPod Library. To accomplish this, I am utilizing sectionForObject:collationStringSelector: in a loop through all mediaItems from a song query. The following code executes in viewDidLoad in order to obtain the number of sections and number of rows per section of the table:

        // Go through the collection of songs and collate them into sections A-Z
        for (MPMediaItemCollection *arrayItem in itemsFromSongQuery) {
            MPMediaItem *representativeItem = [arrayItem representativeItem];   // grab the next object from the collection
            NSString *songName = [representativeItem valueForProperty: MPMediaItemPropertyTitle];
            MPMediaItemArtwork *artWork = [representativeItem valueForProperty: MPMediaItemPropertyArtwork];

            channelButtonTitles *aChannelButtonTitle = [[channelButtonTitles alloc] init];  // create an object to hold both the title and section number
            aChannelButtonTitle.channelTitle = songName;                                                               // save the song name string in this object
            aChannelButtonTitle.channelSubTitle = [representativeItem valueForProperty: MPMediaItemPropertyArtist];    // save the artists string in this object
            aChannelButtonTitle.artwork = [artWork imageWithSize:CGSizeMake(30, 30)];                                   // save the artwork UIImage object
            aChannelButtonTitle.persistentID = [representativeItem valueForProperty:MPMediaEntityPropertyPersistentID]; // save this unique number in case the user wants to play this

            // Then determine which section number it belongs to.
            // The collator will use the channelTitle property of the aChannelButtonTitle object to make this determination
            NSInteger sect = [theCollation sectionForObject:aChannelButtonTitle collationStringSelector:@selector(channelTitle)];
            aChannelButtonTitle.sectionNumber = sect;   // Save the section number that this title string should be assigned to.

            [tempTitles addObject:aChannelButtonTitle]; // Copy the channelButtonTitles object that contains the title and section number in the temp array
            [aChannelButtonTitle release];              // Release the channelButtonTitles object
        }

Unfortunately, for 2000 songs, this takes nearly 15 seconds. The time to allocate the channelButtonTitle object and the execution time of the sectionForObject:collationStringSelector: are both negligible.

It seems that much of this time is spent in the lines that obtain valueForProperty, although the line:

MPMediaItem *representativeItem = [arrayItem representativeItem];

takes 5 seconds.

How can I improve the performance of creating the indexed sections (or at least improve the time that the user is waiting for something to happen)?

Answer 1

An even better way to handle this is to use an NSOperationQueue and NSOperation. A nice tutorial that is relevant to this question can be found here.

Answer 2

For lack of any other suggestions, I found that I can cut the time in half by moving the following 2 lines into tableView:cellForRowAtIndexPath:

        MPMediaItemArtwork *artWork = [representativeItem valueForProperty: MPMediaItemPropertyArtwork];
        aChannelButtonTitle.artwork = [artWork imageWithSize:CGSizeMake(30, 30)];                                   // save the artwork UIImage object
        aChannelButtonTitle.channelSubTitle = [representativeItem valueForProperty: MPMediaItemPropertyArtist];    // save the artists string in this object

Thus, viewDidLoad still captures the song title (for the purpose of preparing the indexed table) and the persistentID (to enable tableView:cellForRowAtIndexPath: to easily grab the artwork and subtitle). It still could use some improvement, but its better.