我如何提高性能为UITableView获得索引节

Question

我正在为表视图创建索引部分，以显示iPod库中的歌曲标题。为了实现这一点，我利用sectionForObject:collationStringSelector:在循环中遍历来自歌曲查询的所有mediaItems。以下代码在viewDidLoad中执行，以获得表的节数和每个节的行数:

        // 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
        }

不幸的是，对于2000首歌曲，这需要近15秒。分配channelButtonTitle对象的时间和sectionForObject:collationStringSelector:的执行时间都可以忽略不计。

似乎大部分时间都花在了获取valueForProperty的行上，尽管:

MPMediaItem *representativeItem = [arrayItem representativeItem];

耗时5秒。

我如何提高创建索引段的性能(或者至少改善用户等待某些事情发生的时间)?

Answer 1

由于缺乏任何其他建议，我发现可以通过将以下两行移动到tableView:cellForRowAtIndexPath:

将时间缩短一半

        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

因此，viewDidLoad仍然捕获歌曲标题(为了准备索引表的目的)和persistentID(使tableView:cellForRowAtIndexPath:能够轻松抓取插图和副标题)。它仍然需要一些改进，但它已经更好了。

Answer 2

一个更好的处理方法是使用NSOperationQueue和NSOperation。在这里可以找到一个与这个问题相关的很好的教程