Question

我正在为表格视图创建索引部分，以显示iPod库中的歌曲标题。为了实现这一点，我正在使用sectionForObject：collationStringSelector：循环遍历歌曲查询中的所有mediaItems。以下代码在viewDidLoad中执行，以获取表的每个部分的节数和行数：

        // 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，这需要将近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。

似乎大部分时间花在了获得valueForProperty的行中，尽管这行：

MPMediaItem *representativeItem = [arrayItem representativeItem];

需要5秒钟。

如何提高创建索引部分的性能（或者至少缩短用户等待事情发生的时间）？

Answer 1

由于缺乏任何其他建议，我发现我可以通过将以下2行移动到tableView：cellForRowAtIndexPath：

来缩短时间。

        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仍会捕获歌曲标题（用于准备索引表）和persistentID（以启用tableView：cellForRowAtIndexPath：轻松抓取图稿和副标题）。它仍然可以使用一些改进，但它更好。

Answer 2

更好的方法是使用NSOperationQueue和NSOperation。可以找到与此问题相关的精彩教程here。

如何提高获取UITableView的索引部分的性能

2 个答案: