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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。为此，我使用 sectionForObject:collat​​ionStringSelector: 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，这需要将近 15 秒。分配 channelButtonTitle 对象的时间和 sectionForObject:collat​​ionStringSelector: 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，尽管这行：

MPMediaItem *representativeItem = [arrayItem representativeItem];

需要 5 秒。

如何提高创建索引部分的性能（或至少提高用户等待某事发生的时间）？

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2 回答 2

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由于没有任何其他建议，我发现我可以通过将以下 2 行移动到 tableView:cellForRowAtIndexPath 中来将时间缩短一半：

        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 仍然会捕获歌曲标题（用于准备索引表）和 persistentID（以启用 tableView:cellForRowAtIndexPath: 以轻松获取艺术品和字幕）。它仍然可以使用一些改进，但更好。

于 2013-04-17T14:35:27.157 回答
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处理这个问题的更好方法是使用 NSOperationQueue 和 NSOperation。可以在此处找到与此问题相关的不错的教程。

于 2013-04-24T17:07:28.373 回答