使用 NSKeyedUnarchiver 解码时出现问题

Question

我正在 XCode 3.2.3 上编写一个针对 iOS 4.0 的应用程序，当应用程序关闭时，我使用 NSCoder 协议存储一些数据。保存似乎工作正常，问题是从保存的文件中检索数据。我的保存方法如下所示：

-(void)saveMusicalWorksToMemory{
    // create the save paths
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory =  [paths objectAtIndex:0];
    NSString *mwPath = [documentsDirectory stringByAppendingPathComponent:kMusicalWorksLocalFileSuffix];
    NSString *fwPath = [documentsDirectory stringByAppendingPathComponent:kFeaturedWorksLocalFileSuffix];


    NSMutableData *musicalWorksData;
    NSMutableData *featuredWorksData;
    NSKeyedArchiver *mwEncoder;
    NSKeyedArchiver *fwEncoder;

    musicalWorksData = [NSMutableData data];
    featuredWorksData = [NSMutableData data];

    mwEncoder = [[NSKeyedArchiver alloc] initForWritingWithMutableData:musicalWorksData];
    fwEncoder = [[NSKeyedArchiver alloc] initForWritingWithMutableData:featuredWorksData];

    // encode the objects
    [mwEncoder encodeObject:musicalWorks forKey:kMusicalWorksArchiveKey];
    [fwEncoder encodeObject:featuredWorks forKey:kFeaturedWorksArchiveKey];
    [mwEncoder finishEncoding];
    [fwEncoder finishEncoding];

    // write to files
    [musicalWorksData writeToFile:mwPath atomically:YES];
    [featuredWorksData writeToFile:fwPath atomically:YES];

    [mwEncoder release];
    [fwEncoder release];
}

这是我从文件中读取数据的方法：

-(void)loadMusicalWorksFromMemory{
    // get the path to the locally saved data
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory =  [paths objectAtIndex:0];
    NSString *mwPath = [documentsDirectory stringByAppendingPathComponent:kMusicalWorksLocalFileSuffix];
    NSString *fwPath = [documentsDirectory stringByAppendingPathComponent:kFeaturedWorksLocalFileSuffix];

    NSFileManager *fileManager = [NSFileManager defaultManager];
    if([fileManager fileExistsAtPath:mwPath] && [fileManager fileExistsAtPath:fwPath]){
        NSMutableData *mwData;
        NSMutableData *fwData;
        NSKeyedUnarchiver *mwDecoder;
        NSKeyedUnarchiver *fwDecoder;
        NSMutableArray *tempMusicalWorks;
        NSMutableArray *tempFeaturedWorks;

        // unarchive the file data
        mwData = [NSData dataWithContentsOfFile:mwPath];
        fwData = [NSData dataWithContentsOfFile:fwPath];
        mwDecoder = [[NSKeyedUnarchiver alloc] initForReadingWithData:mwData];
        fwDecoder = [[NSKeyedUnarchiver alloc] initForReadingWithData:fwData];

        tempMusicalWorks = [mwDecoder decodeObjectForKey:kMusicalWorksArchiveKey];
        tempFeaturedWorks = [fwDecoder decodeObjectForKey:kFeaturedWorksArchiveKey];

        [mwDecoder finishDecoding];
        [fwDecoder finishDecoding];
        [mwDecoder release];
        [fwDecoder release];

        // restore the content
        [self setMusicalWorks:tempMusicalWorks];
        [self setFeaturedWorks:tempFeaturedWorks];


    }else{
        NSLog(@"MusicalWorkManager::loadMusicalWorksFromMemory - No file found at given path");
    }
}

奇怪的是，当从内存中读取数据并进行调试时，我发现 mwData 和 fwData 实际上不为零，并且大致正确尺寸比例（大约2:1）。问题是解码的数组（tempMusicalWorks 和 tempFeaturedWorks）的条目为零（但不是零）。这让我相信问题实际上出在解码上。

作为记录，我还检查了项目是否保存在 saveMusicalWorksToMemory() 方法中。我通过此方法传入的数组确实填充了内容。另外，我还检查了我是否为正在编码和解码的对象正确实现了 initWithCoder() 和encodeWithCoder() 方法。

有什么提示吗？我真的对这个迷失了。

谢谢！

-马特