objective-c - objectForKey :NSFileType returns NULL

Question

我正在研究一种方法，可以从用户 mac 上其他已安装的磁盘中识别出 kindle。如果无法通过名称识别 kindle，我的应用程序将通过查找包含 kindle 使用的文件类型(例如 .prc、.azw、.mobi、.mbp 等)的磁盘来确定哪个安装的磁盘是 kindle )。这是代码:

    NSLog(@"Scanning every file");
    NSMutableArray *mountedDisks = [[NSMutableArray alloc] init];
    mountedDisks = [workspace mountedRemovableMedia];

    NSMutableArray *subpaths = [[NSMutableArray alloc] init];

    int currentSubpath;
    int proprietaryFilesFound;

    while ([mountedDisks count] > currentDisk && [mountedDisks count] != 0)
    {
        subpaths = [manager subpathsAtPath:[mountedDisks objectAtIndex:currentDisk]];
        currentSubpath = 0;
        proprietaryFilesFound = 0;
        NSLog(@"Entered outer loop");

         while ([subpaths count] > currentSubpath && [subpaths count] != 0 && [[[manager attributesOfFileSystemForPath:[mountedDisks objectAtIndex:currentDisk] error:NULL] objectForKey:NSFileSystemSize] longLongValue] / 1073741824 <= 5 && proprietaryFilesFound < 7)
        {
            NSLog(@"Scanning %@, filetype is %@", [subpaths objectAtIndex:currentSubpath], [[manager attributesOfItemAtPath:[subpaths objectAtIndex:currentSubpath] error:NULL] objectForKey:NSFileType]);
            if ([[[manager attributesOfItemAtPath:[subpaths objectAtIndex:currentSubpath] error:NULL] objectForKey:NSFileType] isEqual: @"azw"] || [[[manager attributesOfItemAtPath:[subpaths objectAtIndex:currentSubpath] error:NULL] objectForKey:NSFileType] isEqual: @"mbp"] || [[[manager attributesOfItemAtPath:[subpaths objectAtIndex:currentSubpath] error:NULL] objectForKey:NSFileType] isEqual: @"prc"] || [[[manager attributesOfItemAtPath:[subpaths objectAtIndex:currentSubpath] error:NULL] objectForKey:NSFileType] isEqual: @"mobi"])
            {
                proprietaryFilesFound++;
                NSLog(@"Proprietary file found");
            }
            currentSubpath++;
        }
        currentDisk++;
    }

不幸的是，当我运行以下代码行时，返回 NULL。

[[manager attributesOfItemAtPath:[subpaths objectAtIndex:currentSubpath] error:NULL] objectForKey:NSFileType]

我已经使用 C++ 很长时间了，但我对 Objective C 和 cocoa 还很陌生，所以任何帮助将不胜感激，如果这是一个菜鸟问题，我提前道歉。

Answer 1

不要将 NULL 传递给错误参数，而是将指针传递给 NSError 并查看返回的错误。例如:

NSError *error;
[[manager attributesOfItemAtPath:[subpaths objectAtIndex:currentSubpath] error:&error] objectForKey:NSFileType];
NSLog(@"%@", error);