如何查找iphone磁盘空间？

我正在创建一个iphone性能应用程序，我想知道iphone上可用的磁盘空间。。。我知道很多应用程序都能做到这一点，但我似乎在任何地方都找不到这一点

+(float)getTotalDiskSpaceInBytes {
    float totalSpace = 0.0f;
    NSError *error = nil;
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSDictionary *dictionary = [[NSFileManager defaultManager] attributesOfFileSystemForPath:[paths lastObject] error: &error];

    if (dictionary) {
        NSNumber *fileSystemSizeInBytes = [dictionary objectForKey: NSFileSystemSize];
        totalSpace = [fileSystemSizeInBytes floatValue];
    } else {
        DLog(@"Error Obtaining File System Info: Domain = %@, Code = %@", [error domain], [error code]);
    }

    return totalSpace;
}