objective-c中读取下载文件

代码：

-(void)downloadFile
{
NSURLRequest request = [NSURLRequest requestWithURL:[NSURL URLWithString:@"http://myserver.com/website/file.txt"]];
AFURLConnectionOperation *operation =   [[AFHTTPRequestOperation alloc] initWithRequest:request];
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *filePath = [[paths objectAtIndex:0] stringByAppendingPathComponent:@"file.txt"];
operation.outputStream = [NSOutputStream outputStreamToFileAtPath:filePath append:NO];
[operation setCompletionBlock:^{
        NSLog(@"downloadComplete!");
        NSString *path;
        path = [[NSBundle mainBundle] pathForResource: @"file" ofType: @"txt"];
        NSString *data = [self readFile: path];
        NSLog(@"%@",data);
}];
[operation start];
}
-(NSString *)readFile:(NSString *)fileName

{
    NSLog(@"readFile");

    NSString *appFile = fileName;
    NSFileManager *fileManager=[NSFileManager defaultManager];
    if ([fileManager fileExistsAtPath:appFile])
    {
        NSError *error= NULL;
        NSString *resultData = [NSString stringWithContentsOfFile: appFile encoding: NSUTF8StringEncoding error: &error];
        if (error == NULL)
            return resultData;
    }
    return NULL;
}

代码能成功下载文件，但是不能读取，返回空。不知道哪出错了，请各位高手帮忙看看。

修改这行代码：

path = [ [NSBundle mainBundle] pathForResource: @"file" ofType: @"txt"];

改为：

path = filePath;