//创建文件
NSString *documentsDirectory = [NSHomeDirectory() stringByAppendingPathComponent:@"Documents"];
    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSString *filePath = [documentsDirectory stringByAppendingPathComponent:@"xxx.txt"];
    NSLog(@"-----%@",filePath);
    if(![fileManager fileExistsAtPath:filePath]) //如果文件xxx不存在
    {
        NSLog(@"xxx.txt is not exist");
        if( [fileManager createFileAtPath:filePath contents:nil attributes:nil]) //创建文件         
            {
             //
            }
      }
      else {
                 //
      }

//数据写入文件
  NSString *buf=@"数据";
                [buf writeToFile:filePath atomically:YES encoding:NSUTF8StringEncoding error:nil];

//从文件读数据
NSString * buf = [NSString stringWithContentsOfFile:filePath encoding:NSUTF8StringEncoding error:nil];