+(void)writefile:(NSString *)string fileName:(NSString *)fileName{

    NSLog(@"fileName==%@",fileName);
    NSArray *paths  = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask,YES);
    NSString *homePath = [paths objectAtIndex:0];
    NSString *filePath = [homePath stringByAppendingPathComponent:fileName];

    NSFileManager *fileManager = [NSFileManager defaultManager];
    //如果不存在
    if(![fileManager fileExistsAtPath:filePath]){
        NSLog(@"-------文件不存在，写入文件----------");
        NSError *error;
        if([string writeToFile:filePath atomically:YES encoding:NSUTF8StringEncoding error:&error]){
            NSLog(@"------写入文件------success");
        }else{
            NSLog(@"------写入文件------fail,error==%@",error);
        }
    }else{//追加写入文件，而不是覆盖原来的文件
        NSLog(@"-------文件存在，追加文件----------");
        NSFileHandle *fileHandle = [NSFileHandle fileHandleForUpdatingAtPath:filePath];
        [fileHandle seekToEndOfFile];  //将节点跳到文件的末尾
        NSData* stringData  = [string dataUsingEncoding:NSUTF8StringEncoding];
        [fileHandle writeData:stringData]; //追加写入数据
        [fileHandle closeFile];
    }
}