void Start()
 {
     enemyCountText = GameObject.Find("EnemyCountText").GetComponent <Text>();
     enemySpawn     = GameObject.Find("EnemySpawn").GetComponent <EnemySpawn>();
 }
 //预留接口
 public void Init(EnemySpawn spawn, float speed, Player player)
 {
     this.spawn  = spawn;
     this.speed  = speed;
     this.player = player;
 }