2-15 Enemy Attack Spheres And Spawning Enemy Projectile

本章要來介紹當Player進入Enemy的攻擊範圍後，要自動召喚出Projectile攻擊Player。上一章已經跟大家講過如何製作Projectile以及Interface的功用，如不清楚請先閱讀上一章的內容唷。
2-14 Using Interfaces In C#

https://rpgcorecombat.blogspot.tw/2017/12/2-14-using-interfaces-in-c.html

首先，我們可以撰寫Gizmos程式碼畫出攻擊範圍跟移動範圍，方便我們在遊戲中進行調整。原本程式碼中的變數只有attackRadius，請大家在自己新增chaseRadius。

如下圖，當玩家進入藍色框線的位置 時敵人就會靠近，進入到紅色框線的範圍時就會開始進行攻擊。

接下來，將Projectile的Rigidbody設為Use Gravity並把Is Kinematic關掉。

並在Projectile中新增Projectile Speed變數，用作發射Projectile時的速度。

然後在Enemy中新增一個Empty GameObject取名為Projectile Spawn Point，這是用來生成Projectile的定位座標。

我將Projectile Spawn Point放置在頭頂上方。

然後，在Enemy中建立ProjecilteToUse跟ProjectileSocket兩個變數，並將Projectile跟Projectile Spawn Point拉進去。

以下提供程式碼：
Enemy.cs：

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.AI;
using UnityStandardAssets.Characters.ThirdPerson;

public class Enemy : MonoBehaviour, IDamageable {

 [SerializeField] float maxHealthPoints = 100f;
 [SerializeField] float attackRadius = 3.0f;
 [SerializeField] float chaseRadius = 10.0f;
 [SerializeField] float damagePerShot = 9f;
 [SerializeField] GameObject projectileToUse;
 [SerializeField] GameObject projectileSocket;

 float currentHealthPoint = 100f;
 AICharacterControl aiCharacterControl = null;
 GameObject player;

 public float healthAsPercentage{
  get{ return currentHealthPoint / maxHealthPoints; }
 }

 void Start(){
  aiCharacterControl = GetComponent ();
  player = GameObject.FindGameObjectWithTag ("Player");
 }

 void Update(){
  // 計算Player跟Enemy的距離
  float distanceToPlayer = Vector3.Distance (player.transform.position, transform.position);
  // 若彼此之間的距離小於attackRadius，就讓Enemy開始攻擊Player
  if (distanceToPlayer <= attackRadius) {
   SpawnProjectile();
  } 
  // 若彼此之間的距離小於chaseRadius，就讓Enemy開始追蹤Player
  if (distanceToPlayer <= chaseRadius) {
   aiCharacterControl.SetTarget (player.transform);
  } else {
   aiCharacterControl.SetTarget (transform);
  }
 }

 void SpawnProjectile(){
  // Instantiate可以生成GameObject，Quaternion.identity為方向不進行任何轉向
  GameObject newProjectile = Instantiate(projectileToUse, projectileSocket.transform.position, Quaternion.identity);
  // 取得Projectile
  Projectile projectileComponent = newProjectile.GetComponent ();
  // 設定Projectile的攻擊威力
  projectileComponent.damageCaused = damagePerShot;
  // 計算發射Projectile到Player之間的單位向量
  Vector3 unitVectorToPlayer = (player.transform.position - projectileSocket.transform.position).normalized;
  float projectileSpeed = projectileComponent.projectileSpeed;
  // 將單位向量乘以發射速度，透過velocity將Projectile發射出去吧！
  newProjectile.GetComponent ().velocity = unitVectorToPlayer * projectileSpeed;
 }

 public void TakeDamage(float damage){
  // Math.Clamp可以確保數值運算後不會低於最小值或者高於最大值
  currentHealthPoint = Mathf.Clamp (currentHealthPoint - damage, 0f, maxHealthPoints);
 }

 void OnDrawGizmos(){
  //繪製攻擊範圍
  Gizmos.color = new Color(255f, 0f, 0f, 0.5f);
  Gizmos.DrawWireSphere (transform.position, attackRadius);

  //繪製移動範圍
  Gizmos.color = new Color(0f, 0f, 255f, 0.5f);
  Gizmos.DrawWireSphere (transform.position, chaseRadius);
 }
}

Projectile.cs：

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Projectile : MonoBehaviour {
 
 public float damageCaused;
 public float projectileSpeed;

 void OnTriggerEnter(Collider collider){
  // 取得Component為IDamageable
  Component damageableComponent = collider.gameObject.GetComponent (typeof(IDamageable));
  // 若IDamageable存在
  if (damageableComponent) {
   // 呼叫damageableComponent的TakeDamage方法
   (damageableComponent as IDamageable).TakeDamage (damageCaused);
  }
 }
}

最後執行遊戲，大家可以看見玩家進入紅色的攻擊範圍後，敵人開始發射Projectile攻擊玩家。