出售

Coddy Unity 塔防游戏课程的第 19 课，共 25 课。

出售防御塔会退还其花费的一半，包括升级费用，因此移动防御塔会有花费，但不会退还全部费用。防御塔会累计记录花在它上面的金额：建造时的价格，加上每次支付的升级费用。退款金额是总额的一半，整数除法会向下取整。自行车租赁也会累计同类金额：

public void ExtendRide(int extraHours)
{
    paid += extraHours * pricePerHour;
}

public int Deposit => paid / 4;

出售会销毁防御塔，而格子的 tower 字段仍然指向它。Unity 的 == 会把已销毁的对象视为 null，因此从下一帧开始，pad.tower == null 就为 true，格子也会自动恢复为空闲状态，不需要额外代码。面板则不同：它仍然显示已出售的防御塔，点击“升级”会访问一个已销毁的对象，并引发 MissingReferenceException。出售时不会选中任何对象，因此面板会隐藏。

挑战

中等

以塔花费的一半价格出售它。

在 Tower 中，保留 private int spent;：在 Start 中记录 data.cost，并在 Upgrade() 中记录每次升级的价格。添加 public int SellValue => spent / 2;。
在 BuildManager 中，面板填充时，在序列化的 TMP_Text sellLabel 中显示 Sell 25（出售价格）。
编写 public void SellSelected()：返还出售价格，销毁这座塔，并取消选择。

按下 Play，然后建造、升级并出售一座塔。接着点击 Check。

亲手试试

using UnityEngine;
using UnityEngine.EventSystems;
using UnityEngine.InputSystem;
using UnityEngine.UI;
using TMPro;

public class BuildManager : MonoBehaviour
{
    [SerializeField] private GameManager game;
    [SerializeField] private Tower[] shop;
    [SerializeField] private Button[] shopButtons;
    [SerializeField] private Color chosenColor = new Color(1f, 0.85f, 0.3f);
    [SerializeField] private GameObject towerPanel;
    [SerializeField] private TMP_Text towerNameText;
    [SerializeField] private TMP_Text towerStatsText;
    [SerializeField] private Button upgradeButton;
    [SerializeField] private TMP_Text upgradeLabel;

    private Tower chosen;
    private Tower selected;

    void Start()
    {
        for (int i = 0; i < shop.Length; i++)
        {
            Tower prefab = shop[i];
            shopButtons[i].GetComponentInChildren<TMP_Text>().text = prefab.data.towerName + "\n" + prefab.data.cost;
            shopButtons[i].onClick.AddListener(() => Choose(prefab));
        }
        Choose(shop[0]);
        Select(null);
    }

    void Choose(Tower prefab)
    {
        chosen = prefab;
        for (int i = 0; i < shop.Length; i++)
        {
            shopButtons[i].image.color = shop[i] == chosen ? chosenColor : Color.white;
        }
    }

    void Update()
    {
        if (!Mouse.current.leftButton.wasPressedThisFrame || EventSystem.current.IsPointerOverGameObject())
        {
            return;
        }

        Ray ray = Camera.main.ScreenPointToRay(Mouse.current.position.ReadValue());
        if (!Physics.Raycast(ray, out RaycastHit hit))
        {
            Select(null);
            return;
        }

        Tower tower = hit.collider.GetComponentInParent<Tower>();
        if (tower != null)
        {
            Select(tower);
            return;
        }

        Select(null);
        if (hit.collider.TryGetComponent(out BuildPad pad))
        {
            Build(pad);
        }
    }

    void Build(BuildPad pad)
    {
        if (pad.tower != null)
        {
            return;
        }

        if (!game.TrySpend(chosen.data.cost))
        {
            Debug.Log("Not enough money");
            return;
        }

        pad.tower = Instantiate(chosen, pad.transform.position, Quaternion.identity);
    }

    void Select(Tower tower)
    {
        selected = tower;
        towerPanel.SetActive(tower != null);
        if (tower != null)
        {
            ShowPanel();
        }
    }

    void ShowPanel()
    {
        TowerData data = selected.data;
        int level = selected.Level;
        towerNameText.text = data.towerName + " level " + level;
        if (level < data.maxLevel)
        {
            towerStatsText.text = "Damage " + data.DamageAt(level) + " > " + data.DamageAt(level + 1)
                + "\nRange " + data.RangeAt(level) + " > " + data.RangeAt(level + 1);
            upgradeLabel.text = "Upgrade " + data.UpgradeCost(level);
            upgradeButton.interactable = true;
        }
        else
        {
            towerStatsText.text = "Damage " + data.DamageAt(level) + "\nRange " + data.RangeAt(level);
            upgradeLabel.text = "Max level";
            upgradeButton.interactable = false;
        }
    }

    public void UpgradeSelected()
    {
        if (selected.Level >= selected.data.maxLevel)
        {
            return;
        }

        if (game.TrySpend(selected.data.UpgradeCost(selected.Level)))
        {
            selected.Upgrade();
            ShowPanel();
        }
    }
}

Unity 塔防游戏中的所有课

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