Chapter 8.12: Building a Trading System

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Table of Contents

• What We Are Building
• Step 1: Data Model (3_Game)
• Step 2: RPC Constants (3_Game)
• Step 3: Server-Side Shop Manager (4_World)
• Step 4: Client-Side Shop UI (5_Mission)
• Step 5: Layout File
• Step 6: Mission Hook and Keybind
• Step 7: Currency Item
• Step 8: Shop Config JSON
• Step 9: Build and Test
• Security Considerations
• Complete Code Reference
• Best Practices / Common Mistakes / What You Learned

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What We Are Building

Players press F6 to open a shop menu, browse items by category (Weapons, Food, Medical), and buy/sell using a currency item. The server validates every transaction -- the client never decides prices or spawns items.

CLIENT                                SERVER
1. Press F6 --> REQUEST_SHOP_DATA ->  2. Load config, count currency
                                         SHOP_DATA_RESPONSE ->
3. Show categories + items
   Click Buy --> BUY_ITEM (cls,qty) -> 4. Validate, remove currency, spawn
                                         TRANSACTION_RESULT ->
5. Show result, update balance

Key rule: Client sends (className, quantity) only. Server looks up the price.

Mod Structure

ShopDemo/
    mod.cpp
    GUI/layouts/shop_menu.layout
    Scripts/config.cpp
        3_Game/ShopDemo/  ShopDemoRPC.c  ShopDemoData.c
        4_World/ShopDemo/ ShopDemoManager.c
        5_Mission/ShopDemo/ ShopDemoMenu.c  ShopDemoMission.c

---

Step 1: Data Model (3_Game)

Scripts/3_Game/ShopDemo/ShopDemoData.c

class ShopItem
{
    string ClassName;
    string DisplayName;
    int BuyPrice;
    int SellPrice;
    void ShopItem() { ClassName = ""; DisplayName = ""; BuyPrice = 0; SellPrice = 0; }
};

class ShopCategory
{
    string Name;
    ref array<ref ShopItem> Items;
    void ShopCategory() { Name = ""; Items = new array<ref ShopItem>; }
};

class ShopConfig
{
    string CurrencyClassName;
    ref array<ref ShopCategory> Categories;
    void ShopConfig() { CurrencyClassName = "GoldCoin"; Categories = new array<ref ShopCategory>; }
};

Keep SellPrice < BuyPrice always to prevent infinite money loops.

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Step 2: RPC Constants (3_Game)

Scripts/3_Game/ShopDemo/ShopDemoRPC.c

class ShopDemoRPC
{
    static const int REQUEST_SHOP_DATA   = 79101;  // Client -> Server
    static const int BUY_ITEM            = 79102;
    static const int SELL_ITEM           = 79103;
    static const int SHOP_DATA_RESPONSE  = 79201;  // Server -> Client
    static const int TRANSACTION_RESULT  = 79202;
};

---

Step 3: Server-Side Shop Manager (4_World)

Scripts/4_World/ShopDemo/ShopDemoManager.c

class ShopDemoManager
{
    private static ref ShopDemoManager s_Instance;
    static ShopDemoManager Get() { if (!s_Instance) s_Instance = new ShopDemoManager(); return s_Instance; }

    protected ref ShopConfig m_Config;
    protected string m_ConfigPath;
    void ShopDemoManager() { m_ConfigPath = "$profile:ShopDemo/ShopConfig.json"; }

    void Init()
    {
        m_Config = new ShopConfig();
        if (FileExist(m_ConfigPath))
        {
            string err;
            if (!JsonFileLoader<ShopConfig>.LoadFile(m_ConfigPath, m_Config, err))
                CreateDefaultConfig();
        }
        else CreateDefaultConfig();
        Print("[ShopDemo] Init: " + m_Config.Categories.Count().ToString() + " categories");
    }

    ShopConfig GetConfig() { return m_Config; }

    protected void CreateDefaultConfig()
    {
        m_Config = new ShopConfig();
        m_Config.CurrencyClassName = "GoldCoin";
        ShopCategory c1 = new ShopCategory(); c1.Name = "Weapons";
        AddItem(c1, "IJ70", "IJ-70 Pistol", 50, 25);
        AddItem(c1, "KA74", "KA-74", 200, 100);
        AddItem(c1, "Mosin9130", "Mosin 91/30", 150, 75);
        m_Config.Categories.Insert(c1);
        ShopCategory c2 = new ShopCategory(); c2.Name = "Food";
        AddItem(c2, "SodaCan_Cola", "Cola", 5, 2);
        AddItem(c2, "TunaCan", "Tuna Can", 8, 4);
        AddItem(c2, "Apple", "Apple", 3, 1);
        m_Config.Categories.Insert(c2);
        ShopCategory c3 = new ShopCategory(); c3.Name = "Medical";
        AddItem(c3, "BandageDressing", "Bandage", 10, 5);
        AddItem(c3, "Morphine", "Morphine", 30, 15);
        AddItem(c3, "SalineBagIV", "Saline Bag IV", 25, 12);
        m_Config.Categories.Insert(c3);
        SaveConfig();
    }

    protected void AddItem(ShopCategory cat, string cls, string disp, int buy, int sell)
    {
        ShopItem si = new ShopItem();
        si.ClassName = cls; si.DisplayName = disp; si.BuyPrice = buy; si.SellPrice = sell;
        cat.Items.Insert(si);
    }

    protected void SaveConfig()
    {
        MakeDirectory("$profile:ShopDemo");
        string err;
        JsonFileLoader<ShopConfig>.SaveFile(m_ConfigPath, m_Config, err);
    }

    int CountPlayerCurrency(PlayerBase player)
    {
        if (!player) return 0;
        int total = 0;
        array<EntityAI> items = new array<EntityAI>;
        player.GetInventory().EnumerateInventory(InventoryTraversalType.PREORDER, items);
        for (int i = 0; i < items.Count(); i++)
        {
            EntityAI ent = items.Get(i);
            if (!ent || ent.GetType() != m_Config.CurrencyClassName) continue;
            ItemBase ib = ItemBase.Cast(ent);
            if (ib && ib.HasQuantity()) total = total + ib.GetQuantity();
            else total = total + 1;
        }
        return total;
    }

    protected bool RemoveCurrency(PlayerBase player, int amount)
    {
        int remaining = amount;
        array<EntityAI> items = new array<EntityAI>;
        player.GetInventory().EnumerateInventory(InventoryTraversalType.PREORDER, items);
        for (int i = 0; i < items.Count(); i++)
        {
            if (remaining <= 0) break;
            EntityAI ent = items.Get(i);
            if (!ent || ent.GetType() != m_Config.CurrencyClassName) continue;
            ItemBase ib = ItemBase.Cast(ent);
            if (!ib) continue;
            if (ib.HasQuantity())
            {
                int qty = ib.GetQuantity();
                if (qty <= remaining) { remaining = remaining - qty; ib.DeleteSafe(); }
                else { ib.SetQuantity(qty - remaining); remaining = 0; }
            }
            else { remaining = remaining - 1; ib.DeleteSafe(); }
        }
        return (remaining <= 0);
    }

    protected bool GiveCurrency(PlayerBase player, int amount)
    {
        EntityAI spawned = player.GetInventory().CreateInInventory(m_Config.CurrencyClassName);
        if (!spawned)
            spawned = EntityAI.Cast(GetGame().CreateObjectEx(m_Config.CurrencyClassName, player.GetPosition(), ECE_PLACE_ON_SURFACE));
        if (!spawned) return false;
        ItemBase ci = ItemBase.Cast(spawned);
        if (ci && ci.HasQuantity()) ci.SetQuantity(amount);
        return true;
    }

    protected ShopItem FindShopItem(string className)
    {
        for (int c = 0; c < m_Config.Categories.Count(); c++)
            for (int i = 0; i < m_Config.Categories.Get(c).Items.Count(); i++)
                if (m_Config.Categories.Get(c).Items.Get(i).ClassName == className)
                    return m_Config.Categories.Get(c).Items.Get(i);
        return null;
    }

    void HandleBuy(PlayerBase player, string className, int quantity)
    {
        PlayerIdentity id = player.GetIdentity();
        if (quantity <= 0 || quantity > 10) { SendResult(player, false, "Invalid quantity.", 0); return; }
        ShopItem si = FindShopItem(className);
        if (!si) { SendResult(player, false, "Item not in shop.", 0); return; }
        int cost = si.BuyPrice * quantity;
        int balance = CountPlayerCurrency(player);
        if (balance < cost) { SendResult(player, false, "Need " + cost.ToString() + ", have " + balance.ToString(), balance); return; }
        if (!RemoveCurrency(player, cost)) { SendResult(player, false, "Currency removal failed.", CountPlayerCurrency(player)); return; }
        for (int i = 0; i < quantity; i++)
        {
            EntityAI sp = player.GetInventory().CreateInInventory(className);
            if (!sp) sp = EntityAI.Cast(GetGame().CreateObjectEx(className, player.GetPosition(), ECE_PLACE_ON_SURFACE));
        }
        int nb = CountPlayerCurrency(player);
        SendResult(player, true, "Bought " + quantity.ToString() + "x " + si.DisplayName + " for " + cost.ToString(), nb);
        Print("[ShopDemo] " + id.GetName() + " bought " + quantity.ToString() + "x " + className);
    }

    void HandleSell(PlayerBase player, string className, int quantity)
    {
        PlayerIdentity id = player.GetIdentity();
        if (quantity <= 0 || quantity > 10) { SendResult(player, false, "Invalid quantity.", 0); return; }
        ShopItem si = FindShopItem(className);
        if (!si || si.SellPrice <= 0) { SendResult(player, false, "Cannot sell this.", 0); return; }
        int removed = 0;
        array<EntityAI> items = new array<EntityAI>;
        player.GetInventory().EnumerateInventory(InventoryTraversalType.PREORDER, items);
        for (int i = 0; i < items.Count(); i++)
        {
            if (removed >= quantity) break;
            EntityAI ent = items.Get(i);
            if (ent && ent.GetType() == className) { ent.DeleteSafe(); removed = removed + 1; }
        }
        if (removed <= 0) { SendResult(player, false, "You don't have that item.", CountPlayerCurrency(player)); return; }
        int payout = si.SellPrice * removed;
        GiveCurrency(player, payout);
        SendResult(player, true, "Sold " + removed.ToString() + "x " + si.DisplayName + " for " + payout.ToString(), CountPlayerCurrency(player));
        Print("[ShopDemo] " + id.GetName() + " sold " + removed.ToString() + "x " + className);
    }

    protected void SendResult(PlayerBase player, bool success, string message, int newBalance)
    {
        if (!player || !player.GetIdentity()) return;
        Param3<bool, string, int> r = new Param3<bool, string, int>(success, message, newBalance);
        GetGame().RPCSingleParam(player, ShopDemoRPC.TRANSACTION_RESULT, r, true, player.GetIdentity());
    }
};

modded class PlayerBase
{
    override void OnRPC(PlayerIdentity sender, int rpc_type, ParamsReadContext ctx)
    {
        super.OnRPC(sender, rpc_type, ctx);
        if (!GetGame().IsServer()) return;
        switch (rpc_type)
        {
            case ShopDemoRPC.REQUEST_SHOP_DATA: OnShopDataReq(sender); break;
            case ShopDemoRPC.BUY_ITEM: OnBuyReq(sender, ctx); break;
            case ShopDemoRPC.SELL_ITEM: OnSellReq(sender, ctx); break;
        }
    }

    protected void OnShopDataReq(PlayerIdentity requestor)
    {
        PlayerBase player = PlayerBase.GetPlayerByUID(requestor.GetId());
        if (!player) return;
        ShopDemoManager mgr = ShopDemoManager.Get();
        ShopConfig cfg = mgr.GetConfig();
        // Serialize: "CatName|cls,name,buy,sell;cls2,...\nCat2|..."
        string payload = "";
        for (int c = 0; c < cfg.Categories.Count(); c++)
        {
            ShopCategory cat = cfg.Categories.Get(c);
            if (c > 0) payload = payload + "\n";
            payload = payload + cat.Name + "|";
            for (int i = 0; i < cat.Items.Count(); i++)
            {
                ShopItem si = cat.Items.Get(i);
                if (i > 0) payload = payload + ";";
                payload = payload + si.ClassName + "," + si.DisplayName + "," + si.BuyPrice.ToString() + "," + si.SellPrice.ToString();
            }
        }
        Param2<int, string> data = new Param2<int, string>(mgr.CountPlayerCurrency(player), payload);
        GetGame().RPCSingleParam(player, ShopDemoRPC.SHOP_DATA_RESPONSE, data, true, requestor);
    }

    protected void OnBuyReq(PlayerIdentity sender, ParamsReadContext ctx)
    {
        Param2<string, int> d = new Param2<string, int>("", 0);
        if (!ctx.Read(d)) return;
        PlayerBase p = PlayerBase.GetPlayerByUID(sender.GetId());
        if (p) ShopDemoManager.Get().HandleBuy(p, d.param1, d.param2);
    }

    protected void OnSellReq(PlayerIdentity sender, ParamsReadContext ctx)
    {
        Param2<string, int> d = new Param2<string, int>("", 0);
        if (!ctx.Read(d)) return;
        PlayerBase p = PlayerBase.GetPlayerByUID(sender.GetId());
        if (p) ShopDemoManager.Get().HandleSell(p, d.param1, d.param2);
    }
};

modded class MissionServer
{
    override void OnInit() { super.OnInit(); ShopDemoManager.Get().Init(); }
};

Key decisions: Currency removed before spawning items (prevents duplication). Always DeleteSafe() for networked items. Quantity clamped to 1-10 to prevent abuse.

---

Step 4: Client-Side Shop UI (5_Mission)

Scripts/5_Mission/ShopDemo/ShopDemoMenu.c

class ShopDemoMenu extends ScriptedWidgetEventHandler
{
    protected Widget m_Root, m_CategoryPanel, m_ItemPanel;
    protected TextWidget m_BalanceText, m_DetailName, m_DetailBuyPrice, m_DetailSellPrice, m_StatusText;
    protected ButtonWidget m_BuyButton, m_SellButton, m_CloseButton;
    protected bool m_IsOpen;
    protected int m_Balance;
    protected string m_SelClass;
    protected ref array<string> m_CatNames;
    protected ref array<ref array<ref ShopItem>> m_CatItems;
    protected ref array<Widget> m_DynWidgets;

    void ShopDemoMenu()
    {
        m_IsOpen = false; m_Balance = 0; m_SelClass = "";
        m_CatNames = new array<string>; m_CatItems = new array<ref array<ref ShopItem>>;
        m_DynWidgets = new array<Widget>;
    }
    void ~ShopDemoMenu() { Close(); }

    void Open()
    {
        if (m_IsOpen) return;
        m_Root = GetGame().GetWorkspace().CreateWidgets("ShopDemo/GUI/layouts/shop_menu.layout");
        if (!m_Root) { Print("[ShopDemo] Layout failed!"); return; }
        m_BalanceText     = TextWidget.Cast(m_Root.FindAnyWidget("BalanceText"));
        m_CategoryPanel   = m_Root.FindAnyWidget("CategoryPanel");
        m_ItemPanel       = m_Root.FindAnyWidget("ItemPanel");
        m_DetailName      = TextWidget.Cast(m_Root.FindAnyWidget("DetailName"));
        m_DetailBuyPrice  = TextWidget.Cast(m_Root.FindAnyWidget("DetailBuyPrice"));
        m_DetailSellPrice = TextWidget.Cast(m_Root.FindAnyWidget("DetailSellPrice"));
        m_StatusText      = TextWidget.Cast(m_Root.FindAnyWidget("StatusText"));
        m_BuyButton       = ButtonWidget.Cast(m_Root.FindAnyWidget("BuyButton"));
        m_SellButton      = ButtonWidget.Cast(m_Root.FindAnyWidget("SellButton"));
        m_CloseButton     = ButtonWidget.Cast(m_Root.FindAnyWidget("CloseButton"));
        if (m_BuyButton) m_BuyButton.SetHandler(this);
        if (m_SellButton) m_SellButton.SetHandler(this);
        if (m_CloseButton) m_CloseButton.SetHandler(this);
        m_Root.Show(true); m_IsOpen = true;
        GetGame().GetMission().PlayerControlDisable(INPUT_EXCLUDE_ALL);
        GetGame().GetUIManager().ShowUICursor(true);
        if (m_StatusText) m_StatusText.SetText("Loading...");
        Man player = GetGame().GetPlayer();
        if (player) { Param1<bool> p = new Param1<bool>(true); GetGame().RPCSingleParam(player, ShopDemoRPC.REQUEST_SHOP_DATA, p, true); }
    }

    void Close()
    {
        if (!m_IsOpen) return;
        for (int i = 0; i < m_DynWidgets.Count(); i++) if (m_DynWidgets.Get(i)) m_DynWidgets.Get(i).Unlink();
        m_DynWidgets.Clear();
        if (m_Root) { m_Root.Unlink(); m_Root = null; }
        m_IsOpen = false;
        GetGame().GetMission().PlayerControlEnable(true);
        GetGame().GetUIManager().ShowUICursor(false);
    }

    bool IsOpen() { return m_IsOpen; }
    void Toggle() { if (m_IsOpen) Close(); else Open(); }

    void OnShopDataReceived(int balance, string payload)
    {
        m_Balance = balance;
        if (m_BalanceText) m_BalanceText.SetText("Balance: " + balance.ToString());
        m_CatNames.Clear(); m_CatItems.Clear();
        TStringArray lines = new TStringArray;
        payload.Split("\n", lines);
        for (int c = 0; c < lines.Count(); c++)
        {
            string line = lines.Get(c);
            int pp = line.IndexOf("|");
            if (pp < 0) continue;
            m_CatNames.Insert(line.Substring(0, pp));
            ref array<ref ShopItem> ci = new array<ref ShopItem>;
            TStringArray iStrs = new TStringArray;
            line.Substring(pp + 1, line.Length() - pp - 1).Split(";", iStrs);
            for (int i = 0; i < iStrs.Count(); i++)
            {
                TStringArray parts = new TStringArray;
                iStrs.Get(i).Split(",", parts);
                if (parts.Count() < 4) continue;
                ShopItem si = new ShopItem();
                si.ClassName = parts.Get(0); si.DisplayName = parts.Get(1);
                si.BuyPrice = parts.Get(2).ToInt(); si.SellPrice = parts.Get(3).ToInt();
                ci.Insert(si);
            }
            m_CatItems.Insert(ci);
        }
        // Build category buttons
        if (m_CategoryPanel)
        {
            for (int b = 0; b < m_CatNames.Count(); b++)
            {
                ButtonWidget btn = ButtonWidget.Cast(GetGame().GetWorkspace().CreateWidget(WidgetType.ButtonWidgetTypeID, 0, b*0.12, 1, 0.10, WidgetFlags.VISIBLE, ARGB(255,60,60,60), 0, m_CategoryPanel));
                if (btn) { btn.SetText(m_CatNames.Get(b)); btn.SetHandler(this); btn.SetName("CatBtn_"+b.ToString()); m_DynWidgets.Insert(btn); }
            }
        }
        if (m_CatNames.Count() > 0) SelectCategory(0);
        if (m_StatusText) m_StatusText.SetText("");
    }

    void SelectCategory(int idx)
    {
        if (idx < 0 || idx >= m_CatItems.Count()) return;
        for (int r = m_DynWidgets.Count()-1; r >= 0; r--)
        { Widget w = m_DynWidgets.Get(r); if (w && w.GetName().IndexOf("ItemBtn_")==0) { w.Unlink(); m_DynWidgets.Remove(r); } }
        array<ref ShopItem> items = m_CatItems.Get(idx);
        for (int j = 0; j < items.Count(); j++)
        {
            ShopItem si = items.Get(j);
            ButtonWidget ib = ButtonWidget.Cast(GetGame().GetWorkspace().CreateWidget(WidgetType.ButtonWidgetTypeID, 0, j*0.08, 1, 0.07, WidgetFlags.VISIBLE, ARGB(255,45,45,50), 0, m_ItemPanel));
            if (ib) { ib.SetText(si.DisplayName+" [B:"+si.BuyPrice.ToString()+" S:"+si.SellPrice.ToString()+"]"); ib.SetHandler(this); ib.SetName("ItemBtn_"+si.ClassName); m_DynWidgets.Insert(ib); }
        }
        m_SelClass = "";
        if (m_DetailName) m_DetailName.SetText("Select an item");
    }

    override bool OnClick(Widget w, int x, int y, int button)
    {
        if (w == m_CloseButton) { Close(); return true; }
        if (w == m_BuyButton) { DoBuySell(ShopDemoRPC.BUY_ITEM); return true; }
        if (w == m_SellButton) { DoBuySell(ShopDemoRPC.SELL_ITEM); return true; }
        string wn = w.GetName();
        if (wn.IndexOf("CatBtn_")==0) { SelectCategory(wn.Substring(7,wn.Length()-7).ToInt()); return true; }
        if (wn.IndexOf("ItemBtn_")==0) { SelectItem(wn.Substring(8,wn.Length()-8)); return true; }
        return false;
    }

    void SelectItem(string cls)
    {
        for (int c = 0; c < m_CatItems.Count(); c++)
            for (int i = 0; i < m_CatItems.Get(c).Count(); i++)
            {
                ShopItem si = m_CatItems.Get(c).Get(i);
                if (si.ClassName == cls) {
                    m_SelClass = cls;
                    if (m_DetailName) m_DetailName.SetText(si.DisplayName);
                    if (m_DetailBuyPrice) m_DetailBuyPrice.SetText("Buy: " + si.BuyPrice.ToString());
                    if (m_DetailSellPrice) m_DetailSellPrice.SetText("Sell: " + si.SellPrice.ToString());
                    return;
                }
            }
    }

    protected void DoBuySell(int rpcId)
    {
        if (m_SelClass == "") { if (m_StatusText) m_StatusText.SetText("Select an item first."); return; }
        Man player = GetGame().GetPlayer();
        if (!player) return;
        Param2<string, int> d = new Param2<string, int>(m_SelClass, 1);
        GetGame().RPCSingleParam(player, rpcId, d, true);
        if (m_StatusText) m_StatusText.SetText("Processing...");
    }

    void OnTransactionResult(bool success, string message, int newBalance)
    {
        m_Balance = newBalance;
        if (m_BalanceText) m_BalanceText.SetText("Balance: " + newBalance.ToString());
        if (m_StatusText) m_StatusText.SetText(message);
    }
};

---

Step 5: Layout File

GUI/layouts/shop_menu.layout

Three columns: Categories (left 20%), Items (center 46%), Details (right 26%).

FrameWidgetClass ShopMenuRoot {
 size 0.7 0.7 position 0.15 0.15 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0
 {
  ImageWidgetClass Background { size 1 1 position 0 0 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0 color 0.08 0.08 0.1 0.92 }
  TextWidgetClass ShopTitle { size 0.5 0.06 position 0.02 0.02 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0 text "Shop" "text halign" left "text valign" center color 1 0.85 0.3 1 font "gui/fonts/MetronBook" }
  TextWidgetClass BalanceText { size 0.35 0.06 position 0.63 0.02 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0 text "Balance: --" "text halign" right "text valign" center color 0.3 1 0.3 1 font "gui/fonts/MetronBook" }
  FrameWidgetClass CategoryPanel { size 0.2 0.82 position 0.02 0.1 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0
   { ImageWidgetClass CatBg { size 1 1 position 0 0 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0 color 0.12 0.12 0.14 0.8 } }
  }
  FrameWidgetClass ItemPanel { size 0.46 0.82 position 0.24 0.1 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0
   { ImageWidgetClass ItemBg { size 1 1 position 0 0 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0 color 0.1 0.1 0.12 0.8 } }
  }
  FrameWidgetClass DetailPanel { size 0.26 0.82 position 0.72 0.1 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0
   {
    ImageWidgetClass DetailBg { size 1 1 position 0 0 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0 color 0.12 0.12 0.14 0.8 }
    TextWidgetClass DetailName { size 0.9 0.08 position 0.05 0.05 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0 text "Select an item" "text halign" center "text valign" center color 1 1 1 1 font "gui/fonts/MetronBook" }
    TextWidgetClass DetailBuyPrice { size 0.9 0.06 position 0.05 0.16 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0 text "Buy: --" "text halign" left "text valign" center color 0.3 1 0.3 1 font "gui/fonts/MetronBook" }
    TextWidgetClass DetailSellPrice { size 0.9 0.06 position 0.05 0.24 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0 text "Sell: --" "text halign" left "text valign" center color 1 0.85 0.3 1 font "gui/fonts/MetronBook" }
    ButtonWidgetClass BuyButton { size 0.8 0.08 position 0.1 0.5 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0 text "Buy" "text halign" center "text valign" center color 0.2 0.7 0.2 1.0 }
    ButtonWidgetClass SellButton { size 0.8 0.08 position 0.1 0.62 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0 text "Sell" "text halign" center "text valign" center color 0.8 0.6 0.1 1.0 }
    TextWidgetClass StatusText { size 0.9 0.06 position 0.05 0.82 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0 text "" "text halign" center "text valign" center color 0.9 0.9 0.9 1 font "gui/fonts/MetronBook" }
   }
  }
  ButtonWidgetClass CloseButton { size 0.05 0.04 position 0.935 0.015 hexactpos 0 vexactpos 0 hexactsize 0 vexactsize 0 text "X" "text halign" center "text valign" center color 1.0 0.3 0.3 1.0 }
 }
}

---

Step 6: Mission Hook and Keybind

Scripts/5_Mission/ShopDemo/ShopDemoMission.c

modded class MissionGameplay
{
    protected ref ShopDemoMenu m_ShopDemoMenu;

    override void OnInit() { super.OnInit(); m_ShopDemoMenu = new ShopDemoMenu(); }

    override void OnMissionFinish()
    {
        if (m_ShopDemoMenu) { m_ShopDemoMenu.Close(); m_ShopDemoMenu = null; }
        super.OnMissionFinish();
    }

    override void OnKeyPress(int key)
    {
        super.OnKeyPress(key);
        if (key == KeyCode.KC_F6 && m_ShopDemoMenu) m_ShopDemoMenu.Toggle();
    }

    override void OnRPC(PlayerIdentity sender, Object target, int rpc_type, ParamsReadContext ctx)
    {
        super.OnRPC(sender, target, rpc_type, ctx);
        if (rpc_type == ShopDemoRPC.SHOP_DATA_RESPONSE)
        {
            Param2<int, string> d = new Param2<int, string>(0, "");
            if (ctx.Read(d) && m_ShopDemoMenu) m_ShopDemoMenu.OnShopDataReceived(d.param1, d.param2);
        }
        if (rpc_type == ShopDemoRPC.TRANSACTION_RESULT)
        {
            Param3<bool, string, int> r = new Param3<bool, string, int>(false, "", 0);
            if (ctx.Read(r) && m_ShopDemoMenu) m_ShopDemoMenu.OnTransactionResult(r.param1, r.param2, r.param3);
        }
    }
};

For released mods, use inputs.xml so players can remap the key:

<?xml version="1.0" encoding="UTF-8"?>
<modded>
 <inputs><actions>
  <input name="UAShopToggle" loc="Toggle Shop" type="button" default="Keyboard:KC_F6" group="modded" />
 </actions></inputs>
</modded>

---

Step 7: Currency Item

You can use any existing item -- set CurrencyClassName to "Rag" in the JSON and rags become money. For a custom coin, see Chapter 8.2: Custom Item.

---

Step 8: Shop Config JSON

Auto-generated at $profile:ShopDemo/ShopConfig.json on first server start. Edit prices, add categories/items, restart server. Always keep SellPrice < BuyPrice.

---

Step 9: Build and Test

1. Pack ShopDemo/ into PBO, add to server+client @ShopDemo/addons/, add -mod=@ShopDemo
2. Spawn currency, press F6, browse, buy/sell
3. Check server log for [ShopDemo] lines

Test Case	Expected
Buy with no currency	"Need X, have 0"
Buy unknown class (hacked)	"Item not in shop"
Sell item not owned	"You don't have that item"
Inventory full on buy	Item drops on ground

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Security Considerations

1. NEVER trust client-sent prices. Client sends (className, qty) only. Server looks up price.
2. Delete before spawn. Remove currency first, then create items. Prevents duplication.
3. Validate existence. Confirm item is in inventory before giving sell currency.
4. Log everything. Print player name, item, amount for every transaction.
5. Quantity bounds. Reject qty <= 0 or qty > 10.
6. Rate limit in production: 500ms cooldown per player per transaction.

---

Complete Code Reference

File	Layer	Purpose
ShopDemoRPC.c	3_Game	RPC ID constants
ShopDemoData.c	3_Game	Data classes: ShopItem, ShopCategory, ShopConfig
ShopDemoManager.c	4_World	Server: config, buy/sell logic, inventory, RPC handlers
ShopDemoMenu.c	5_Mission	Client: UI, dynamic widgets, RPC send/receive
ShopDemoMission.c	5_Mission	Mission hook: init, keybind, RPC routing
shop_menu.layout	GUI	3-panel layout

---

Best Practices

• Server is the single source of truth. Client is a display terminal.
• Use DeleteSafe() not Delete(). Handles network sync and locked slots.
• Data classes in 3_Game. Visible to both 4_World and 5_Mission.
• Always call super in overrides. Breaking the chain breaks other mods.
• Clean up dynamic widgets. Every CreateWidget needs Unlink on close.

Theory vs Practice

Concept	Theory	Reality
JsonFileLoader.LoadFile()	Loads cleanly	Trailing commas cause silent failures. Validate JSON externally.
String RPC serialization	Simple	500+ items may hit size limits. Paginate for large shops.
CreateInInventory()	Always works	Returns null if inventory full. Always check.
Listen server testing	Fast iteration	Hides network bugs. Test on dedicated server.

What You Learned

• JSON config loading with JsonFileLoader<T> and auto-generation of defaults
• Singleton pattern for server-side game managers
• Inventory enumeration, counting, deletion (DeleteSafe), and spawning
• String serialization of complex data over RPC (categories, items, prices)
• Dynamic widget creation for data-driven UI
• Full buy/sell transaction flow with server-only authority
• Security principles for multiplayer economy systems

Common Mistakes

Mistake	Fix
Client sends price	Send (className, qty) only. Server decides price.
Spawn before paying	Remove currency first, then create items.
Skip super.OnRPC()	Always call super -- other mods need the chain.
Delete() on networked items	Use DeleteSafe().
Ignore CreateInInventory return	Check for null, fall back to ground spawn.
Redeclare vars in else-if	Declare once before the if-chain (Enforce Script rule).

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Previous: Chapter 8.11: Clothing Mod