Trade with another player

Goal

Two players in a room trade, all or nothing: one sells a gem, the other pays coins, and either both happen or neither does. What each player owns lives in their player document, which the room's store keeps between visits, so a gem bought today is still yours tomorrow.

A trade is two steps. G offers your gem to the next player for gemPrice coins; H accepts the offer made to you. The accept becomes one ctx.data.exchange, and both players hear how it ended. An offer the documents no longer match (the buyer spent the coins, the seller sold the gem) is stale and does nothing.

This builds on Play with friends: the game already declares multiplayer. The collect kit (templates/collect) trades pets the same way, with offer and accept messages.

Files you will edit

src/game.ts

Steps

Add the price to rules, beside the others.

rules: {
  // ...the rules already there
  gemPrice: 5,
},

Keep the open offer at module scope, below the imports. Add applyTransfer and type TransferDoc to the file's gameable import. The offer is one object, made once; init resets it (step 4).

/** A player's document: what the room's store keeps for them. */
interface Purse {
  coins: number;
  gems: string[];
}

/** The one open offer: `from` gives `give` and takes `take` from `to`. */
const offer = { open: false, from: 0, to: 0, give: {} as TransferDoc, take: {} as TransferDoc };

Trade on the authority. It reads each player's own keys from their handle, so nothing a page sends can make an offer for someone else. applyTransfer is the rule the store runs: number fields move amounts, list fields move items, and it returns null when a side is short.

/** Authority: new purses, offers, accepts, and how each trade ended. */
function trading(ctx: GameContext): void {
  for (let i = 0; i < ctx.events.length; i += 1) {
    const e = ctx.events[i];
    // A first-time player starts with 10 coins and a gem of their own.
    if (e.tag === 'player-joined' && ctx.players.get(e.val.player)?.data === null) {
      ctx.data.save(e.val.player, { coins: 10, gems: [`gem-${e.val.name}`] });
    }
  }
  const list = ctx.players.list;
  for (let i = 0; i < list.length; i += 1) {
    const p = list[i];
    const purse = p.data as Purse | null;
    // G: offer my first gem to the next player, for gemPrice coins.
    if (p.input.pressed('G') && purse !== null && purse.gems.length > 0 && list.length > 1) {
      const buyer = list[(i + 1) % list.length].id;
      const price = Number(ctx.rules.gemPrice);
      Object.assign(offer, {
        open: true,
        from: p.id,
        to: buyer,
        give: { gems: [purse.gems[0]] },
        take: { coins: price },
      });
      ctx.net.send('offered', { from: p.id, price }, { to: buyer });
    }
    // H: accept the offer made to me, if the documents still match it.
    if (p.input.pressed('H') && offer.open && offer.to === p.id) {
      offer.open = false;
      const seller = ctx.players.get(offer.from)?.data;
      if (applyTransfer(seller, purse, offer.give, offer.take) === null) continue; // stale
      ctx.data.exchange(offer.from, offer.to, offer.give, offer.take);
    }
  }
  const results = ctx.data.results();
  for (let i = 0; i < results.length; i += 1) {
    ctx.net.send('traded', { ok: results[i].ok, reason: results[i].reason });
  }
}

On success both documents already show the trade when the result arrives, and the room has written both. On a failure (stale, when the store's copy moved on; refused, when a side is short there) neither document changed.

Reset the offer in init, beside the resets already there.

init: (ctx) => {
  offer.open = false;
  // ...the resets already there
},

Wire it in systems, on the authority.

systems: [
  // ...the systems already there
  { run: trading, on: 'authority' },
],

Verify

npm test
npx gameable serve --direct
npm run dev

Open a room in two tabs, as in Play with friends. Press G in the first: the second is sent offered. Press H in the second: both are sent traded with ok: true, and the gem has changed hands (log ctx.players.get(id).data on the authority to see it). Press H again: nothing, the offer is gone. Close both tabs and open the same room in the same two browsers again: the gem is still where the trade left it, because the store keys a document by the player, not the seat. A room server started without GAMEABLE_PG_URL keeps documents in memory until it stops; Play Solo is a room of one, so it has nobody to trade with.

Trade with another player ​

Goal ​

Files you will edit ​

Steps ​

Verify ​

See also ​

Trade with another player

Goal

Files you will edit

Steps

Verify

See also