State Smart Contract

In our first greetings smart contract, we created a greet function and exposed it using publicFacet so that it can be remotely called. However, if you notice, there is no state in our smart contract that is preserved between calls. Contracts can use ordinary variables and data structures for state.

In our second example smart contract, we will manage a list of rooms. We want everyone with access to publicFacet to be able to create a new room, and also get current count of rooms. We maintain state using Map data structure as below:

const rooms = new Map();

Anyone can add new rooms by making a call to makeRoom which is defined as:

const makeRoom = id => {
  let count = 0;
  const room = Far('Room', {
    getId: () => id,
    incr: () => (count += 1),
    decr: () => (count -= 1),
  });
  rooms.set(id, room);
  return room;
};

Using makeRoom creates a new room, exposing these functions to be invoked on the newly added room, getId, incr, and decr. As you can see this pattern follows the Object Capability model, as whoever receives the room by invoking makeRoom, will now have access to these three methods. Following this, rooms.set(id, room) adds the newly created room, into the contract's map state variable. A call to getRoomCount function returns the number of rooms in this map.

const getRoomCount = () => rooms.size;

Putting it all together:

import { Far } from '@endo/far';

export const start = () => {
  const rooms = new Map();

  const getRoomCount = () => rooms.size;
  const makeRoom = id => {
    let count = 0;
    const room = Far('Room', {
      getId: () => id,
      incr: () => (count += 1),
      decr: () => (count -= 1),
    });
    rooms.set(id, room);
    return room;
  };

  return {
    publicFacet: Far('RoomMaker', { getRoomCount, makeRoom }),
  };
};

Let us save this contract as 02-state.js and creating a simple test to validate its functionality:

test('state', async t => {
  const { publicFacet } = state.start();
  const actual = await E(publicFacet).getRoomCount();
  t.is(actual, 0);
  await E(publicFacet).makeRoom(2);
  t.is(await E(publicFacet).getRoomCount(), 1);
});

This test asserts that in the beginning the number of rooms is zero and after a call to makeRoom, the number of rooms changes to one. If you're having trouble, check out the tut-02-state branch in the example repo.

Heap state is persistent

Ordinary heap state persists between contract invocations.

We'll discuss more explicit state management for large numbers of objects (virtual objects) and objects that last across upgrades (durable objects) later.