This document describes Empirica's server, client and shared APIs.

Server

Empirica.gameInit(callback)

The gameInit callback is called just before a game starts, when all players are ready, and it must create rounds and stages for the game.

One (and one only) gameInit callback is required for Empirica to work.

The callback receives one argument, the game object, which gives acces to the players and the treatment for this game.

It also offers the addRound() method, which allows to add a round to the game. The returned Round object will implement the addStage(stageArgs) method, which allows to add a Stage to the Round. The stageArgs object to be passed to the stage creation method must contain:

• name: the name used to identify this stage in the UI code

• displayName: which will be showed to the UI to players

• durationInSeconds: the stage duration, in seconds

Note that the Game has not yet been created when the callback is called, and you do not have access to the other properties of the Game which will be created subsequently.

Example

Empirica.gameInit(game => {
  game.players.forEach((player, i) => {
    player.set("avatar", `/avatars/jdenticon/${player._id}`);
    player.set("score", 0);
  });
​
  _.times(10, i => {
    const round = game.addRound();
    round.addStage({
      name: "response",
      displayName: "Response",
      durationInSeconds: 120
    });
​
    if (game.treatment.playerCount > 1) {
      round.addStage({
        name: "response",
        displayName: "Response",
        durationInSeconds: 120
      });
    }
  });
});

Game Hooks

Game hooks are optional methods attached to various events throughout the game life cycle to update data on the server-side.

Contrary to client side data updates, sever-side updates are synchronous, there is no risk of conflicting updates, and important calculations can be taken at precise points along the game.

Empirica.onGameStart(callback)

onGameStart is triggered once per game, before the game starts, and before the first onRoundStart. It receives the game object. Contrary to gameInit, the Game has been created at this point.

Example

Empirica.onGameStart(game => {
  if (game.treatment.myFactor === "fourtytwo") {
    game.set("maxScore", 100);
  } else {
    game.set("maxScore", 0);
  }
});

Empirica.onRoundStart(callback)

onRoundStart is triggered before each round starts, and before onStageStart. It receives the same options as onGameStart, and the round that is starting.

Example

Empirica.onRoundStart((game, round) => {
  round.set("scoreToReach", game.get("maxScore"));
});

Empirica.onStageStart(callback)

onRoundStart is triggered before each stage starts. It receives the same options as onRoundStart, and the stage that is starting.

Example

Empirica.onStageStart((game, round, stage) => {
  stage.set("randomColor", myRandomColorGenerator());
});

Empirica.onStageEnd(callback)

onStageEnd is triggered after each stage. It receives the current game, the current round, and stage that just ended.

Example

Empirica.onStageEnd((game, round, stage) => {
  stage.set("scoreGroup", stage.get("score") > 10 ? "great" : "not_great");
});

Empirica.onRoundEnd(callback)

onRoundEnd is triggered after each round. It receives the current game, and the round that just ended.

Example

Empirica.onRoundEnd((game, round) => {
  let maxScore = 0;
  game.players.forEach(player => {
    const playerScore = player.round.get("score") || 0;
    if (playerScore > maxScore) {
      maxScore = playerScore;
    }
  });
  round.set("maxScore", maxScore);
});

Empirica.onGameEnd(callback)

onGameEnd is triggered when the game ends. It receives the game that just ended.

Example

Empirica.onGameEnd(game => {
  let maxScore = 0;
  game.rounds.forEach(round => {
    const roundMaxScore = round.get("maxScore") || 0;
    if (roundMaxScore > maxScore) {
      maxScore = roundMaxScore;
    }
  });
  game.set("maxScore", maxScore);
});

Change Callbacks

​onSet, onAppend and onChange are called on every single update made by all players in each game, so they can rapidly become computationally expensive and have the potential to seriously slow down the app. Use wisely.

It is very useful to be able to react to each update a user makes. Try nontheless to limit the amount of computations and database saves done in these callbacks. You can also try to limit the amount of calls to set() and append() you make (avoid calling them on a continuous drag of a slider for example) and inside these callbacks use the key argument at the very beginning of the callback to filter out which keys your need to run logic against.

If you are not using these callbacks, comment them out, so the system does not call them for nothing.

Empirica.onSet(callback)

onSet is called when the experiment code call the .set() method on games, rounds, stages, players, playerRounds or playerStages.

Example

Empirica.onSet((
  game,
  round,
  stage,
  player, // Player who made the change
  target, // Object on which the change was made (eg. player.set() => player)
  targetType, // Type of object on which the change was made (eg. player.set() => "player")
  key, // Key of changed value (e.g. player.set("score", 1) => "score")
  value, // New value
  prevValue // Previous value
) => {
  // Example filtering
  if (key !== "value") {
    return;
  }
​
  // Do some important calculation
});

Empirica.onAppend(callback)

onSet is called when the experiment code call the .append() method on games, rounds, stages, players, playerRounds or playerStages.

Example

Empirica.onAppend((
  game,
  round,
  stage,
  player, // Player who made the change
  target, // Object on which the change was made (eg. player.set() => player)
  targetType, // Type of object on which the change was made (eg. player.set() => "player")
  key, // Key of changed value (e.g. player.set("score", 1) => "score")
  value, // New value
  prevValue // Previous value
) => {
  // Note: `value` is the single last value (e.g 0.2), while `prevValue` will
  // be an array of the previsous values (e.g. [0.3, 0.4, 0.65]).
});

Empirica.onChange(callback)

onChange is called when the experiment code call the .set() or the .append() method on games, rounds, stages, players, playerRounds or playerStages.

onChange is useful to run server-side logic for any user interaction. Note the extra isAppend boolean that will allow to differenciate sets and appends.

Example

Empirica.onChange((
  game,
  round,
  stage,
  player, // Player who made the change
  target, // Object on which the change was made (eg. player.set() => player)
  targetType, // Type of object on which the change was made (eg. player.set() => "player")
  key, // Key of changed value (e.g. player.set("score", 1) => "score")
  value, // New value
  prevValue, // Previous value
  isAppend // True if the change was an append, false if it was a set
) => {
  Game.set("lastChangeAt", new Date().toString());
});

Empirica.onSubmit(callback)

onSubmit is called when the experiment code call the .submit() on a Stage.

Note that onSubmit is only called if .submit() is explicitely called on the Stage object. Players for which the stage times out naturally, onSubmit will not be triggered.

Example

Empirica.onSubmit((game, round, stage, player) => {
  stage.set("lastSubmitAt", new Date().toString());
});

Adding Bots

Adding bots to a game is as simple as defining a few callbacks. You can add different bots with different behaviors.

Empirica.bot(name, configuration)

The bot method allows to add a bot with name (e.g. "Alice"), while the configuration is a set of callbacks that will allow to configure how the bot is suppose to react in certain conditions.

The configuration has the follows callbacks:

• onStageTick: called during each stage at 1 second interval

• onStageStart: CURRENTLY NOT SUPPORTED called at the beginning of each

  stage (after onRoundStart/onStageStart)

• onStageEnd: CURRENTLY NOT SUPPORTED called at the end of each stage

  (after onStageEnd, before onRoundEnd if it's the enf of the round)

• onPlayerChange: CURRENTLY NOT SUPPORTED called each time any (human)

  player has changed a value

All callbacks are called with the following arguments:

• bot: is the Player object representing this bot

• game: the current Game​

• round: the current Round​

• stage: the current Stage​

• secondsRemaining: the number of remaining seconds in the stage

Example

Empirica.bot("bob", {
  onStageTick(bot, game, round, stage, secondsRemaining) {
    let score = 0;
    game.players.forEach(player => {
      if (player === bot) {
        return;
      }
      const playerScore = player.get("score");
      if (playerScore > score) {
        score = playerScore
      }
    });
    bot.set("score", score+1);
  }
};)

Client

Empirica.round(Component)

Set the Round Component that will contain all of the UI logic for your game.

Props

Component will receive the following props:

Example

const Round = ({ player, game, round, stage }) => (
  <div className="round">
    <div className="profile">{player.id}: {player.get("score")}</p>
    <div className="stimulus">{stage.get("somePieceOfData...")</p>
    // ... Add round logic here. This is not a good example, we recommend you
    // take a look a Tutorial or a Demo app for better examples.
  </div>
);
Empirica.round(Round);

Empirica.consent(Component)

Optionally set the Consent Component you want to present players before they are allowed to register.

Props

Component will receive the following props:

Example

const Consent = ({ onConsent }) => (
  <div className="consent">
    <p>This experiment is part of...</p>
    <p>
      <button onClick={onConsent}>I CONSENT</button>
    </p>
  </div>
);
Empirica.consent(Consent);

Empirica.introSteps(callback)

Set the intro steps to present to the user after consent and registration, and before they are allowed into the Lobby. These steps might be instructions, a quiz/test, a survey... You may present the steps in multiple pages.

The introSteps callback should return an array of 0 or more React Components to show the user in order.

Props

Component will receive the following props:

N.B.: The game given here only has the treatment field defined as the game has not yet been created.

Example

Empirica.introSteps((game, player) => {
  const steps = [InstructionStepOne];
  if (game.treatment.playerCount > 1) {
    steps.push(InstructionStepTwo);
  }
  steps.push(Quiz);
  return steps;
});

N.B.: InstructionStepOne or Quiz, in this example, are components that are not implemented in this example, they are simply React Components.

Empirica.exitSteps(callback)

Set the exit steps to present to the user after the game has finished successfully (all rounds finished) or not (lobby timeout, cancelled game,...)

The exitSteps callback should return an array of 1 or more React Components to show the user in order.

Props

Component will receive the following props:

Example

Empirica.exitSteps((game, player) => {
  if (player.exitStatus !== "finished") {
    return [Sorry];
  }
  return [ExitSurvey, Thanks];
});

N.B.: ExitSurvey or Thanks, in this example, are components that are not implemented in this example, they are simply React Components.

Empirica.lobby(Component)

Optionally set the Lobby Component to replace the default lobby.

Props

Component will receive the following props:

Example

const Lobby = ({ player, gameLobby }) => (
  <header className="lobby">
    <h1>Please wait until the game is ready...</h1>
    <p>
      There are {gameLobby.readyCount} players ready out of{" "}
      {gameLobby.treatment.playerCount} expected total.
    </p>
  </header>
);
Empirica.lobby(Lobby);

Empirica.header(Component)

Optionally set the Header Component to replace the default app header.

Props

Component will NOT receive any props.

Example

const Header = () => (
  <header className="app-header">
    <img src="/my-logo.png" />
    <h1>My Experiment</h1>
  </header>
);
Empirica.header(Header);

Empirica.breadcrumb(Component)

Optionally set the Breadcrumb Component to replace the default Round/Stage progress indicator. This is the UI that shows which are the current Round and Stage, between the page Header and the Round

Props

Component will receive the following props:

Example

const Breadcrumb = ({ round, stage }) => (
  <ul className="breadcrumb">
    <li>Round {round.index + 1}</li>
    {round.stages.map(s => (
      <li key={s.name} className={s.name === stage.name ? "current" : ""}>
        {s.displayName}
      </li>
    ))}
  </ul>
);
Empirica.breadcrumb(Breadcrumb);

Empirica.routes()

routes are the entry point for the Empirica app. It is required to be used as part of the React render tree for Empirica to work properly and the example below usually does not need changing, other than the HTML node to attach to (document.getElementById("app") here).

N.B.: This must be called after any other Empirica calls (Empirica.round(), Empirica.introSteps(), ...).

Example

Meteor.startup(() => {
  render(Empirica.routes(), document.getElementById("app"));
});

Shared

Game object

Round object

Stage object

Player object

GameLobby object