PhET-iO Developer's Guide

After you are familiar with the PhET-iO features and demos at, and after you have obtained a license for PhET-iO development, you can use this guide to start developing your own wrappers. A wrapper is an HTML page that embeds a PhET-iO simulation in an iframe and provides added value, such as simulation customization, data logging, dynamic feedback, classroom activities, etc. The wrapper communicates with the simulation iframe using a JavaScript API called SimIFrameClient.js

Getting started

Here is an example wrapper that shows how to connect to a PhET-iO from a wrapper, with hooks for initialization, customization, listening for data streams, etc. You can generate a template like this for a particular simulation by launching the "Instance Proxies" wrapper, then pressing the "Generate HTML" button.

Copyright 2016, University of Colorado Boulder
This PhET-iO file requires a license
For licensing, please contact
Minimal template for developing a wrapper.  Used as a template for devguide.
@author Sam Reid (PhET Interactive Simulations)
  <title>PhET-iO Wrapper Harness</title>

<!-- Specify the simulation to run in the iframe. -->
<iframe id="sim" width="768" height="464"></iframe>

<!--Load PhET-iO scripts.-->
<script type="text/javascript"
<script type="text/javascript"
<script type="text/javascript"
<script type="text/javascript"


  var sim = WrapperUtils.getSim( 'faradays-law', '1.3.2-phetio' );

  // Construct the sim iframe client that can be used to send messages to the sim.
  var simIFrameClient = new SimIFrameClient( document.getElementById( 'sim' ) );

  // Choose details of how the sim is launched and fill in callbacks for events
  simIFrameClient.launchSim( sim.URL, {

    // Choose whether the sim should emit states and/or input events in addition to the instance-based messages
    // Can be overriden by query parameters in the wrapper phetioEmitStates and phetioEmitInputEvents
    emitStates: false,
    emitInputEvents: false,

    // Callback for messages from the sim's event stream
    phetioEventsListener: function( message ) {
      console.log( 'message emitted: ', JSON.parse( message ) );

    // Add customization that will be applied as the simulation launches
    expressions: [], // Takes the form { phetioID: {string}, method: {string}, args: {Object[]} }.

    // Callback when the sim is initialized
    onSimInitialized: function() {
      console.log( 'sim initialized' );

      // Send other commands to simIFrameClient as desired, using simIFrameClient.invoke or invokeSequence
  } );

Note that this example code is hard-coded to use "faradays-law" version "1.3.2-phetio". If you use this as a starting point for wrapper development, you'll need to replace all occurrences of those strings with the strings appropriate for your simulation and version. Alternatively, you can launch the Instance Proxies wrapper and press the "Generate HTML" button to create a template like the one above with customizations provided by the Instance Proxies wrapper.

A good place to start is by downloading the example above (or one generated by Instance Proxies => "Generate HTML") and launching it on your development machine. Try printing messages to the console from the phetioEventsListener callback. Once that is working, you can start changing the commands sent across the simIFrameClient and switch to a different sim/version. The links for the SimIFrameClient, WrapperUtils and simulation itself should be taken from the same version of the simulation, this will ensure they are using a compatible communication protocol.

To send one command to one simulation instance, use simIFrameClient.invoke() as shown in the example above. To invoke multiple commands, use simIFrameClient.invokeSequence(), passing in an argument like startupCustomization in the example above.

Instrumented Instances

Each instrumented instance in a simulation is associated with an identifier (called a phetioID) and a type. The phetioID:

  • appears in messages produced by the object in the data log
  • is used to configure the object and
  • is used to interact with the object dynamically

The type describes the nature of the object as well as methods for interacting with the object. The full identifier for an object is given by a dot-delimited string. For example, the in Faraday's Law, the magnet's position has the phetioID:


Note that all identifiers are prefixed by the name of the simulation and the name of the screen within the simulation to facilitate cross-simulation studies. To identify the names and types of all of the instrumented instances in a simulation, launch the "Instance Proxies" wrapper. To see what methods are available for a type, please refer to the "Types" section below.

Query Parameters

Several query parameters govern the behavior of an instrumented PhET simulation:

  • phetioEmitStates: Outputs the full state at the end of every frame.
  • phetioEmitDeltas: Outputs state keyframes every 10 seconds and deltas every frame
  • phetioEmitEmptyDeltas: When emitting states using phetio.js, emit deltas that are empty, to simplify playback in some systems.
  • phetioLog: If set to console, will stream phetioEvents to console. This is useful for understanding or debugging the phetioEvent log output in real-time. If using Chrome or Firefox, using phetioLog=lines provides a colorized single-line output.
  • phetioExpressions: Evaluate expressions on phetio wrapper objects, like:
  • screens: Choose the screens that will be available in the sim. Specify the number of the screen (1-indexed) to just get that screen, and no home screen, i.e ?screens=2. Specify any number of screen indices separated by a comma "," to add desired screens. NOTE: some sims use a period "." to separate screen indices, but we are working toward standardizing to use commas. For example, if you wanted to run Forces and Motion Basics with only the Net Force screen and the Friction screen, you would use "?screens=1,3" (or "?screens=1.3" for older versions). You can also reorder the numbers to change the selection order in the home screen and navbar.

Example Wrappers

Additionally, when you obtain a license for PhET-iO development, you will receive a password to access the protected wrappers for each PhET-iO simulation. The wrappers demonstrate the PhET-iO functionality of the simulation, provide information for developing wrappers and provide a starting point for development.

Data Stream

Here is (the beginning of) a sample data log from the Color Vision simulation:

  "messageIndex": 0,
  "eventType": "model",
  "phetioID": "sim",
  "componentType": "TSim",
  "event": "simStarted",
  "time": 1447285103220,
  "parameters": {
    "sessionID": null,
    "simName": "‪Color Vision‬",
    "simVersion": "1.1.0-phet-io.3",
    "url": "",
    "userAgent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_1) AppleWebKit/537.36 (KHTML, like Gecko)
    Chrome/46.0.2490.80 Safari/537.36",
    "provider": "PhET Interactive Simulations, University of Colorado Boulder"
  "messageIndex": 1,
  "eventType": "user",
  "phetioID": "homeScreen.singleBulbScreenLargeButton",
  "componentType": "TPushButton",
  "event": "fired",
  "time": 1447285111392,
  "children": [
      "messageIndex": 2,
      "eventType": "model",
      "phetioID": "sim.showHomeScreen",
      "componentType": "TProperty",
      "event": "changed",
      "time": 1447285111393,
      "parameters": {
        "oldValue": true,
        "newValue": false
  "messageIndex": 3,
  "eventType": "user",
  "phetioID": "colorVision.singleBulbScreen.photonRadioButton",
  "componentType": "TRadioButton",
  "event": "fired",
  "time": 1447285114104,
  "parameters": {
    "value": "photon"
  "children": [
      "messageIndex": 4,
      "eventType": "model",
      "phetioID": "colorVision.singleBulbScreen.beamType",
      "componentType": "TProperty",
      "event": "changed",
      "time": 1447285114104,
      "parameters": {
        "oldValue": "beam",
        "newValue": "photon"

Here is a brief description of each of the fields in a message:

Attribute Description
messageIndex The number of the message, starting with 0. Can be used to verify that the correct number of messages was received.
eventType {user|model} whether the message was originated by a user action or by the simulation itself. For instance, a button press is a user event while a state change in the simulation is a model event (even if it was triggered by a user event)
phetioID The unique identifier for the object sending the message. It usually starts with ${simulation-name}.${screen-name} except for globals outside of screens, or objects in the navigation bar.
componentType The name of the component type of the object that sent the message. Component type names begin with a capital letter T because they refer to Type wrappers.
event The name of the event that was fired by the object.
time The number of milliseconds since the epoch (in 1960) as reported by the client machine. No effort is made to verify that this matches any other clock.
parameters (optional) Key value pairs that describe the details of the event.
children (optional) Events that were triggered while this event was being processed.

The events are delivered as JSON objects, and must be parsed using a JSON parser. JSON and JavaScript do not guarantee the order of the fields, so the data stream cannot be parsed as simple text under the assumption that fields appear in a deterministic ordering.

iframe API

The iframe API can be used to communicate with a running simulation. Methods can be called on any of the simulation instances. Commands can be sent to them, or values retrieved from them. Here's a full example of how to use the sim iframe API to toggle whether a simulation is active (running) or inactive (frozen), which also demonstrates how to send commands to the simulation and receive messages back from the simulation.

// Get a reference to the iframe for the sim
var simFrame = document.getElementById( 'sim' );

// This sim does not need any listeners to be added before the sim launches,
// so it can be phetioStandalone
simFrame.src = sim.URL + '&phetioStandalone';

// Construct the sim iframe client that can be used to send messages to the sim.
var simIFrameClient = new SimIFrameClient( simFrame );

// Keep track of whether the sim is active or not
var lastActiveValue = true;

// When the user presses the "Toggle Active" button, send a command to the iframe
document.getElementById( 'toggleActiveButton' ).addEventListener( 'click', function() {

  // Send a message using the simIFrameClient to invoke a method on an instance
  simIFrameClient.invoke( sim.camelCaseName + '', 'setValue', [ !lastActiveValue ] );
} );

// When the sim launches, wire up a listener to the active Property,
// so we can report its value and use the correct value when toggling
simIFrameClient.onSimInitialized( function() {

  // Send a message using the simIFrameClient to link up to the active Property
  simIFrameClient.invoke( sim.camelCaseName + '', 'link', [ function( active ) {
    lastActiveValue = active;
    document.getElementById( 'readout' ).innerHTML = 'Active: ' + active;
  } ] );
} );

There are two ways to pass a function across the API, one which runs the function in the parent frame and one which runs the function in the sim frame. For instance, in the above example the args has a function wrapped in an array. This will run in the parent frame. If passing an object literal with {phetioID, method, args}, it will be invoked synchronously in the simulation frame, see the source for state.html for an example.

Startup Sequence

In order to prevent missing any events sent from the simulation to the wrapper (in the case of multi-threaded browser support for multiple frames), PhET-iO provides a startup sequence with a step where the wrapper can register any listeners with phet-io, before the simulation launches. This pattern is implemented in the example provided above. Here is an overview of the startup sequence:

  1. the wrapper starts up and initializes its state
  2. the wrapper creates a listener with `window.addEventListener( 'message',...)`
  3. the wrapper sets the source of the simulation iframe
  4. the simulation preload files initialize (including phetio.js and related files)
  5. the simulation creates a listener with `window.addEventListener( 'message',...)`
  6. the simulation does postMessage indicating that it is ready for phet-io messages
  7. the wrapper receives the postMessage and sends a composite command to the simulation frame, containing adding any desired listeners, setting any desired startup configuration, etc and ending with the directive to start launching the requirejs portion of the simulation. Alternatively, this could be a sequence of individual messages, but the wrapper would have to wait for the response message for each before sending the final go-ahead to launch the remainder of the simulation.
  8. the simulation receives the message to launch the requirejs portion and finishes launching normally.

// Wait for the phase where we can add listeners
simIFrameClient.onPhETiOInitialized( function() {

  // Add any desired listeners to phet-io (cannot communicate with sim instances at this
  // point since they have not been created yet
  simIFrameClient.invoke( 'phetio', 'addPhETIOEventsListener', [ function( message ) {
      textarea.innerHTML += '\n' + message;
      textarea.scrollTop = textarea.scrollHeight;
    } ], function() {

      // after that is complete, launch the simulation
      simIFrameClient.invoke( 'phetio', 'launchSimulation', [] );
} );


Each instrumented instance in a PhET-iO simulation has a type which provides a set of methods which can be called from query parameters or across the iframe api. The events field declares which event names can be emitted by the type in the phetioEvents log. The "T" prefix in front of each type indicates that it is a thin "phet-io" wrapper type instead of the actual implementation simulation type. This section provides documentation for all instrumented types, including the events that can appear in the data stream and the methods associated with each type.

TAccordionBox (extends TNode)
A traditional accordionBox
events: expanded,collapsed
TAquaRadioButton (extends TRadioButton)
A radio button which looks like the Mac "Aqua" radio buttons
events: fired
setCircleButtonVisible: (TBoolean) ➞ TVoid
Sets whether the circular part of the radio button will be displayed.
TArray (extends TObject)
A wrapper for the built-in JS array type, with the element type specified.
TBarrierRectangle (extends TNode)
Shown when a dialog is present, so that clicking on the invisible barrier rectangle will dismiss the dialog
events: fired
TBoolean (extends TObject)
Wrapper for the built-in JS boolean type (true/false)
TBounds2 (extends TObject)
a 2-dimensional bounds rectangle
TBounds3 (extends TObject)
a 3-dimensional bounds (bounding box)
TButtonListener (extends TObject)
Button listener
events: up,over,down,out,fire
TCheckBox (extends TNode)
A traditional checkbox
events: toggled
isChecked: () ➞ TBoolean
Returns true if the checkbox is checked, false otherwise
link: (TFunctionWrapper) ➞ TVoid
Link a listener to the underlying checked TProperty. The listener receives an immediate callback with the current value (true/false)
setChecked: (TBoolean) ➞ TVoid
Sets whether the checkbox is checked or not
TColor (extends TObject)
A color, with rgba
TComboBox (extends TNode)
A traditional combo box
events: fired,popupShown,popupHidden
TComboBoxItemNode (extends TNode)
A traditional item node for a combo box
events: fired
TDerivedProperty (extends TObject)
Like TProperty, but not settable. Instead it is derived from other TDerivedProperty or TProperty instances
events: changed
getValue: () ➞ TObject
Gets the current value
link: (TFunctionWrapper) ➞ TVoid
Adds a listener which will receive notifications when the value changes and an immediate callback with the current value upon linking.
unlink: (TFunctionWrapper) ➞ TVoid
Removes a listener that was added with link
TDialog (extends TPanel)
A dialog panel
TEvents (extends TObject)
Event system, with multiple channels defined by string keys
events: documentation,fromStateObject,typeName,methods,supertype,getMethodDeclaration,allMethods
addListener: (TString,TFunctionWrapper) ➞ TVoid
Adds a listener to the specified event channel
removeListener: (TString,TFunctionWrapper) ➞ TVoid
Removes a listener that was added with addListener
TFaucetNode (extends TNode)
Faucet that emits fluid, typically user-controllable
events: startTapToDispense,endTapToDispense
TFocus (extends TObject)
A wrapper type for the instance in the simulation which currently has keyboard focus.
TFont (extends TObject)
Font handling for text drawing. Options:
style: normal // normal | italic | oblique
variant: normal // normal | small-caps
weight: normal // normal | bold | bolder | lighter | 100 | 200 | 300 | 400 | 500 | 600 | 700 | 800 | 900
stretch: normal // normal | ultra-condensed | extra-condensed | condensed | semi-condensed | semi-expanded | expanded | extra-expanded | ultra-expanded
size: 10px // absolute-size | relative-size | length | percentage -- unitless number interpreted as px. absolute suffixes: cm, mm, in, pt, pc, px. relative suffixes: em, ex, ch, rem, vw, vh, vmin, vmax.
lineHeight: normal // normal | number | length | percentage -- NOTE: Canvas spec forces line-height to normal
family: sans-serif // comma-separated list of families, including generic families (serif, sans-serif, cursive, fantasy, monospace). ideally escape with double-quotes

TFunctionWrapper (extends TObject)
Wrapper for the built-in JS function type
THSlider (extends TNode)
A traditional slider component, with a knob and possibly tick marks
setMajorTicksVisible: (TBoolean) ➞ TVoid
Set whether the major tick marks should be shown
setMinorTicksVisible: (TBoolean) ➞ TVoid
Set whether the minor tick marks should be shown
THSliderTrack (extends TNode)
The track for a knob of a traditional slider
TJoistButton (extends TNode)
The buttons used in the home screen and navigation bar
events: fired
TMenuItem (extends TNode)
The item buttons shown in a popup menu
events: fired
TNavigationBarScreenButton (extends TNode)
A pressable button in the simulation's navigation bar
events: fired
TNode (extends TObject)
The base type for graphical and potentially interactive objects
addPickableListener: (TFunctionWrapper) ➞ TVoid
Adds a listener for when pickability of the node changes
addVisibleListener: (TFunctionWrapper) ➞ TVoid
Adds a listener for when visibility of the node changes
isPickable: () ➞ TBoolean
Gets whether the node is pickable (and hence interactive)
isVisible: () ➞ TBoolean
Gets a Boolean value indicating whether the node can be seen and interacted with
setOpacity: (TNumber) ➞ TVoid
Set opacity between 0-1 (inclusive)
setPickable: (TBoolean) ➞ TVoid
Set whether the node will be pickable (and hence interactive)
setRotation: (TNumber) ➞ TVoid
Set the rotation of the node, in radians
setVisible: (TBoolean) ➞ TVoid
Set whether the node will be visible (and interactive)
TNumber (extends TObject)
Wrapper for the built-in JS number type (floating point, but also represents integers)
TNumberControl (extends TNode)
A numeric readout with a background
The root of the wrapper object hierarchy
TObservableArray (extends TObject)
An array that sends notifications when its values have changed.
events: itemAdded,itemRemoved
TOnOffSwitch (extends TNode)
A traditional switch component
events: toggled
TOptionsDialog (extends TDialog)
A dialog panel
TPanel (extends TNode)
A container for other TNodes
TParticle (extends TObject)
The model for a single particle such as an electron, proton, or neutron.
TParticleAtom (extends TObject)
A model of an atom that tracks and arranges the subatomic particles, i.e. protons, neutrons, and electrons, of which it is comprised. When particles are added, they are moved into the appropriate places. This object also keeps track of things like atomic number, mass number, and charge.
TPhETIO (extends TObject)
Mediator for the phet-io module, with systemwide methods for communicating with the sim or other globals
events: simStarted,state,stateDelta,frameCompleted,stepSimulation,inputEvent,displaySize
addExpressions: (TArray) ➞ TVoid
Set expressions to take effect before the simulation is launched, similar to phetioExpressions, see TObject
addInstanceAddedListener: (TFunctionWrapper) ➞ TVoid
Adds a listener that receives a callback whenever a new sim instance has been prepared for interoperability
addInstanceRemovedListener: (TFunctionWrapper) ➞ TVoid
Removes a listener that was added with addInstanceAddedListener
addPhETIOEventsListener: (TFunctionWrapper) ➞ TVoid
Adds a listener to the phetioEvents event channel, for data analysis
endEvent: (TNumber) ➞ TVoid
End a message
getPhETIOIDs: () ➞ TArray
Gets a list of all of the wrapped instances which are available for interoperability.
getRandomSeed: () ➞ TNumber
Get the random seed, used for replicable playbacks
getState: () ➞ TObject
Gets the full state of the simulation
getValues: (TArray) ➞ TObject
Get the current values for multiple Property/DerivedProperty at the same time. Useful for collecting data to be plotted, so values will be consistent.
invokeInputEvent: (TString) ➞ TVoid
Plays back a recorded input event into the simulation.
isInteractive: () ➞ TBoolean
Gets whether the sim can be interacted with (via mouse/touch).
launchSimulation: () ➞ TVoid
Finish launching the simulation, called from a wrapper after all cross-frame initialization is complete
rasterizeDisplay: (TFunctionWrapper) ➞ TVoid
Rasterize the display, asynchronously
setDisplaySize: (TNumber,TNumber) ➞ TVoid
set the size of the visible region for the simulation
setInteractive: (TBoolean) ➞ TVoid
Sets whether the sim can be interacted with (via mouse/touch).
setPlaybackMode: (TBoolean) ➞ TVoid
Disable the sim clock driver so that time will only pass as it is played back from a log file.
setRandomSeed: (TNumber) ➞ TVoid
Set the random seed, used for replicable playbacks
setSimStartedMetadata: (TObject) ➞ TVoid
Set additional data that is added to the simStarted event.
setState: (TObject) ➞ TVoid
Sets the full state of the simulation
simulateError: () ➞ TVoid
Simulate an error for testing purposes
startEvent: (TObject) ➞ TNumber
Begin a message
stepSimulation: (TNumber) ➞ TVoid
Steps one frame of the simulation.
triggerEvent: (TObject) ➞ TVoid
Start and end a message
TPhetMenu (extends TNode)
The PhET Menu in the bottom right of the screen
TProperty (extends TObject)
Model values that can send out notifications when the value changes. This is different from the traditional observer pattern in that listeners also receive a callback with the current value when the listeners are registered.
events: changed
getValue: () ➞ TObject
Gets the current value.
link: (TFunctionWrapper) ➞ TVoid
Add a listener which will be called when the value changes. The listener also gets an immediate callback with the current value.
setValue: (TObject) ➞ TVoid
Sets the value of the property, and triggers notifications if the value is different
unlink: (TFunctionWrapper) ➞ TVoid
Removes a listener
TPushButton (extends TNode)
A pressable button in the simulation
events: fired
addListener: (TFunctionWrapper) ➞ TVoid
Adds a listener that is called back when the button is pressed.
fire: () ➞ TVoid
Performs the action associated with the button
TRadioButton (extends TNode)
A traditional radio button
events: fired
TRadioButtonGroupMember (extends TNode)
A traditional radio button
events: fired
TRandom (extends TObject)
Generates pseudorandom values
TResetAllButton (extends TPushButton)
The round (typically orange) button that restores the simulation screen to its initial state
events: fired
TRoundMomentaryButton (extends TNode)
Button that performs an action while it is being pressed, and stops the action when released
events: pressed,released,releasedDisabled
TScreen (extends TObject)
A single screen for a PhET simulation
TScreenButton (extends TNode)
A pressable button in the simulation, in the home screen
events: fired
TSim (extends TObject)
The type for the simulation instance
events: simStarted
addEventListener: (TString,TFunctionWrapper) ➞ TVoid
Add an event listener to the sim instance
disableRequestAnimationFrame: () ➞ TVoid
Prevents the simulation from animating/updating
getScreenshotDataURL: () ➞ TString
Gets a base64 representation of a screenshot of the simulation as a data url
TSimIFrameAPI (extends TObject)
Instance of a SimIFrameAPI.
events: invoked
TSphereBucket (extends TObject)
A model of a bucket into which spherical objects can be placed.
TString (extends TObject)
Wrapper for the built-in JS string type
TTandem (extends TObject)
Tandems give sim elements their phet-io identifiers
TEmitter (extends TObject)
Emitters indicate when events have occurred, with optional arguments describing the event
events: emitted
addListener: (TFunctionWrapper) ➞ TVoid
Add a listener which will be called when the emitter emits.
TTandemSimpleDragHandler (extends TObject)
Drag listener for objects that can be dragged by the user.
events: dragStarted,dragged,dragEnded
TText (extends TNode)
The tandem wrapper type for the scenery Text node
events: textChanged
addTextChangedListener: (TFunctionWrapper) ➞ TVoid
Add a listener for when the text has changed.
getFontOptions: () ➞ TFont
Get font options for this TText instance as an object
getMaxWidth: () ➞ TNumber
Get maximum width of text box in px
getText: () ➞ TString
Get the text content
setFontOptions: (TFont) ➞ TVoid
Set font options for this TText instance, e.g. {size: 16, weight: bold}
setMaxWidth: (TNumber) ➞ TVoid
Set maximum width of text box in px. If text is wider than maxWidth at its default font size, it is scaled down to fit.
setText: (TString) ➞ TVoid
Set the text content
TToggleButton (extends TNode)
A button that toggles state (in/out) when pressed
events: toggled
TVector2 (extends TObject)
A numerical object with x/y scalar values
TVector3 (extends TObject)
Basic 3-dimensional vector, represented as (x,y,z)
TVerticalCheckBoxGroup (extends TNode)
A vertical group of checkboxes
TVoid (extends TObject)
Type for which there is no instance, usually to mark functions without a return value
TWavelengthSlider (extends TNode)
A slider that shows wavelengths for selection


SimIFrameAPI Invocation Error
Thrown when a SimIFrameClient.invoke() call causes an internal sim error. In general this is best to report back to the PhET-iO team.

Tips and Tricks

Reset Issues

If you are planning to customize a simulation with a reset button, then special action must be taken. Resetting normally with remove the customizations, restoring the sim to its original state. If you don't need the reset button, then you can hide it using the api for TNode.setVisible(). Otherwise you will need to add a listener to the reset button to apply the customizations on the button fire event. This could be done successfully in SimIFrameClient.onSimInitialized(). Note that you can add SimIFrameClient callbacks from the launchSim options.

var sim = WrapperUtils.getSim( 'faradays-law', '1.3.2-phetio' );

// customizations to be added to the sim on startup.
var expressions = [
    phetioID: 'faradaysLaw.faradaysLawScreen.model.magnetModel.showFieldLinesProperty',
    method: 'setValue',
    args: [ true ]
    phetioID: 'faradaysLaw.faradaysLawScreen.model.showSecondCoilProperty',
    method: 'setValue',
    args: [true]

simIFrameClient.launchSim( sim.URL, {
  expressions: expressions,
  onSimInitialized: function() {

  // When the sim is reset, re-apply the initial customization.
    [ function() {
      simIFrameClient.invokeSequence( expressions );
    } ] );
} );