PhET-iO Developer's Guide

Using this developer's guide, you can learn how to develop HTML files that harness the power of PhET-iO simulations. Simulations can be embedded on your site using iframes and can be configured using query parameters or JavaScript. Interoperability (two-way messaging between the hosting frame and the simulation iframe) is moderated using a high-level JavaScript API.



Configure the simulation to launch in a particular state or with customizations using the Query Parameter API. Simulation can be customized for classrooms or individiual learners, or for usage with a particular activity or question. Setting the start state, hiding controls or fixing values, changing text, choosing just one screen, etc.


Send commands to the simulation and receive messages from the simulation. A versatile API - which enables the surrounding technology to "talk" with the simulation, get data, freeze interaction, set values, take screenshots, or capture or set state.

Streaming Data

An event stream (data log) provides detailed messages about how the student used the simulation and how the simulation responded, this can be used for research purposes. The states of the simulation can also be streamed for analysis or playback.


the state of the simulation can be stored and restored


Using states + deltas, the simulation can be recorded and played back like a movie


Each object in a simulation is associated with an identifier and a type. The identifier (a) appears in messages produced by the object in the data log (b) is used to configure the object and (c) 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. For instance, a user interface component commonly found in PhET Simulations is a "reset all" button, which restores the simulation to its initial state. The identifier and type for a reset all button in a simulation may be given like this:

// The Reset All button, which appears in the bottom right of the screen
        resetAllButton: TResetAllButton

The identifier can be used to refer globally to the instance after it has been created, and the type shows what kinds of events and methods are provided by the object. In this case, the TResetAllButton provides an event to the data stream when the button is pressed.

// Names of events emitted by the button
  events: [ 'fired' ]

Here is a larger example of the identifiers and names for PhET's Faraday's Law Simulation:

phetio.api = PhETIOCommon.createAPI( {

// Simulation-specific content
  faradaysLaw: PhETIOCommon.createSim( [ 'faradaysLaw' ], {

    // Content in the screen
    faradaysLawScreen: {

      // Physical values in the model and settings
      model: {
        showSecondCoilProperty: TProperty( TBoolean ),
        showMagnetArrowsProperty: TProperty( TBoolean ),
        magnetModel: {
          positionProperty: TProperty( TVector2 ),
          flippedProperty: TProperty( TBoolean ),
          showFieldLinesProperty: TProperty( TBoolean )
        voltmeterModel: {
          thetaProperty: TProperty( TNumber ) // the angle of the needle, in radians

      // Visible objects
      view: {
        controlPanel: {
          resetAllButton: TResetAllButton,
          flipMagnetButton: TPushButton,
          showFieldCheckBox: TCheckBox,
          singleCoilRadioButton: TRadioButton( TBoolean ),
          doubleCoilRadioButton: TRadioButton( TBoolean )
        magnet: TNode.extend( {
          dragHandler: TTandemDragHandler
        } )
} )

The full identifier for an object is given by the full path to reach the object. For example, the magnet's position is


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.

Query Parameters

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

  • phet-io.emitStates: Outputs the full state at the end of every frame.
  • phet-io.emitDeltas: Outputs state keyframes every 10 seconds and deltas every frame
  • phet-io.emitEmptyDeltas: When emitting states using phetio.js, emit deltas that are empty, to simplify playback in some systems.
  • phet-io.log: 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 phet-io.log=lines provides a colorized single-line output.
  • Will output type documentation to the console, see
  • phet-io.expressions: Evaluate expressions on phetio wrapper objects, like:
    Described in more detail below.

Query Parameter API for instrumented instances

Instrumented objects in a simulation can be customized through a query parameter named phet-io.expressions. Here is an example of that query parameter:


The list of available methods and types are provided below in the types section. A complete an example of this is provided in customization.html. Please be aware that different browsers have different limits on the length of query strings. If a query string becomes too long, it will simply be truncated, leading to errors during parsing. Internet Explorer has the lowest maximum query string length at approximately 2000 characters. This limit can be reached easily if setting many state variables through the query string. An alternative to configuration with query strings is to configure the simulation after it launches using the iframe API.

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.

var sim = WrapperUtils.getSim( 'concentration' );

// 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 phet-io.standalone
simFrame.src = sim.URL + '&phet-io.standalone';

// 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. 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 requirejs portion 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 of the common types, and was generated by running an instrumented simulation with the query parameter ? The list of all common types is given below:

A wrapper for the built-in JS array type, with the element type specified.
setValue: () ➔ TVoid
Sets the value of all elements in the array
Wrapper for the built-in JS boolean type (true/false)
A pressable button in the simulation
events: fired
fire: () ➔ TVoid
Fire the button's action, as if the button has been pressed and released
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
A color, with rgba
A traditional combo box
events: fired,popupShown,popupHidden
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
Event system, with multiple channels defined by string keys
events: documentation , typeName, methods, supertype, getMethodDeclaration, extend
addListener: (TString, TFunctionWrapper) ➔ TVoid
Adds a listener to the specified event channel
removeListener: (TString, TFunctionWrapper) ➔ TVoid
Removes a listener that was added with addListener
Faucet that emits fluid, typically user-controllable
events: dragStarted, dragged, dragEnded, startTapToDispense, endTapToDispense
The item buttons shown in a popup menu
events: fired
Button that performs an action while it is being pressed, and stops the action when released
events: pressed, released, releasedDisabled
The base type for graphical and potentially interactive objects
setRotation: (TNumber) ➔ TVoid
Set the rotation of the node, in radians
setVisible: (TBoolean) ➔ TVoid
Set whether the node will be visible (and interactive)
Wrapper for the built-in JS number type (floating point, but also represents integers)
The root of the wrapper object hierarchy
An array that sends notifications when its values have changed.
events: itemAdded
setValue: (TObject) ➔ TVoid
Set all of the elements simultaneously
A traditional switch component
events: toggled
A container for other TNodes
The buttons used in the home screen and navigation bar
events: fired
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
A traditional radio button
events: fired
The round (typically orange) button that restores the simulation screen to its initial state
events: fired
The type for the simulation instance
events: simStarted, state, stateDelta, frameCompleted, stepSimulation, inputEvent
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
A traditional slider component, with a knob and possibly tick marks
events: dragStarted, dragged, dragEnded
setMajorTicksVisible: (TBoolean) ➔ TVoid
Set whether the major tick marks should be shown
setMinorTicksVisible: (TBoolean) ➔ TVoid
Set whether the minor tick marks should be shown
Wrapper for the built-in JS string type
Mediator for the phet-io module, with systemwide methods for communicating with the sim or other globals
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
getState: () ➔ TObject
Gets the full state of the simulation
getPhETIOIDs: () ➔ TArray
Gets a list of all of the wrapped instances which are available for interoperability.
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.
setState: (TObject) ➔ TVoid
Sets the full state of the simulation
stepSimulation: (TNumber) ➔ TVoid
Steps one frame of the simulation.
A button that toggles state (in/out) when pressed
events: toggled
A numerical object with x/y scalar values


A good place to start is by downloading one of the example wrapper files, such as EventWrapper. Save this to your hard drive by clicking "View Page Source", then copying and pasting the page source to a file. After downloading the file, the relative paths must be replaced with absolute paths:

  • Replace "../../lib/SimIFrameClient.js" with ""
  • Replace "../../lib/WrapperUtils.js" with ""
  • Replace sim.URL with ""

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.

Tips and Tricks

Reset Issues

When the user presses the Reset All button in the simulation, it may overwrite some of the customization that has been previously applied (such as making a Node visible). Two workarounds for this problem are:

  • Remove the Reset All button
  • Add a listener to the Reset All button