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The Snap! API

Jens Mönig, Bernat Romagosa, January 07, 2021

This document describes how Snap! can be accessed from an outside program to start scripts, send and retrieve information. The model use case is embedding interactive Snap! projects in other websites such as MOOCs or other adaptive learning platforms.

This experimental Snap! API is a set of methods for an IDE_Morph containing a Snap! project. These methods are maintained to work with future versions of Snap! They can be used to trigger scripts, get feedback from running scripts, and access the project's global variables.

Currently the API consists of the following methods:

Broadcast Messages (and optionally wait)

  • IDE_Morph.prototype.broadcast()

Listen to Messages

  • IDE_Morph.prototype.addMessageListenerForAll()
  • IDE_Morph.prototype.addMessageListener()
  • IDE_Morph.prototype.getMessages()

Access Global Variables

  • IDE_Morph.prototype.getVarNames()
  • IDE_Morph.prototype.getVar()
  • IDE_Morph.prototype.setVar()

Create and Modify Lists

  • IDE_Morph.prototype.newList()

Access the Serialized Project

  • IDE_Morph.prototype.getProjectXML()
  • IDE_Morph.prototype.loadProjectXML()
  • IDE_Morph.prototype.unsavedChanges()

Referencing the IDE

Getting hold of an ide can usually be achieved by evaluating:

var ide = world.children[0];

The model case in mind is embedding Snap! in an iframe:

<!DOCTYPE html>
<html>
    <head>
        <title>Snap! iFrame</title>
    </head>
    <body>
        <iframe id="inlineFrameExample"
            title="Inline Frame Example"
            width="1024"
            height="720"
            src="snap.html">
        </iframe>
    </body>
</html>

In such a set up the ide can be accessed through the contentWindow property, e.g.

var ide = document.getElementsByTagName("iframe")[0].contentWindow.world.children[0];

Cross-domain iframes

If the iframe and the container do not share domains, you won't be able to reach the world and, thus, the API. For that particular case, you should use the postMessage mechanism, as follows:

document.querySelector('iframe').contentWindow.postMessage(
    { selector: <API selector>, params: <param array> },
    '*'
);

For instance, to get the value of a variable named "foo", you would do:

document.querySelector('iframe').contentWindow.postMessage(
    { selector: 'getVar', params: [ 'foo' ] },
    '*'
);

The way to capture the return values of these messages from the page containing the iframe is to define an onmessage listener:

winndow.addEventListener('message',function(e) {
    console.log('the response to', e.data.selector, 'is', e.data.response);
},false);

Note that e.data.selector carries the original selector back, so you can tie it to the request, while e.data.response carries the return value of the API method call.

Interacting with the IDE

IDE_Morph.prototype.broadcast()

The broadcast() method triggers all scripts whose hat block listens to the specified message. An optional callback can be added to be run after all triggered scripts have terminated.

syntax

ide.broadcast(message [, callback]);

parameters

  • message
    • string, the message to be sent to all listeners
  • callback | optional
    • function to execute after all scripts terminate, no arguments

return value

undefined

IDE_Morph.prototype.addMessageListenerForAll()

The addMessageListenerForAll() method sets up a function that will be called whenever a message is broadcast. The function takes one argument, the message being broadcast, and can be used to react to any message. Multiple message listeners can be set up, they all get executed in the order in which they were added.

syntax

ide.addMessageListenerForAll(callback);

parameters

  • callback
    • function to execute whenever a message is sent, takes one argument: The message string

return value

undefined

IDE_Morph.prototype.addMessageListener()

The addMessageListener() method sets up a function that will be called whenever the specified message is broadcast. Multiple message listeners can be set up per message, they all the executed in the order in which they were added.

syntax

ide.addMessageListener(message, callback);

parameters

  • message
    • string, the message to which the listener will react. If the message is an empty string the callback will be executed at any broadcast, passing the message as argument
  • callback
    • function to execute whenever the specified message is sent, takes no argument, except when the message to listen to is the empty string, then it takes the message as argument

return value

undefined

IDE_Morph.prototype.getMessages()

The getMessage() method returns a new Array that contains all the message strings that occur in the project, both in hat blocks and in broadcast blocks.

syntax

ide.getMessages();

return value

an Array of Strings, or an empty Array

IDE_Morph.prototype.getVarNames()

The getVarNames() method returns a new Array that contains all the global variable names in the project.

syntax

ide.getVarNames();

return value

an Array of Strings, or an empty Array

IDE_Morph.prototype.getVar()

The getVar() method returns the value of the global variable indicated by the specified name.

syntax

ide.getVar(name);

return value

whatever value the variable holds.

IDE_Morph.prototype.setVar()

The setVar() methods assigns a value to the a global variable specified by name.

syntax

ide.setVar(name, value);

return value

undefined

IDE_Morph.prototype.newList()

The newList() methods returns a new Snap! list. Optionally a source array containing the list elements can be specified.

syntax

ide.newList([array]);

return value

a new Snap! List

IDE_Morph.prototype.getProjectXML()

the getProjectXML() method returns a string in XML format representing the serialized project currently loaded into the IDE.

syntax

ide.getProjectXML();

return value

an XML String

IDE_Morph.prototype.loadProjectXML()

the loadProjectXML() method replaces the current project of the IDE with another serialized one encoded in a string in XML format. Note that no user acknowledgement is required, all unsaved edits to the prior project are lost.

syntax

ide.loadProjectXML(projectData);

parameters

  • projectData
    • XML string representing a serialized project

return value

unefined

IDE_Morph.prototype.unsavedChanges()

the unsavedChanges() method return a Boolean value indicating whether the currently edited project has been modifed since it was last saved.

syntax

ide.unsavedChanges();

return value

a Boolean

Manipulating Lists

Snap! lists can be accessed and manipulated through a set of methods described in the file lists.js