scratch-vm/README.md

scratch-vm

Scratch VM is a library for representing, running, and maintaining the state of computer programs written using Scratch Blocks.

Installation

This requires you to have Git and Node.js installed.

In your own node environment/application:

npm install https://github.com/LLK/scratch-vm.git

If you want to edit/play yourself:

git clone https://github.com/LLK/scratch-vm.git
cd scratch-vm
npm install

Development Server

This requires Node.js to be installed.

For convenience, we've included a development server with the VM. This is sometimes useful when running in an environment that's loading remote resources (e.g., SVGs from the Scratch server).

Running the Development Server

Open a Command Prompt or Terminal in the repository and run:

npm start

Or on Windows:

StartServerWindows.bat

Playground

To run the Playground, make sure the dev server's running and go to http://localhost:8080/ - you will be directed to the playground, which demonstrates various tools and internal state.

Standalone Build

npm run build

<script src="/path/to/vm.js"></script>
<script>
    var vm = new window.VirtualMachine();
    // do things
</script>

How to include in a Node.js App

For an extended setup example, check out the /playground directory, which includes a fully running VM instance.

var VirtualMachine = require('scratch-vm');
var vm = new VirtualMachine();

// Block events
workspace.addChangeListener(vm.blockListener);

// Run threads
vm.start();

Abstract Syntax Tree

Overview

The Virtual Machine constructs and maintains the state of an Abstract Syntax Tree (AST) by listening to events emitted by the scratch-blocks workspace via the blockListener. Each target (code-running object, for example, a sprite) keeps an AST for its blocks. At any time, the current state of an AST can be viewed by inspecting the vm.runtime.targets[...].blocks object.

Anatomy of a Block

The VM's block representation contains all the important information for execution and storage. Here's an example representing the "when key pressed" script on a workspace:

{
  "_blocks": {
    "Q]PK~yJ@BTV8Y~FfISeo": {
      "id": "Q]PK~yJ@BTV8Y~FfISeo",
      "opcode": "event_whenkeypressed",
      "inputs": {
      },
      "fields": {
        "KEY_OPTION": {
          "name": "KEY_OPTION",
          "value": "space"
        }
      },
      "next": null,
      "topLevel": true,
      "parent": null,
      "shadow": false,
      "x": -69.333333333333,
      "y": 174
    }
  },
  "_scripts": [
    "Q]PK~yJ@BTV8Y~FfISeo"
  ]
}

Testing

npm test

npm run coverage

Donate

We provide Scratch free of charge, and want to keep it that way! Please consider making a donation to support our continued engineering, design, community, and resource development efforts. Donations of any size are appreciated. Thank you!