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](https://github.com/LLK/scratch-blocks).

[![Build Status](https://travis-ci.org/LLK/scratch-vm.svg?branch=develop)](https://travis-ci.org/LLK/scratch-vm)
[![Dependency Status](https://david-dm.org/LLK/scratch-vm.svg)](https://david-dm.org/LLK/scratch-vm)
[![devDependency Status](https://david-dm.org/LLK/scratch-vm/dev-status.svg)](https://david-dm.org/LLK/scratch-vm#info=devDependencies)

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

In your own node environment/application:
```bash
npm install https://github.com/LLK/scratch-vm.git
```
If you want to edit/play yourself:
```bash
git clone git@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:
```bash
npm start
```
Or on Windows:
```bash
StartServerWindows.bat
```

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

![VM Playground Screenshot](https://i.imgur.com/nOCNqEc.gif)


## Standalone Build
```bash
make build
```

```html
<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.
```js
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](https://en.wikipedia.org/wiki/Abstract_syntax_tree) (AST) by listening to events emitted by the [scratch-blocks](https://github.com/LLK/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:
```json
{
  "_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
```bash
make test
```

```bash
make coverage
```

## Donate
We provide [Scratch](https://scratch.mit.edu) free of charge, and want to keep it that way! Please consider making a [donation](https://secure.donationpay.org/scratchfoundation/) to support our continued engineering, design, community, and resource development efforts. Donations of any size are appreciated. Thank you!