# Extensions

Extensions add new functionality to the viewer by extending existing functionality. This can be something as simple as adding a new button to the toolbar, or as complex as a new viewport capable of rendering volumes in 3D. 

## Overview

At a glance, an extension is a class or object that has a `getExtensionId()` method, and one or more "module" methods. You can find an abbreviated extension below, or [view the source](https://github.com/OHIF/Viewers/blob/react/extensions/ohif-cornerstone-extension/src/OHIFCornerstoneExtension.js#L32-L65) of our `cornerstone` viewport extension.

```js
class myCustomExtension {

    /** Required */
    getExtensionId: () => 'my-extension-id';

    /** React component that receives props from ConnectLayoutManager
     *  If more than one viewport module is registered, SopClassHandler
     *  is used to help determine which component is used */
    getViewportModule: () => reactViewportComponent;

    /** React component that adds buttons/behavior to the viewer Toolbar */ 
    getToolbarModule: () => reactToolbarComponent;

    /** Provides a whitelist of SOPClassUIDs the viewport is capable of rendering.
     *  Can modify default behavior for methods like `getDisplaySetFromSeries` */
    getSopClassHandler: () => {
        id: 'some-other-unique-id',
        type: PLUGIN_TYPES.SOP_CLASS_HANDLER,
        sopClassUids: ['string'],
        getDisplaySetFromSeries: (series, study, dicomWebClient, authorizationHeaders) => ...
    };

    // Not yet used
    getPanelModule: () => null;
}
```

### Modules

Modules, or [PLUGIN_TYPES](https://github.com/OHIF/ohif-core/blob/43c08a29eff3fb646a0e83a03a236ddd84f4a6e8/src/plugins.js#L1-L6), help us determine where, when, and how a plugin should be used. For example, 

#### Viewport

A React Component

```js
    studies: PropTypes.object,
    displaySet: PropTypes.object,
    viewportIndex: PropTypes.number,
    children: PropTypes.node,
    customProps: PropTypes.object
```

#### Toolbar

...

#### SopClassHandler

...

#### Panel

...

### Registering Extensions

Extensions are registered for the application at startup. The `ExtensionManager`, exposed by `ohif-core`, registers a list of extensions with our application's store. Each module provided by the extension becomes available via `state.plugins.availablePlugins`, and consists of three parts: id, type (PLUGIN_TYPE), and the return value of the module method.

In a future version, we will likely expose a way to provide the extensions you would like included at startup.

_app.js_
```js
import { createStore, combineReducers } from 'redux';
import OHIF from 'ohif-core';
import OHIFCornerstoneExtension from 'ohif-cornerstone-extension';

const combined = combineReducers(OHIF.redux.reducers);
const store = createStore(combined);
const extensions = [ new OHIFCornerstoneExtension() ];

// Dispatches the `addPlugin` action to the store
// Adding extension modules to `state.plugins.availablePlugins`
ExtensionManager.registerExtensions(store, extensions);
```


## OHIF Maintained Extensions

A small number of powerful extensions for popular use cases are maintained by OHIF. They're co-located in the [`OHIF/Viewers`](https://github.com/OHIF/Viewers/tree/react/) repository, in the top level [`extensions/`](https://github.com/OHIF/Viewers/tree/react/extensions) directory. 


{% include "./_maintained-extensions-table.md" %}



https://github.com/OHIF/ohif-core/blob/43c08a29eff3fb646a0e83a03a236ddd84f4a6e8/src/redux/reducers/plugins.js#L7-L30

- Connected Layout Manager
    - AvailablePlugins that match Viewport type

ToolbarRow
    - AvailablePlugins that match Toolbar type