# Main Extension

The Main Extension API is the interface to Lens's main process.
Lens runs in both main and renderer processes.
The Main Extension API allows you to access, configure, and customize Lens data, add custom application menu items, and run custom code in Lens's main process.

## `LensMainExtension` Class

### `onActivate()` and `onDeactivate()` Methods

To create a main extension simply extend the `LensMainExtension` class:

```typescript
import { LensMainExtension } from "@k8slens/extensions";

export default class ExampleExtensionMain extends LensMainExtension {
  onActivate() {
    console.log('custom main process extension code started');
  }

  onDeactivate() {
    console.log('custom main process extension de-activated');
  }
}
```

Two methods enable you to run custom code: `onActivate()` and `onDeactivate()`.
Enabling your extension calls `onActivate()` and disabling your extension calls `onDeactivate()`.
You can initiate custom code by implementing `onActivate()`.
Implementing `onDeactivate()` gives you the opportunity to clean up after your extension.

Disable extensions from the Lens Extensions page:

1. Navigate to **File** > **Extensions** in the top menu bar.
(On Mac, it is **Lens** > **Extensions**.)
2. Click **Disable** on the extension you want to disable.

The example above logs messages when the extension is enabled and disabled.
To see standard output from the main process there must be a console connected to it.
Achieve this by starting Lens from the command prompt.

The following example is a little more interesting.
It accesses some Lens state data, and it periodically logs the name of the cluster that is currently active in Lens.

```typescript
import { LensMainExtension, Store } from "@k8slens/extensions";

export default class ActiveClusterExtensionMain extends LensMainExtension {

  timer: NodeJS.Timeout

  onActivate() {
    console.log("Cluster logger activated");
    this.timer = setInterval(() => {
      if (!Store.ClusterStore.getInstance().active) {
        console.log("No active cluster");
        return;
      }
      console.log("active cluster is", Store.ClusterStore.getInstance().active.contextName)
    }, 5000)
  }

  onDeactivate() {
      clearInterval(this.timer)
      console.log("Cluster logger deactivated");
  }
}
```

For more details on accessing Lens state data, please see the [Stores](../stores) guide.

### `appMenus`

The Main Extension API allows you to customize the UI application menu.
Note that this is the only UI feature that the Main Extension API allows you to customize.
The following example demonstrates adding an item to the **Help** menu.

``` typescript
import { LensMainExtension } from "@k8slens/extensions";

export default class SamplePageMainExtension extends LensMainExtension {
  appMenus = [
    {
      parentId: "help",
      label: "Sample",
      click() {
        console.log("Sample clicked");
      }
    }
  ]
}
```

`appMenus` is an array of objects that satisfy the `MenuRegistration` interface.
`MenuRegistration` extends React's `MenuItemConstructorOptions` interface.
The properties of the appMenus array objects are defined as follows:

* `parentId` is the name of the menu where your new menu item will be listed.
Valid values include: `"file"`, `"edit"`, `"view"`, and `"help"`.
`"lens"` is valid on Mac only.
* `label` is the name of your menu item.
* `click()` is called when the menu item is selected.
In this example, we simply log a message.
However, you would typically have this navigate to a specific page or perform another operation.
Note that pages are associated with the [`LensRendererExtension`](renderer-extension.md) class and can be defined in the process of extending it.
