# Stores

Stores are components that persist and synchronize state data. Lens uses a number of stores to maintain various kinds of state information, including:

* The `ClusterStore` manages cluster state data (such as cluster details), and it tracks which cluster is active.
* The `WorkspaceStore` manages workspace state data (such as the workspace name), and and it tracks which clusters belong to a given workspace.
* The `ExtensionStore` manages custom extension state data.

This guide focuses on the `ExtensionStore`.

## ExtensionStore

Extension developers can create their own store for managing state data by extending the `ExtensionStore` class.
This guide shows how to create a store for the [`appPreferences`](../renderer-extension#apppreferences) guide example, which demonstrates how to add a custom preference to the **Preferences** page.
The preference is a simple boolean that indicates whether or not something is enabled.
However, in the example, the enabled state is not stored anywhere, and it reverts to the default when Lens is restarted.

`Store.ExtensionStore`'s child class will need to be created before being used.
It is recommended to call the inherited static method `getInstanceOrCreate()` only in one place, generally within you extension's `onActivate()` method.
It is also recommenced to delete the instance, using the inherited static method `resetInstance()`, in your extension's `onDeactivate()` method.
Everywhere else in your code you should use the `getInstance()` static method.
This is so that your data is kept up to date and not persisted longer than expected.

The following example code creates a store for the `appPreferences` guide example:

``` typescript
import { Store } from "@k8slens/extensions";
import { observable, toJS, makeObservable } from "mobx";

export type ExamplePreferencesModel = {
  enabled: boolean;
};

export class ExamplePreferencesStore extends Store.ExtensionStore<ExamplePreferencesModel> {

  @observable  enabled = false;

  private constructor() {
    super({
      configName: "example-preferences-store",
      defaults: {
        enabled: false
      }
    });
    makeObservable(this);
  }

  protected fromStore({ enabled }: ExamplePreferencesModel): void {
    this.enabled = enabled;
  }

  toJSON(): ExamplePreferencesModel {
    return {
      enabled: toJS(this.enabled)
    };
  }
}
```

First, our example defines the extension's data model using the simple `ExamplePreferencesModel` type.
This has a single field, `enabled`, which represents the preference's state.
`ExamplePreferencesStore` extends `Store.ExtensionStore`, which is based on the `ExamplePreferencesModel`.
The `enabled` field is added to the `ExamplePreferencesStore` class to hold the "live" or current state of the preference.
Note the use of the `observable` decorator on the `enabled` field.
The [`appPreferences`](../renderer-extension#apppreferences) guide example uses [MobX](https://mobx.js.org/README.html) for the UI state management, ensuring the checkbox updates when it's activated by the user.

Next, our example implements the constructor and two abstract methods.
The constructor specifies the name of the store (`"example-preferences-store"`) and the default (initial) value for the preference state (`enabled: false`).
Lens internals call the `fromStore()` method when the store loads.
It gives the extension the opportunity to retrieve the stored state data values based on the defined data model.
The `enabled` field of the `ExamplePreferencesStore` is set to the value from the store whenever `fromStore()` is invoked.
The `toJSON()` method is complementary to `fromStore()`.
It is called when the store is being saved.
`toJSON()` must provide a JSON serializable object, facilitating its storage in JSON format.
The `toJS()` function from [`mobx`](https://mobx.js.org/README.html) is convenient for this purpose, and is used here.

Finally, `ExamplePreferencesStore` is created by calling `ExamplePreferencesStore.getInstanceOrCreate()`, and exported for use by other parts of the extension.
Note that `ExamplePreferencesStore` is a singleton.
Calling this function will create an instance if one has not been made before.
Through normal use you should call `ExamplePreferencesStore.getInstance()` as that will throw an error if an instance does not exist.
This provides some logical safety in that it limits where a new instance can be created.
Thus it prevents an instance from being created when the constructor args are not present at the call site.

If you are doing some cleanup it is recommended to call `ExamplePreferencesStore.getInstance(false)` which returns `undefined` instead of throwing when there is no instance.

The following example code, modified from the [`appPreferences`](../renderer-extension#apppreferences) guide demonstrates how to use the extension store.
`ExamplePreferencesStore` must be loaded in the main process, where loaded stores are automatically saved when exiting Lens.
This can be done in `./main.ts`:

``` typescript
import { LensMainExtension } from "@k8slens/extensions";
import { ExamplePreferencesStore } from "./src/example-preference-store";

export default class ExampleMainExtension extends LensMainExtension {
  async onActivate() {
    await ExamplePreferencesStore.getInstanceOrCreate().loadExtension(this);
  }
}
```

Here, `ExamplePreferencesStore` loads with `ExamplePreferencesStore.getInstanceOrCreate().loadExtension(this)`, which is conveniently called from the `onActivate()` method of `ExampleMainExtension`.
Similarly, `ExamplePreferencesStore` must load in the renderer process where the `appPreferences` are handled.
This can be done in `./renderer.ts`:

``` typescript
import { LensRendererExtension } from "@k8slens/extensions";
import { ExamplePreferenceHint, ExamplePreferenceInput } from "./src/example-preference";
import { ExamplePreferencesStore } from "./src/example-preference-store";
import React from "react";

export default class ExampleRendererExtension extends LensRendererExtension {

  async onActivate() {
    await ExamplePreferencesStore.getInstanceOrCreate().loadExtension(this);
  }

  appPreferences = [
    {
      title: "Example Preferences",
      components: {
        Input: () => <ExamplePreferenceInput />,
        Hint: () => <ExamplePreferenceHint/>
      }
    }
  ];
}
```

Again, `ExamplePreferencesStore.getInstanceOrCreate().loadExtension(this)` is called to load `ExamplePreferencesStore`, this time from the `onActivate()` method of `ExampleRendererExtension`.

`ExamplePreferenceInput` is defined in `./src/example-preference.tsx`:

``` typescript
import { Component } from "@k8slens/extensions";
import { observer } from "mobx-react";
import React from "react";
import { ExamplePreferencesStore } from "./example-preference-store";

@observer
export class ExamplePreferenceInput extends React.Component {

  render() {
    return (
      <Component.Checkbox
        label="I understand appPreferences"
        value={ExamplePreferencesStore.getInstace().enabled}
        onChange={v => { ExamplePreferencesStore.getInstace().enabled = v; }}
      />
    );
  }
}

export class ExamplePreferenceHint extends React.Component {
  render() {
    return (
      <span>This is an example of an appPreference for extensions.</span>
    );
  }
}
```

The only change here is that `ExamplePreferenceProps` defines its `preference` field as an `ExamplePreferencesStore` type.
Everything else works as before, except that now the `enabled` state persists across Lens restarts because it is managed by the
`ExamplePreferencesStore`.
