Controlling mobx style `appState` from cypress tests

Courtesy : @Rebvado :rose:

* SETUP ONCE (app code): In your application code store `appState` on window when running under cypress.

```ts
export const appState = new AppState();

if ((global as any).Cypress) {
(global as any).appState = appState
}
```
* SETUP ONCE (test code): Create a `appStateUtil.ts` to load `appState` into a cypress context. If a test is running without loading a running version of the app (e.g. a unit test), then we just defer to loading the `appState` directly.

```ts
import { appState as INTERNAL_STATE, AppState } from "../../../app/state/appState";
export let appState!: AppState;
/**
* Either loads appState from window for e2e tests
* Or the imported one for unit tests
*/
export const loadAppState = () => {
cy.window().then(global => {
appState = (global as any).appState;
if (!appState) {
appState = INTERNAL_STATE;
}
});
}
```

* USE in e2e test:

```ts
import { appState } from './appStateUtil';

beforeEach(() => {
cy.visit('http://localhost:8080'); // Some url for your e2e test
loadAppState();
})

it('should work', () => {
appState.directlyMoveToSomeState();
})
```

* USE in unit test (without the need to *visit* the app):

```ts
import { appState } from './appStateUtil';

beforeEach(() => {
loadAppState();
})

it('should work', () => {
appState.directlyMoveToSomeState();
})
```

# More

You can create utilities around `appState` that work in *both* `full e2e` and `unit tests` by using `appState` from `testUtil` e.g.

```ts
import { appState } from '../appStateUtil';
async function fillSomeForm() {
await appState.doSomething();
await appState.doSomethingElse();
// etc.
}
```

Start by locating the book's relevant Cypress or testing entry point, then review the app/state/appState path and the appStateUtil.ts pattern shown in the issue. Done means the appState setup and usage for both end-to-end and unit tests are documented in the appropriate place, with the intended scope confirmed.

Written by the indexing model from the issue text.