# Rethinking Navigation

An exploration of a component-first API for React Navigation for building more dynamic navigation solutions.

## Considerations

- Should play well with static type system
- Navigation state should be contained in root component (helpful for stuff such as deep linking)
- Component-first API

## Building blocks

### `NavigationContainer`

Component which wraps the whole app. It stores the state for the whole navigation tree.

### `useNavigationBuilder`

Hook which can access the navigation state from the context. Along with the state, it also provides some helpers to modify the navigation state provided by the router. All state changes are notified to the parent `NavigationContainer`.

### Router

The router object provides various helper methods to deal with the state and actions, a reducer to update the state as well as some action creators.

The router is responsible for handling actions dispatched by calling methods on the `navigation` object. If the router cannot handle an action, it can return `null`, which would propagate the action to other routers until it's handled.

### Navigator

Navigators bundle a `router` and a view which takes the navigation state and decides how to render it.

A simple navigator could look like this:

```js
function StackNavigator({ initialRouteName, children, ...rest }) {
  // The `navigation` object contains the navigation state and some helpers (e.g. push, pop)
  // The `descriptors` object contains the screen options and a helper for rendering a screen
  const { state, navigation, descriptors } = useNavigationBuilder(StackRouter, {
    initialRouteName,
    children,
  });

  return (
    // The view determines how to animate any state changes
    <StackView
      state={state}
      navigation={navigation}
      descriptors={descriptors}
      {...rest}
    />
  );
}

export default createNavigator(StackNavigator);
```

The navigator can render a screen by calling `descriptors[route.key].render()`. Internally, the descriptor adds appropriate wrappers to handle nested state.

## Initial state

In the current implementation of React Navigation, the initial state is extracted from the navigator definitions. This is possible because they are defined statically. In our case, it's not possible because the screens are rendered dynamically.

Turns out we don't really need the initial state in the `NavigationContainer`. This state is the default state, so we can store `undefined` instead, and let the navigators initialize their initial state themselves. Next time an action modifies the state, we update the value in the container.

If an initial state is specified, e.g. as a result of `Linking.getInitialURL()`, the child navigators will use that state, instead of having to initialize it themselves.

## Basic usage

```js
const Stack = StackNavigator();
const Tab = TabNavigator();

function App() {
  return (
    <NavigationContainer>
      <Stack.Navigator initialRouteName="home">
        <Stack.Screen name="settings" component={Settings} />
        <Stack.Screen
          name="profile"
          component={Profile}
          options={{ title: 'John Doe' }}
        />
        <Stack.Screen name="home">
          {() => (
            <Tab.Navigator initialRouteName="feed">
              <Tab.Screen name="feed" component={Feed} />
              <Tab.Screen name="article" component={Article} />
              <Tab.Screen name="notifications">
                {props => <Notifications {...props} />}
              </Tab.Screen>
            </Tab.Navigator>
          )}
        </Stack.Screen>
      </Stack.Navigator>
    </NavigationContainer>
  );
}
```

Navigators need to have `Screen` components as their direct children. These components don't do anything by themselves, but the navigator can extract information from these and determine what to render. Implementation-wise, we use `React.Children` API for this purpose.

The content to render can be specified in 2 ways:

1. React component in `component` prop (recommended)
2. Render callback as children

When a React component is specified, the navigator takes care of adding a `React.memo` to prevent unnecessary re-renders. However, it's not possible to pass extra props to the component this way. It's preferable to use the context API for such cases instead of props.

A render callback which doesn't have such limitation and is easier to use for this purpose. However, performance optimization for the component is left to the user in such case.

The rendered component will receives a `navigation` prop with various helpers and a `route` prop which represents the route being rendered.

## Setting screen options

In React Navigation, screen options can be specified in a static property on the component (`navigationOptions`). This poses few issues:

- It's not possible to configure options based on props, state or context
- To update the props based on an action in the component (such as button press), we need to do it in a hacky way by changing params
- It breaks when used with HOCs which don't hoist static props, which is a common source of confusion

Instead of a static property, we expose a method to configure screen options:

```js
function Selection({ navigation }) {
  const [selectedIds, setSelectedIds] = React.useState([]);

  navigation.setOptions({
    title: `${selectedIds.length} items selected`,
  });

  return <SelectionList onSelect={id => setSelectedIds(ids => [...ids, id])} />;
}
```

This allows options to be changed based on props, state or context, and doesn't have the disadvantages of static configuration.

## Type-checking

The library exports few helper types. Each navigator also need to export a custom type for the `navigation` prop which should contain the actions they provide, .e.g. `push` for stack, `jumpTo` for tab etc.

Currently type checking and intelliSense works for route name and params. The user has to define a type alias with a list of routes along with the type of params they use.

For our example above, we need 2 separate types for stack and tabs:

```ts
type StackParamList = {
  settings: undefined;
  profile: { userId: string };
  home: undefined;
};

type TabParamList = {
  feed: undefined;
  article: undefined;
  notifications: undefined;
};
```

In a component, it's possible to annotate the `navigation` and `route` props using these types:

```ts
function Profile({
  navigation,
  route,
}: {
  navigation: StackNavigationProp<StackParamList, 'profile'>;
  route: RouteProp<StackParamList, 'profile'>;
}) {
  // Content
}
```

For nested navigators, the `navigation` prop is a combination of multiple `navigation` props, so we need to combine multiple types to type them:

```ts
function Feed({
  navigation,
}: {
  navigation: CompositeNavigationProp<
    TabNavigationProp<TabParamList, 'feed'>,
    StackNavigationProp<StackParamList>
  >;
}) {
  // Content
}
```

Annotating the `navigation` prop will be enough for provide type-checking for actions such as `navigate` etc. Annotating `route` will provide type-checking for accessing `params` for the current route.

It's also possible to type-check the navigator to some extent. To do this, we need to pass a generic when creating the navigator object:

```ts
const Stack = StackNavigator<StackParamList>();
```

And then we can use it:

```js
<Stack.Navigator initialRouteName="profile">
  <Stack.Screen name="settings" component={Settings} />
  <Stack.Screen
    name="profile"
    component={Profile}
    options={{ title: 'My profile' }}
  />
  <Stack.Screen name="home" component={Home} />
</Stack.Navigator>
```

Unfortunately it's not possible to verify that the type of children elements are correct since [TypeScript doesn't support type-checking JSX elements](https://github.com/microsoft/TypeScript/issues/21699).