Header Interaction with Screen Component

See the result at https://expo.io/@idkjs/ReasonHeaderScreenDemo

This is the js version from the docs at here

class HomeScreen extends React.Component {
  static navigationOptions = ({ navigation }) => {
    return {
      headerTitle: <LogoTitle />,
      headerRight: (
        <Button
          onPress={navigation.getParam('increaseCount')}
          title="+1"
          color="#fff"
        />
      ),
    };
  };

  componentDidMount() {
    this.props.navigation.setParams({ increaseCount: this._increaseCount });
  }

  state = {
    count: 0,
  };

  _increaseCount = () => {
    this.setState({ count: this.state.count + 1 });
  };

  /* later in the render function we display the count */
}

In the ReasonML version I could not figure out how to use share the value of the HomeScreen state with the incrementing button module. I took a hint from the doc which say:

React Navigation doesn't guarantee that your screen component will be mounted before the header. Because the increaseCount param is set in componentDidMount, we may not have it available to us in navigationOptions. This usually will not be a problem because onPress for Button and Touchable components will do nothing if the callback is null. If you have your own custom component here, you should make sure it behaves as expected with null for its press handler prop.

As an alternative to setParams, you could use a state management library (such as Redux or MobX) and communicate between the header and the screen in the same way you would with two distinct components.

So I figured I need some sort of state management solution which I have never used at this point. I remembered [@ken_wheeler] talking about how great unstated was so I checked github to see if anyone had tried it in react-native. I found @@mirshko's unleaded repo and go the guidance I needed. While looking around for that I found re-unstated-next by Yujia which implements unstated in ReasonML. You can find that implemented src/UnstatedDemo.re.

Anyway, here is how I got the state shared using a library as suggested by the docs. I created the CounterState.re module to house my counter's state.

// straight from https://github.com/Raincal/re-unstated-next/blob/master/example/Index.re
type counter = {
  count: int,
  decrement: unit => unit,
  increment: unit => unit,
};

let useState = initial => {
  React.useReducer((_, action) => action, initial);
};

let useCounter = (~initialState=0, ()) => {
  let (count, setCount) = useState(initialState);
  let decrement = () => setCount(count - 1);
  let increment = () => setCount(count + 1);
  {count, decrement, increment};
};

module Counter =
  UnstatedNext.CreateContainer({
    type state = int;
    type value = counter;
    let useHook = useCounter;
  });

In HomeScreen module I opened that module with open CounterState the created an IncButton module for the button I want to pass to headerRight in NavigationOptions in the HomeScreen module. I did this because I need to use CounterStates useContainer function and didn't want to figure out passing directly in headerRight. I then passed the IncButton to headerRight.

This is the IncButton that goes in the header to the right and accesses the state container.

module IncButton = {
  [@react.component]
  let make = () => {
    let counter = Counter.useContainer();
    <Button title="+" color="#fff" onPress={_ => counter.increment()} />;
  };
};

  make->NavigationOptions.(setNavigationOptions(t(
    // ~title="Home",
    // headerTitle instead of title
      ~headerTitle=NavigationOptions.HeaderTitle.element(<LogoTitle />),
      ~headerRight=<Button title="Info"
        color="#fff" onPress={_e =>
          Alert.alert(~title="This is a button!", ());
        }
      />,
     ())));

Then in the HomeScreen module we render the current count in the ui with this line where access the count value on Counter.useContainer.

...
<Text>
  {"Count: " ++ string_of_int(Counter.useContainer().count) |> React.string}
</Text>
...

The trick to getting this all to work is to make sure that all these modules have access to the state container. I do that by wrapping my reactnavigation AppContainer in <Container.Provider>, the state container;s provider.

@react.component]
let make = () =>
  <Counter.Provider initialState=0> <AppContainer /> </Counter.Provider>;

This is what it looks like: