Communitication Between a Header Button and Screen in Reason React Navigation

This post will cover Header Interaction with Screen Component based on the React Navigation docs here.

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

This is the js version from the docs 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.

This is straight for re-unstated-next

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 then HomeScreen module I opened that module with open CounterState then 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 CounterState's useContainer function and didn't want to figure out passing directly in headerRight. So instead, I created the button module and passed in 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()} />;
  };
};

Then you pass it to headerRight like so:

  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!", ());
        }
      />,
     ())));

In the HomeScreen module we render the current count in the ui with this line where we 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 same state container. I do that by wrapping my AppContainer in <Container.Provider>, the state container's provider wrapper.

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

This is what it looks like:

Show me the code.