Advanced example for manually updating subscriptions in response to props changes in an async-safe way

## updating-subscriptions-when-props-change-example.js
// This is an advanced example! It is not typically required for application code.
// If you are using a library like Redux or MobX, use the container component provided by that library.
// If you are authoring such a library, use the technique shown below.

// This example shows how to safely update subscriptions in response to props changes.
// In this case, it is important to wait until `componentDidUpdate` before removing a subscription.
// In the event that a render is cancelled before being committed, this will prevent us from unsubscribing prematurely.

// We also need to be careful about how we handle events that are dispatched in between
// `getDerivedStateFromProps` and `componentDidUpdate` so that we don't put stale values into the `state`.
// To do this, we should use the callback form of `setState` and compare the event dispatcher to the one currently in `state`.

class ExampleComponent extends React.Component {
  state = {
    dataSource: this.props.dataSource,
    subscribedValue: this.props.dataSource.value,
  };

  static getDerivedStateFromProps(nextProps, prevState) {
    if (nextProps.dataSource !== prevState.dataSource) {
      return {
        dataSource: nextProps.dataSource,
        subscribedValue: nextProps.dataSource.value,
      };
    }

    return null;
  }

  componentDidMount() {
    this.finalizeSubscription();
  }

  componentDidUpdate(prevProps, prevState) {
    if (this.state.dataSource !== prevState.dataSource) {
      // Similar to adding subscriptions,
      // It's only safe to unsubscribe during the commit phase.
      prevState.dataSource.unsubscribe(
        this.handleSubscriptionChange
      );

      this.finalizeSubscription();
    }
  }

  componentWillUnmount() {
    this.state.dataSource.unsubscribe(
      this.handleSubscriptionChange
    );
  }

  finalizeSubscription() {
    // Event listeners are only safe to add during the commit phase,
    // So they won't leak if render is interrupted or errors.
    this.state.dataSource.subscribe(
      this.handleSubscriptionChange
    );

    // External values could change between render and mount,
    // In some cases it may be important to handle this case.
    const subscribedValue = this.state.dataSource.value;
    if (subscribedValue !== this.state.subscribedValue) {
      this.setState({subscribedValue});
    }
  }

  handleSubscriptionChange = dataSource => {
    this.setState(state => {
      // If this event belongs to the current data source, update.
      // Otherwise we should ignore it.
      if (dataSource === state.dataSource) {
        return {
          subscribedValue: dataSource.value,
        };
      }

      return null;
    });
  };
}