Managing Complex UI Component State

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Managing state in React is difficult. While many suggest that lifting state up to the highest level of the app, and frequently even outside of the component tree, most apps aren't that simple.

One case where this becomes complicated is when app authors need to interact with complex UI components within their application. I internally wrestle with this concept frequently as I build these UI components everyday at work.

One of the most recent cases where I am running into this is our new carousel component which I started work on about 6 months ago as of the time of writing this blog post. This component is perhaps the most complicated UI component we have at Wayfair.

It manages quite a bit of internal state, ranging from actual React state to many references to the rendered dom nodes as well. The problem here is that we also want end users (app developers) the ability to control the carousel from their business logic.

We started with the concept that render props and prop getters would be sufficient to allow end users to control the carousel, however over the months of development, we realized that many (maybe 90%) of frontend developers at Wayfair (and really anywhere) don't want to manage a lot of state and helpers that comes with a render prop.

So we mostly scrapped the render prop pattern on the carousel fn-1^{[1]Jump to footnote}. Since then we have continued to iterate on the carousel but we were still blocked by this battle of how to properly let end developers control the carousel while also not needing to manage a ton of internal logic (and possibly duplicating that across the codebase).

Skip forward ~ 2 months

On Memorial day weekend of 2018, I decided to spend some time in Codesandbox to work on this problem. One of the rough patterns that I still don't fully grasp from the Downshift component published by Paypal, is the idea of passing back up functions in event handlers to help compute the next state fn-2^{[2]Jump to footnote}.

I started with a simple example, a component that can be both controlled as well as uncontrolled (a counter component). I broke the problem down into two pieces:

1. Local state management
2. External state management

The common logic between these two pieces is the actual methods behind how to update this state. Locally we call setState with either an object or a function. Using an object for setState generally means that we have all the information we need to determine the next state. Whereas using a function means that we need to know the current state of the world to derive the next state.

As with a counting component, a carousel only really needs to know the current slide, the action the user is taking (i.e. clicking next), and the number of slides to scroll by. With this information (which can be derived by the event as well as the current state and props) we can derive the next active index.

So the solution was a concept I will be calling State Updaters, these are functions which can be (and often are) curried methods that lastly return a function that takes in state and props and returns some new state. They are curried because they need additional information, such as selectors and transformers to return the correct state.

Lets break down this step by step in some code.

class Counter extends React.Component {
  state = {
    count: this.props.defaultCount,
    propCount: this.props.count,
  };

  static getDerivedStateFromProps(nextProps, prevState) {
    /* we'll get to this later */
  }

  // This is used for local control flow
  // We determine if we call setState or not
  isControlled = () => typeof this.props.count !== 'undefined';

  handleClick = event => {
    /* we'll get to this later */
  };

  render() {
    return (
      <Fragment>
        <button onClick={this.handleClick}>Increment Count</button>
        {this.state.count}
      </Fragment>
    );
  }
}

So we start off with some local state, a class method for handling the click event from the button, and an isControlled method for determining if an implementer is controlling the component or not.

The next step is to handle the click logic:

// ...
handleClick = event => {
  // If we aren't controlled (i.e. we manage our own state)
  // call setState with the return value of the state updater
  if (!this.isControlled()) {
    this.setState(stateUpdater());
  }
  // Always call the prop handleClick handler
  // first passing the event, and second passing an object
  // with a stateUpdater argument
  this.props.handleClick(event, {
    // defined in the module
    // could also be exported as well
    stateUpdater,
  });
};
// ...

Once we have the local handler resolved, now we can dive into the state updater method.

// return a function that accepts an object with three key fields
// 1. transformState: this determines if the returned state should be nested
// 2. selectState: this selects the correct UI component state within the parent state
// 3. fieldName: this is a string that is the key inside state that we update
export const stateUpdater = ({
  transformState = state => state,
  selectState = state => state,
  fieldName = 'count',
} = {}) =>
// then return another function that accepts state and props
(state, props) => {
  return transformState({
    ...selectState(state),
    [fieldName]: selectState(state)[fieldName] + 1,
  });
};

Now that we know what the stateUpdater looks like, we can dive back into the event handler above. Inside the !this.isControlled() check, we setState using the returned function after calling stateUpdater with undefined (results in our default argument for transformState, selectState and fieldName).

There is one other thing we need to cover to fully make the UI component controllable:

class Counter extends React.Component {
  // ...
  state = {
    count: this.props.defaultCount,
    // this needs to be an exact reference to the value provided by props
    propCount: this.props.count,
  };

  static getDerivedStateFromProps(nextProps, prevState) {
    // if the exact comparison between propCount in local state
    // and the count provided by the props is false
    // then its time to update the component
    if (prevState.propCount !== nextProps.count) {
      return {
        count: nextProps.count,
        propCount: nextProps.count,
      };
    }
    return null;
  }
  // ...
}

Now we can put all this together into a working demo, which you can find here on Codesandbox.

Footnotes:

👆Back to reference Most of the render prop code still lives in the carousel but it will most likely be removed before we open source.

👆Back to reference Side note, I haven't even validated that this is what Downshift does, I have only some second hand knowledge of their helpers that they pass back up.

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