Compound Components in React: Using the Context API

This post is meant to guide you through a working example on CodeSandbox. I recommend following along on a desktop. 👾

In the first post of this series I introduced compound components, a group of components that work in tandem to produce some common functionality. I explained how to:

  • add “sub-components” to a parent using the static keyword
  • loop through the direct children of a component using React.Children.map
  • identify specific children using the displayName property
  • edit children by passing them additional props using React.cloneElement

These techniques provide a unique ability to abstract away irrelevant implementation details, resulting in a clean API for the end user.

Revisiting drawbacks#

Even though we accomplished what we set out to do in that first post, the solution was quite fickle. Here are a few inconvenient truths posed as questions that you may or may not have asked yourself while following along.

“I have to use this strange displayName property to identify a component?”

“I can only access direct children inside React.Children.map?”

“I have to clone the component I want to pass data to? That doesn’t sound good for performance.”

In my opinion, the biggest drawback surrounds React.Children.map. Not only does it bloat the render method, but it also has the limitation of only giving you access to the direct children of the component you’re rendering.

{React.Children.map(this.props.children, child => {
  /* only direct children */

  /* if statements everywhere! */
  if (child.type.displayName === 'Thing') {}

  if (child.type.displayName === 'OtherThing') {}
})}

This isn’t a long term solution. An alternative one would need to handle the following use cases:

  1. Children should be accessible at all levels of the component tree
  2. A child should be able to explicitly subscribe to a piece of state
  3. A cloning process should not be required to pass data down to children

Does such a solution exist?

React’s Context API#

Enter the Context API, a new addition to the React library in version 16.3. The API allows a component to pass data down to any of its children, whether they are direct or indirect. The official React docs give a great description of what Context is meant for:

Context is designed to share data that can be considered “global” for a tree of React components

There are a slew of great tutorials on this topic, but the offical docs are, in my opinion, the most helpful. I recommend pausing here and brushing up on the concept before moving forward.

Getting started#

I’ve created a starter template on CodeSandbox. It starts exactly where we left off in the last post, with a working implementation of a basic compound component named Chat. If you haven’t read that post it may be helpful to do so in order to gain some context. 😂

In src/index.js the Chat component is being rendered with three children components. Together they make up a single compound component.

// src/index.js

<Chat>
  <Chat.Messages />
  <Chat.Input />
  <Chat.Button />
</Chat>

Because of the nature of this example, these components don’t come with a bundle of options that can be passed as props. The surface API is practically non-existent, however, it should be obvious to anyone reading the code what is happening here.

Creating context#

To start, open up src/components/Chat.js and edit the file so that you are importing the createContext method from the react library. Call the method near the top of the file and set the result equal to a new variable named ChatContext.

// src/components/Chat.js

import React, { Component, createContext } from 'react'

// ...

const ChatContext = createContext()

The createContext method returns an object containing a Provider and Consumer pair. The former exposes data to the latter.

The Chat component will need to be refactored so that it renders the Provider. This means that the unsightly React.Children.map method can finally take hike:

// src/components/Chat.js

// ...

render() {
  const { children } = this.props

  return (
    <ChatContext.Provider>
      <h1>Chatroom</h1>
      {children}
    </ChatContext.Provider>
  )
}

The change above will cause the app to error, but don’t sweat it. The fix will be arriving shortly.

Providing context#

The ChatContext.Provider requires a single prop named value. This prop can be thought of simply as the actual context being provided, and any underlying Consumers will have access to it.

<Provider value={context}>
  /* consumers can access context here */
</Provider>

In order to know what the value prop should be, take a look at all of the props that the sub-components of Chat currently require in order to function.

  • Messages requires a messages prop
  • Input requires a value prop and an onChange prop
  • Button requires an onClick prop

The values of these props all exist somewhere in the Chat component, either in this.state or as class methods. We can combine them into a single object and assign it to the value prop for ChatContext.Provider. This can be done in one of two ways.

The first way is to simply create a new object, we’ll call it context, at the top of the render method with all the necessary data inside of it. Then we can pass that object to ChatContext.Provider:

// src/components/Chat.js

// ...

render() {
  const { children } = this.props;
  const { messages, currentMessage } = this.state;
  const { updateCurrentMessage, add } = this;
  const context = { 
    messages, 
    currentMessage, 
    updateCurrentMessage, 
    add 
  }

  return (
    <ChatContext.Provider value={context}>
      <h1>Chatroom</h1>
      {children}
    </ChatContext.Provider>
  )
}

Although this works, there is a significant downside. The context object must be re-created every time the Chat component renders, even if none of the values inside of the object have changed. This will inevitably cause some unnecessary re-rendering of the components below.

The second and more performant way to handle this would be to add the methods to the state object of Chat, and pass the state directly to ChatContext.Provider:

// src/components/Chat.js

class Chat extends Component {

  // ...

  updateCurrentMessage = event => {/* */};

  add = () => {/* */};

  state = {
    currentMessage: "",
    messages: [],
    updateCurrentMessage: this.updateCurrentMessage,
    add: this.add
  };

  render() {
    const { children } = this.props
    
    <ChatContext.Provider value={this.state}>
      {children}
    </ChatContext.Provider>
  }
}

The first time I saw this my left eye started to twitch. It’s weird. I get it. But this prevents any underlying sub-components from re-rendering unnecessarily.

Consuming context#

The last step in this refactor is to update the sub-components of Chat so that they consume the context created earlier instead of relying on props. In order for this to happen we’ll first need to export ChatContext.Consumer out of Chat.js.

// src/components/Chat.js

// ...

export const ChatConsumer = ChatContext.Consumer

In each sub-component you can now import ChatConsumer and render it as the root element of each.

// src/components/Messages.js

import React from 'react'
import { ChatConsumer } from './Chat'

const Messages = () => (
  <ChatConsumer>
    {({ messages }) => (/* render Messages */)}
  </ChatConsumer>
)
// src/components/Input.js

import React from 'react'
import { ChatConsumer } from './Chat'

const Input = () => (
  <ChatConsumer>
    {({ currentMessage, updateCurrentMessage }) => (/* render Input */)}
  </ChatConsumer>
)
// src/components/Button.js

import React from 'react'
import { ChatConsumer } from './Chat'

const Button = () => (
  <ChatConsumer>
    {({ add }) => (/* render Button */)}
  </ChatConsumer>
)

Note: The Consumer returned by the createContext method uses a render prop. Familiarity with this render prop pattern will definitely be of use here.

Before this refactor, the sub-components of Chat relied on props passed in during the React.cloneElement process. Now, instead of mapping through each child and cloning them, they can each explicitly declare what data they need from the Provider above. This data is a drop-in replacement for the props that were being used before, albeit with a few name changes.

For instance, Button used to expect an onClick prop, which was a reference to the add method on the Chat component. Now it gets direct access to add via Context:

// Before

const Button = ({ onClick }) => (
  <button onClick={onClick}>Send</button>
)


// After

const Button = () = (
  <ChatConsumer>
    {({ add }) => (
      <button onClick={add}>Send</button>
    )}
  </ChatConsumer>
)

The Input component also has a few name changes you’ll need to address. Originally it expected a value and an onChange prop. These were just mappings to this.state.currentMessage and the updateCurrentMessage method on the Chat component respectively:

// Before

const Input = ({ value, onChange }) => (
  <input type="text" value={value} onChange={onChange} />
);


// After

const Input = () => (
  <ChatConsumer>
    {({ currentMessage, updateCurrentMessage }) => (
      <input
        type="text"
        value={currentMessage}
        onChange={updateCurrentMessage}
      />
    )}
  </ChatConsumer>
);

With these changes all errors should be resolved and the application should be working properly as before!

The story so far#

That wasn’t too much work. I’ve definitely had tougher refactors. But, was it worth it? It really depends on your use case. In my opinion, constructing compound components with this strategy is almost always worth it. Especially if you do it this way the first time around.

Let’s hop into src/index.js and see what happens when we apply the same test as the previous post, wrapping Chat.Button in a div element.

// src/index.js

<Chat>
  <Chat.Messages />
  <Chat.Input />
  <div>
    <Chat.Button />
  </div>
</Chat>

Last time we did this, Chat.Button stopped working due to the fact that it wasn’t receiving its onClick This time our app still works!

Context has solved our number one problem: passing information to children no matter where they’re at in the component tree. You can nest that button in a hundred divs and the sucker will still work.

This provides loads of flexiblity to the end user. There is only a single constraint being placed on them, which is that all sub-components of Chat must be rendered beneath Chat. A pretty fair tradeoff I’d say.

Which leads me to my final point of this post. What were to happen if you decided not to render a sub-component, say Chat.Button, underneath Chat?

// Here there be errors, arrrgh!

const App = () => (
  <div>
    <Chat>
      <Chat.Messages />
      <Chat.Input />
    </Chat>
    <Chat.Button />
  </div>
)

Yes, an ugly little error! This is a use case we haven’t planned for, and the chances of this happening in the wild are quite high, especially if you’re working with open source software.

Validating consumers#

This is a nifty trick I picked up from the Advanced React Patterns course given by Kent C Dodds.

Let’s talk about the createContext method real quick. createContext can take an optional argument, defaultValue:

const Context = createContext(defaultValue)

defaultValue comes in to play when a Consumer is rendered outside of a matching Provider.

// a quick example

const { Provider, Consumer } = createContext('red')

const Blue = () => (
  <Provider value='blue'>
    <Consumer>{color => <h1>{color}</h1>}</Consumer>
  </Provider>
)

const Red = () => (
  <Consumer>{color => <h1>{color}</h1>}</Consumer>
)

This is great and all, but a default value is not very helpful in the case of compound components. So what else can we do?

One way to prevent users of Chat from rendering sub-components in the wrong place is to warn them when they’re doing so. This can be done by updating the ChatConsumer to throw an error if no context is found.

// src/components/Chat.js

export const ChatConsumer = ({ children }) => (
  <ChatContext.Consumer>
    {context => {
      if (!context) {
        throw new Error(
          "You do bad thing here!"
        );
      }
      return children(context);
    }}
  </ChatContext.Consumer>
)

ChatConsumer can continue to be used like normal, except now it will throw if it’s rendered out of place. Much more helpful to our users don’t you think? To be even more helpful you may want to craft a more appropriate error message. Something like, Compound components of Chat should render beneath Chat.

Conclusion#

Hopefully this example has given you a better understanding of how compound components can work with the Context API. Possibilities abound!

Reach out to me on Twitter if you have any questions related to this post, or if you just want to talk shop! I would also love to know your thoughts on these walkthrough-style blog posts. Happy coding!