Compound Pattern

Create multiple components that work together to perform a single task

Overview

With the Compound Pattern, we can create multiple components that work together to perform one single task.

Let's say for example that we have a Search input component. When a user clicks on the search input, we show a SearchPopup component that shows some popular locations.

To create this behavior, we can create a FlyOut compound component.

This FlyOut component is an example of a compound component, as it also exposes some sub-components that all work together to toggle and render the FlyOut component.

import React from "react";
import { FlyOut } from "./FlyOut";

export default function SearchInput() {
  return (
    <FlyOut>
      <FlyOut.Input placeholder="Enter an address, city, or ZIP code" />
      <FlyOut.List>
        <FlyOut.ListItem value="San Francisco, CA">
          San Francisco, CA
        </FlyOut.ListItem>
        <FlyOut.ListItem value="Seattle, WA">Seattle, WA</FlyOut.ListItem>
        <FlyOut.ListItem value="Austin, TX">Austin, TX</FlyOut.ListItem>
        <FlyOut.ListItem value="Miami, FL">Miami, FL</FlyOut.ListItem>
        <FlyOut.ListItem value="Boulder, CO">Boulder, CO</FlyOut.ListItem>
      </FlyOut.List>
    </FlyOut>
  );
}

The FlyOut compound component is a stateful component - which means we don't have to add the stateful logic to the SearchInput component.

Implementation

We can implement the Compound pattern using either a Provider, or React.Children.map.

Provider

The FlyOut compound component consists of:

FlyoutContext to keep track of the visbility state of FlyOut
Input to toggle the FlyOut's List component's visibility
List to render the FlyOut's ListItemss
ListItem that gets rendered within the List.

const FlyOutContext = React.createContext();

export function FlyOut(props) {
  const [open, setOpen] = React.useState(false);
  const [value, setValue] = React.useState("");
  const toggle = React.useCallback(() => setOpen((state) => !state), []);

  return (
    <FlyOutContext.Provider value={{ open, toggle, value, setValue }}>
      <div>{props.children}</div>
    </FlyOutContext.Provider>
  );
}

function Input(props) {
  const { value, toggle } = React.useContext(FlyOutContext);

  return <input onFocus={toggle} onBlur={toggle} value={value} {...props} />;
}

function List({ children }) {
  const { open } = React.useContext(FlyOutContext);

  return open && <ul>{children}</ul>;
}

function ListItem({ children, value }) {
  const { setValue } = React.useContext(FlyOutContext);

  return <li onMouseDown={() => setValue(value)}>{children}</li>;
}

FlyOut.Input = Input;
FlyOut.List = List;
FlyOut.ListItem = ListItem;

Although we didn't have to name our compound component's sub-components FlyOut.<ComponentName>, it's an easy way to identify compound components, and only requires a single import.

Another way to implement the Compound pattern, is to use React.Children.map in combination with React.cloneElement. Instead of having to use the Context API like in the previous example, we now have access to these two values through props.

export function FlyOut(props) {
  const [open, setOpen] = React.useState(false);
  const [value, setValue] = React.useState("");
  const toggle = React.useCallback(() => setOpen((state) => !state), []);

  return (
    <div>
      {React.Children.map(props.children, (child) =>
        React.cloneElement(child, { open, toggle, value, setValue })
      )}
    </div>
  );
}

function Input(props) {
  const { value, toggle } = React.useContext(FlyOutContext);

  return <input onFocus={toggle} onBlur={toggle} value={value} {...props} />;
}

function List({ children }) {
  const { open } = React.useContext(FlyOutContext);

  return open && <ul>{children}</ul>;
}

function ListItem({ children, value }) {
  const { setValue } = React.useContext(FlyOutContext);

  return <li onMouseDown={() => setValue(value)}>{children}</li>;
}

FlyOut.Input = Input;
FlyOut.List = List;
FlyOut.ListItem = ListItem;

All children components are cloned, and passed the value of open, toggle, value and setValue.

Tradeoffs

State management: Compound components manage their own internal state, which they share among the several child components. When implementing a compound component, we don't have to worry about managing the state ourselves.

Single import: When importing a compound component, we don't have to explicitly import the child components that are available on that component.

Nested components: When using React.Children.map, only direct children of the parent component will have access to the open and toggle props, meaning we can't wrap any of these components in another component.

function FlyoutMenu() {
  return (
    <FlyOut>
      {/* This breaks, since the direct child of FlyOut is now a div */}
      <div>
        <FlyOut.Input />
        <FlyOut.List>
          <FlyOut.ListItem>San Francisco, CA</FlyOut.ListItem>
          <FlyOut.ListItem>Seattle, WA</FlyOut.ListItem>
        </FlyOut.List>
      </div>
    </FlyOut>
  );
}

Naming collisions: Cloning an element with React.cloneElement performs a shallow merge. If we pass a prop that already exists on the component, in this example open or toggle, a naming collision occurs, and the value of these props will be overwritten with the latest value that we pass.

Challenge

Exercise

Create a FlyOut component that gets rendered by the Input component. This component should contain:

FlyOut.Input to render the input field
FlyOut.List to render the list items
FlyOut.ListItem to render the list items

Compound Pattern

Overview

Implementation

Provider

`React.Children.map`

Tradeoffs

Challenge

Exercise

Solution