hoc-pattern（特殊模式）

HOC Pattern

Table of Contents

• When to Use
• When NOT to Use
• Instructions
• Details
• Source

Within our application, we often want to use the same logic in multiple components. This logic can include applying a certain styling to components, requiring authorization, or adding a global state.

One way of being able to reuse the same logic in multiple components, is by using the higher order component pattern. This pattern allows us to reuse component logic throughout our application.

When to Use

• Use this when the same uncustomized behavior needs to be applied to many components
• This is helpful when a component should work standalone without the added custom logic

When NOT to Use

• When custom hooks can achieve the same result with less nesting and better readability
• In new React 18+ code where hooks are the idiomatic approach to sharing stateful logic
• When the HOC wrapper adds prop-name collisions or obscures the component tree in DevTools

Instructions

• Create a function that takes a component and returns a new component with enhanced behavior
• Avoid naming collisions by renaming or merging props in the HOC
• Prefer React Hooks over HOCs for most new code to avoid wrapper hell and deep nesting
• Compose multiple HOCs carefully and be aware that the order of composition matters

Details

A Higher Order Component (HOC) is a component that receives another component. The HOC contains certain logic that we want to apply to the component that we pass as a parameter. After applying that logic, the HOC returns the element with the additional logic.

Say that we always wanted to add a certain styling to multiple components in our application. Instead of creating a style object locally each time, we can simply create a HOC that adds the style objects to the component that we pass to it:

function withStyles(Component) {
  return props => {
    const style = { padding: '0.2rem', margin: '1rem' }
    return <Component style={style} {...props} />
  }
}

const Button = () => <button>Click me!</button>
const Text = () => <p>Hello World!</p>

const StyledButton = withStyles(Button)
const StyledText = withStyles(Text)

We just created a StyledButton and StyledText component, which are the modified versions of the Button and Text component. They now both contain the style that got added in the withStyles HOC!

Let's improve the user experience a little bit. When we're fetching the data, we want to show a "Loading..." screen to the user. Instead of adding data to the DogImages component directly, we can use a Higher Order Component that adds this logic for us.

Let's create a HOC called withLoader. A HOC should receive a component, and return that component. In this case, the withLoader HOC should receive the element which should display Loading… until the data is fetched.

function withLoader(Element) {
  return (props) => <Element />;
}

However, we don't just want to return the element it received. Instead, we want this element to contain logic that tells us whether the data is still loading or not.

To make the withLoader HOC very reusable, we won't hardcode the Dog API url in that component. Instead, we can pass the URL as an argument to the withLoader HOC, so this loader can be used on any component that needs a loading indicator while fetching data from a different API endpoint.

function withLoader(Element, url) {
  return (props) => {};
}

A HOC returns an element, a functional component props => {} in this case, to which we want to add the logic that allows us to display a text with Loading… as the data is still being fetched. Once the data has been fetched, the component should pass the fetched data as a prop.

We just created a HOC that can receive any component and url.

1. In the useEffect hook, the withLoader HOC fetches the data from the API endpoint that we pass as the value of url. While the data hasn't returned yet, we return the element containing the Loading... text.
2. Once the data has been fetched, we set data equal to the data that has been fetched. Since data is no longer null, we can display the element that we passed to the HOC!

So, how can we add this behavior to our application? In DogImages.js, we no longer want to just export the plain DogImages component. Instead, we want to export the "wrapped" withLoading HOC around the DogImages component.

export default withLoader(
  DogImages,
  "https://dog.ceo/api/breed/labrador/images/random/6"
);

The Higher Order Component pattern allows us to provide the same logic to multiple components, while keeping all the logic in one single place. The withLoader HOC doesn't care about the component or url it receives: as long as it's a valid component and a valid API endpoint, it'll simply pass the data from that API endpoint to the component that we pass.

Composing

We can also compose multiple Higher Order Components. Let's say that we also want to add functionality that shows a Hovering! text box when the user hovers over the DogImages list.

We need to create a HOC that provides a hovering prop to the element that we pass. Based on that prop, we can conditionally render the text box based on whether the user is hovering over the DogImages list.

We can now wrap the withHover HOC around the withLoader HOC.

The DogImages element now contains all props that we passed from both withHover and withLoader. We can now conditionally render the Hovering! text box, based on whether the value of the hovering prop is true or false.

A well-known library used for composing HOCs is recompose. Since HOCs can largely be replaced by React Hooks, the recompose library is no longer maintained.

Hooks

In some cases, we can replace the HOC pattern with React Hooks.

Let's replace the withHover HOC with a useHover hook. Instead of having a higher order component, we export a hook that adds a mouseOver and mouseLeave event listener to the element. We cannot pass the element anymore like we did with the HOC. Instead, we'll return a ref from the hook that should get the mouseOver and mouseLeave events.

The useEffect hook adds an event listener to the component, and sets the value hovering to true or false, depending on whether the user is currently hovering over the element. Both the ref and hovering values need to be returned from the hook: ref to add a ref to the component that should receive the mouseOver and mouseLeave events, and hovering in order to be able to conditionally render the Hovering! text box.

Instead of wrapping the DogImages component with the withHover component, we can simply use the useHover hook within the component directly.

Generally speaking, React Hooks don't replace the HOC pattern.

*"In most cases, Hooks will be sufficient and can help reduce nesting in your tree."* - React Docs

As the React docs tell us, using Hooks can reduce the depth of the component tree. Using the HOC pattern, it's easy to end up with a deeply nested component tree.

<withAuth>
  <withLayout>
    <withLogging>
      <Component />
    </withLogging>
  </withLayout>
</withAuth>

By adding a Hook to the component directly, we no longer have to wrap components.

Using Higher Order Components makes it possible to provide the same logic to many components, while keeping that logic all in one single place. Hooks allow us to add custom behavior from within the component, which could potentially increase the risk of introducing bugs compared to the HOC pattern if multiple components rely on this behavior.

Best use-cases for a HOC:

• The *same, uncustomized* behavior needs to be used by many components throughout the application.
• The component can work standalone, without the added custom logic.

Best use-cases for Hooks:

• The behavior has to be customized for each component that uses it.
• The behavior is not spread throughout the application, only one or a few components use the behavior.
• The behavior adds many properties to the component

Case Study

Some libraries that relied on the HOC pattern added Hooks support after the release. A good example of this is Apollo Client.

One way to use Apollo Client is through the graphql() higher order component. With the graphql() HOC, we can make data from the client available to components that are wrapped by the higher order component! Although we can still use the graphql() HOC currently, there are some downsides to using it.

When a component needs access to multiple resolvers, we need to compose multiple graphql() higher order components in order to do so. Composing multiple HOCs can make it difficult to understand how the data is passed to your components. The order of the HOCs can matter in some cases, which can easily lead to bugs when refactoring the code.

After the release of Hooks, Apollo added Hooks support to the Apollo Client library. Instead of using the graphql() higher order component, developers can now directly access the data through the hooks that the library provides.

By using the useMutation hook, we reduced the amount of code that was needed in order to provide the data to the component.

Besides a reduction in boilerplate, it's also much easier to use the data of multiple resolvers in a component. Instead of having to compose multiple higher order components, we can simply write multiple hooks in the component. Knowing how data gets passed to the component is much easier this way, and improves developer experience when refactoring components, or breaking them down into smaller pieces.

Pros

Using the Higher Order Component pattern allows us to keep logic that we want to re-use all in one place. This reduces the risk of accidentally spreading bugs throughout the application by duplicating code over and over, potentially introducing new bugs each time. By keeping the logic all in one place, we can keep our code DRY and easily enforce separation of concerns.

Cons

The name of the prop that a HOC can pass to an element, can cause a naming collision.

function withStyles(Component) {
  return props => {
    const style = { padding: '0.2rem', margin: '1rem' }
    return <Component style={style} {...props} />
  }
}

const Button = () => <button style={{ color: 'red' }}>Click me!</button>
const StyledButton = withStyles(Button)

In this case, the withStyles HOC adds a prop called style to the element that we pass to it. However, the Button component already had a prop called style, which will be overwritten! Make sure that the HOC can handle accidental name collision, by either renaming the prop or merging the props.

function withStyles(Component) {
  return props => {
    const style = {
      padding: '0.2rem',
      margin: '1rem',
      ...props.style
    }

    return <Component style={style} {...props} />
  }
}

const Button = () => <button style={{ color: 'red' }}>Click me!</button>
const StyledButton = withStyles(Button)

When using multiple composed HOCs that all pass props to the element that's wrapped within them, it can be difficult to figure out which HOC is responsible for which prop. This can hinder debugging and scaling an application easily.

Source

• patterns.dev/react/hoc-pattern

References

• Higher-Order Components - React