React context is a cool feature, and I use it a lot for injecting configuration and making container / child component APIs (think <RadioGroup /> + <RadioButton />). Unfortunately, out of the box Context comes with a limiting and not very convenient API. In most cases, I choose to wrap both the provider and consumer with a custom component and a hook. Some of the issues I highlight are more relevant to library maintainers, but most apply to app development as well.

In this post, we revisit an AdaptivityContext that allows components to read viewport dimension data — pixel width and breakpoint status, isMobile:

const getWidth = () => window.innerWidth;
const isMobile = (w: number) => w < 600;
const AdaptivityContext = createContext({
  w: getWidth(),
  isMobile: isMobile(getWidth),
});

If you've read my post on Context performance issues, you know it is not the best design choice — components that only care about isMobile will still re-render on every width change. Still, suppose that's what we happen to have on our project. How can custom AdaptivityProvider and useAdaptivity help us?

Wrap useContext

In raw context API, the consuming components utilize useContext hook (or a Context.Consumer component, but I don't know why anyone would choose it over the hook today). There's nothing especially wrong with useContext, but we can do so much better with a custom useAdaptivity!

If useContext is used outside Provider, you're left with either a static default value from createContext or cryptic can't read property width of null errors. Sometimes it's enough, but AdaptivityContext is supposed to be dynamic, and we get a lot of "bug reports" that are fixed with a "did you forget the provider?". A custom useAdaptivity gives us two stronger options:

1. Show an explicit error message, like console.error('useAdaptivity must be used inside AdaptivityProvider')
2. Give each component an independent size observer, and make AdaptivityProvider optional for advanced optimizations and overrides.

Next, useContext has a 1:1 relationship to contexts. Fixing AdaptivityContext performance problems involves splitting it into two separate contexts — a frequently-changing one for width, and a more stable one for isMobile. useAdaptivity can subscribe to both contexts — it won't have any performance benefits, but it's backwards compatible and allows users to gradually update their apps to the new API:

const useAdaptivity = () => {
  console.warn('Please migrate to useMobile or useViewport for better performance');
  const viewport = useContext(ViewportContext);
  const mobile = useContext(MobileContext);
  return { ...viewport, ...mobile };
};

Custom useAdaptivity hook also allows for an alternate context injection mechanism, like react-tracked. You can even bind to a global state manager instead of context. Nothing about useAdaptivity implies that it has anything to do with contexts!

So, a custom useAdaptivity hook gives us a lot of freedom — we can modify the contexts as we wish, replace them with other state management mechanism, and we can handle a missing provider as we see fit. That's convincing. What about Provider?

Wrap Context.Provider, too

React.createContext gives you a Context.Provider component you're supposed to use for passing a context value. It lacks some important features, but we can easily fix that by wrapping it into a custom Provider component.Frankly, it's less of a concern than useContext — you often have a single Provider, and it has to be located in some component, so you can't go too wrong. For completeness, here's what I normally do with a custom Provider.

Raw Context.Provider with object context is a performance hazard — if you don't stabilize value reference yourself, every context consumer will re-render on every Provider render, because React updates them every time context value changes under strict equality. I don't know why this feature is not in react core, but it's one good reason to have a custom provider (see my post on custom memo for details on useObjectMemo):

const AdaptivityProvider = ({ children, ...context }) => {
  const contextValue = useObjectMemo(context);
  return (
    <AdaptivityContext.Provider value={contextValue}>
      {children}
    </AdaptivityContext.Provider>
  );
};

Just like useContext, raw Providers have a 1:1 relationship with contexts, making it harder to split / merge the contexts. To fix the coupling of width and isMobile updates, we must split AdaptivityContext into two parts. Easy with a custom provider:

const AdaptivityProvider = ({ children, width, isMobile }) => {
  const viewportValue = useObjectMemo({ width });
  const mobileValue = useObjectMemo({ isMobile });
  return (
    <ViewportSizeContext.Provider value={viewportValue}>
      <MobileContext.Provider value={mobileValue}>
        {children}
      </MobileContext.Provider>
    </ViewportSizeContext.Provider>
  );
};

Just like useAdaptivity, AdaptivityProvider also allows you to replace context with any other state management technology — just throw a <StoreProvider> in there and you're done.

Finally, a custom provider can handle context value in a smarter way — add default options or merge with another provider up the tree. If we had both width and height, we could allow partial overrides — user could use <ViewportSizeProvider width={100}> in a narrow sidebar, while preserving the height value:

const parentViewport = useContext(ViewportSizeContext);
const contextValue = useObjectMemo({
  ...parentWiewport,
  ...size
});

Of course, you could also have a custom mechanism of auto-detecting and updating context values:

useLayoutEffect(() => {
  const cb = () => {
    setDetectedSize(getViewportSize());
  };
  cb();
  window.addEventListener('resize', cb);
  return () => window.removeEventListener('resize', cb);
}, []);
const contextValue = useObjectMemo({
  ...detectedSize,
  ...props
});

You could have amazing combinations of inheritance, auto-detection and overrides. Really, there are endless possibilities once you are the master of your context provider. Just don't settle for raw Context.Provider.

Wrapping both the provider and the consumer of a context into custom hooks gives you a lot of flexibility:

• Merge and split context as you want.
• Replace raw contexts with another state injection technique.
• Stabilize context object value.
• Introduce smart dynamic defaults for context value.
• Inherit from other providers up the tree with partial overrides.
• Warn or fallback on missing provider.

This flexibility is crucial if you're building a library, but it also helps a lot in any non-trivial app. Hope that convinces you! See you later.