Preventing rerenders with React.memo and useContext hook.

[pumanitro]

Do you want to request a feature or report a bug?

bug

What is the current behavior?

I can't rely on data from context API by using (useContext hook) to prevent unnecessary rerenders with React.memo

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem. Your bug will get fixed much faster if we can run your code and it doesn't have dependencies other than React. Paste the link to your JSFiddle (https://jsfiddle.net/Luktwrdm/) or CodeSandbox (https://codesandbox.io/s/new) example below:

React.memo(() => {
const [globalState] = useContext(SomeContext);

render ...

}, (prevProps, nextProps) => {

// How to rely on context in here?
// I need to rerender component only if globalState contains nextProps.value

});

What is the expected behavior?

I should have somehow access to the context in React.memo second argument callback to prevent rendering
Or I should have the possibility to return an old instance of the react component in the function body.

Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?
16.8.4

[gaearon]

This is working as designed. There is a longer discussion about this in #14110 if you're curious.

Let's say for some reason you have AppContext whose value has a theme property, and you want to only re-render some ExpensiveTree on appContextValue.theme changes.

TLDR is that for now, you have three options:

Option 1 (Preferred): Split contexts that don't change together

If we just need appContextValue.theme in many components but appContextValue itself changes too often, we could split ThemeContext from AppContext.

function Button() {
  let theme = useContext(ThemeContext);
  // The rest of your rendering logic
  return <ExpensiveTree className={theme} />;
}

Now any change of AppContext won't re-render ThemeContext consumers.

This is the preferred fix. Then you don't need any special bailout.

Option 2: Split your component in two, put memo in between

If for some reason you can't split out contexts, you can still optimize rendering by splitting a component in two, and passing more specific props to the inner one. You'd still render the outer one, but it should be cheap since it doesn't do anything.

function Button() {
  let appContextValue = useContext(AppContext);
  let theme = appContextValue.theme; // Your "selector"
  return <ThemedButton theme={theme} />
}

const ThemedButton = memo(({ theme }) => {
  // The rest of your rendering logic
  return <ExpensiveTree className={theme} />;
});

Option 3: One component with useMemo inside

Finally, we could make our code a bit more verbose but keep it in a single component by wrapping return value in useMemo and specifying its dependencies. Our component would still re-execute, but React wouldn't re-render the child tree if all useMemo inputs are the same.

function Button() {
  let appContextValue = useContext(AppContext);
  let theme = appContextValue.theme; // Your "selector"

  return useMemo(() => {
    // The rest of your rendering logic
    return <ExpensiveTree className={theme} />;
  }, [theme])
}

---

There might be more solutions in the future but this is what we have now.

Still, note that option 1 is preferable — if some context changes too often, consider splitting it out.

[eps1lon]

Both of these options will bail out of rendering children if theme hasn't changed.

@gaearon Are the Buttons the children or do the Buttons render children? I'm missing some context how these are used.

Using the unstable_Profiler option 2 will still trigger onRender callbacks but not call the actual render logic. Maybe I'm doing something wrong? https://codesandbox.io/s/kxz4o2oyoo https://codesandbox.io/s/00yn9yqzjw

[gaearon]

I updated the example to be clearer.

[gaearon]

Using the unstable_Profiler option 2 will still trigger onRender callbacks but not call the actual render logic. Maybe I'm doing something wrong? https://codesandbox.io/s/kxz4o2oyoo

That's exactly the point of that option. :-)

[pumanitro]

Maybe a good solution for that would be to have the possibility of "taking" the context and rerender component only if given callback return true e.g:
useContext(ThemeContext, (contextData => contextData.someArray.length !== 0 ));

The main problem with hooks that I actually met is that we can't manage from inside of a hook what is returned by a component - to prevent rendering, return memoized value etc.

[gaearon]

If we could, it wouldn't be composable.

https://overreacted.io/why-isnt-x-a-hook/#not-a-hook-usebailout

[steida]

Option 4: Do not use context for data propagation but data subscription. Use useSubscription (because it's hard to write to cover all cases).

[Alfrex92]

There is another way to avoid re-render.
"You need to move the JSX up a level out of the re-rendering component then it won't get re-created each time"

More info here

[jonnolen]

Maybe a good solution for that would be to have the possibility of "taking" the context and rerender component only if given callback return true e.g:
useContext(ThemeContext, (contextData => contextData.someArray.length !== 0 ));

The main problem with hooks that I actually met is that we can't manage from inside of a hook what is returned by a component - to prevent rendering, return memoized value etc.

Instead of a true/false here... could we provide an identity based function that allowed us to subset the data from the context?

const contextDataINeed = useContext(ContextObj, (state) => state['keyICareAbout'])

where useContext wouldn't pop in this component unless the result of the selector fn was different identity wise from the previous result of the same function.

[fuleinist]

found this library that it may be the solution for Facebook to integrate with hooks https://blog.axlight.com/posts/super-performant-global-state-with-react-context-and-hooks/

[fuleinist]

There is another way to avoid re-render.
"You need to move the JSX up a level out of the re-rendering component then it won't get re-created each time"

More info here

Problem is it may be costly to restructure the components tree just to prevent top to bottom re-rendering.

[danielkcz]

@fuleinist Ultimately, it's not that different from MobX, although a lot simplified for a specific use case. MobX already works like that (also using Proxies), the state is mutated and components who use specific bits of the state get re-rendered, nothing else.

[marrkeri]

@gaearon I don't know if I'm missing something, but I have tried yours second and third options and they are not working correctly. Not sure if this is only react chrome extension bug or there is other catch. Here is my simple example of form, where I see rerendering both inputs. In console I see that memo is doing his job but DOM is rerendered all the time. I have tried 1000 items and onChange event is really slow, that's why I think that memo() is not working with context correctly. Thanks for any advice:

Here is demo with 1000 items/textboxes. But in that demo dev tools doesn't show re-render. You have to download sources on local to test it: https://codesandbox.io/embed/zen-firefly-d5bxk

import React, { createContext, useState, useContext, memo } from "react";

const FormContext = createContext();

const FormProvider = ({ initialValues, children }) => {
  const [values, setValues] = useState(initialValues);

  const value = {
    values,
    setValues
  };

  return <FormContext.Provider value={value}>{children}</FormContext.Provider>;
};

const TextField = memo(
  ({ name, value, setValues }) => {
    console.log(name);
    return (
      <input
        type="text"
        value={value}
        onChange={e => {
          e.persist();
          setValues(prev => ({
            ...prev,
            [name]: e.target.value
          }));
        }}
      />
    );
  },
  (prev, next) => prev.value === next.value
);

const Field = ({ name }) => {
  const { values, setValues } = useContext(FormContext);

  const value = values[name];

  return <TextField name={name} value={value} setValues={setValues} />;
};

const App = () => (
  <FormProvider initialValues={{ firstName: "Marr", lastName: "Keri" }}>
    First name: <Field name="firstName" />
    <br />
    Last name: <Field name="lastName" />
  </FormProvider>
);

export default App;

On the other hand this approach without context works correctly, still in debug it is slower than I expected but at least rerender is ok

import React, { useState, memo } from "react";
import ReactDOM from "react-dom";

const arr = [...Array(1000).keys()];

const TextField = memo(
  ({ index, value, onChange }) => (
    <input
      type="text"
      value={value}
      onChange={e => {
        console.log(index);
        onChange(index, e.target.value);
      }}
    />
  ),
  (prev, next) => prev.value === next.value
);

const App = () => {
  const [state, setState] = useState(arr.map(x => ({ name: x })));

  const onChange = (index, value) =>
    setState(prev => {
      return prev.map((item, i) => {
        if (i === index) return { name: value };

        return item;
      });
    });

  return state.map((item, i) => (
    <div key={i}>
      <TextField index={i} value={item.name} onChange={onChange} />
    </div>
  ));
};

const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);

[MarkosKon]

@marrkeri I don't see something wrong in the first code snippet. The component that's highlighted in dev tools is the Field that uses the context, not the TextField which is a memo component and implements the areEqual function.

I think the performance problem in the codesandbox example comes from the 1000 components that use the context. Refactor it to one component that uses the context, say Fields, and return from that component (with a map) a TextField for each value.

[marrkeri]

@marrkeri I don't see something wrong in the first code snippet. The component that's highlighted in dev tools is the Field that uses the context, not the TextField which is a memo component and implements the areEqual function.

I think the performance problem in the codesandbox example comes from the 1000 components that use the context. Refactor it to one component that uses the context, say Fields, and return from that component (with a map) a TextField for each value.

As you said I was under same thinking that should be rerendered every time but () only when value is changed. But it is probably just dev tools problems because I've added padding around and instead of it is rerendered. Check this picture

I haven't catch your second point about refactoring to one component . Could you do snapshot pls? And what do you guys think about maximum displayed number of which are ok without lagging? Is 1000 to much?

[MarkosKon]

@marrkeri I was suggesting something like this: https://codesandbox.io/s/little-night-p985y.

[snawaz]

@steida

Option 4: Do not use context for data propagation but data subscription. Use useSubscription (because it's hard to write to cover all cases).

What does it mean? Could you explain both data propagation and data subscription using examples? As per my understanding they are same: data is propagated using subscribe/publish, right?

[Hypnosphi]

@steida it means that the context value should not be the data, i.e. something that changes, but rather a data source, an object that always stays the same by reference, but has something like subscribe and getCurrentData methods.

[snawaz]

@Hypnosphi

@steida it means that the context value should not be the data, i.e. something that changes, but rather a data source, an object that always stays the same by reference, but has something like subscribe and getCurrentData methods.

OK. Such data sources can be global/singleton objects and that would still work? Why do we need context then?

[gmoniava]

Maybe I am missing something, but @gaearon maybe we should add clarification to Option 1 that it prevents rerenders in some cases; for example here it doesn't help:

import React from 'react';
import './style.css';

const { useState, createContext, useContext, useEffect, useRef } = React;

const ViewContext = createContext();
const ActionsContext = createContext();

function MyContainer() {
  const [contextState, setContextState] = useState();

  return (
    <ViewContext.Provider value={contextState}>
      <ActionsContext.Provider value={setContextState}>
        <MySetCtxComponent />
        <MyViewCtxComponent />
      </ActionsContext.Provider>
    </ViewContext.Provider>
  );
}

function MySetCtxComponent() {
  const setContextState = useContext(ActionsContext);

  const counter = useRef(0);
  console.log('Set');
  useEffect(() => {
    console.log('=======>>>>>>>>>>>>  Use Effect run in MySetCtxComponent');
    const intervalID = setInterval(() => {
      setContextState('New Value ' + counter.current);
      counter.current++;
    }, 1000);

    return () => clearInterval(intervalID);
  }, [setContextState]);

  return <button onClick={() => (counter.current = 0)}>Reset</button>;
}

function MyViewCtxComponent() {
  const contextState = useContext(ViewContext);
  console.log('View');

  return <div>This is the value of the context: {contextState}</div>;
}

export default MyContainer;

One can see both "View" and "Set" are being logged, which means both components got rerendered.

[hrvojegolcic]

Maybe I am missing something, but @gaearon maybe we should add clarification to Option 1 that it prevents rerenders in some cases; for example here it doesn't help:

import React from 'react';
import './style.css';

const { useState, createContext, useContext, useEffect, useRef } = React;

const ViewContext = createContext();
const ActionsContext = createContext();

function MyContainer() {
  const [contextState, setContextState] = useState();

  return (
    <ViewContext.Provider value={contextState}>
      <ActionsContext.Provider value={setContextState}>
        <MySetCtxComponent />
        <MyViewCtxComponent />
      </ActionsContext.Provider>
    </ViewContext.Provider>
  );
}

function MySetCtxComponent() {
  const setContextState = useContext(ActionsContext);

  const counter = useRef(0);
  console.log('Set');
  useEffect(() => {
    console.log('=======>>>>>>>>>>>>  Use Effect run in MySetCtxComponent');
    const intervalID = setInterval(() => {
      setContextState('New Value ' + counter.current);
      counter.current++;
    }, 1000);

    return () => clearInterval(intervalID);
  }, [setContextState]);

  return <button onClick={() => (counter.current = 0)}>Reset</button>;
}

function MyViewCtxComponent() {
  const contextState = useContext(ViewContext);
  console.log('View');

  return <div>This is the value of the context: {contextState}</div>;
}

export default MyContainer;

One can see both "View" and "Set" are being logged, which means both components got rerendered.

Wondering the same, have you found any solution for this specific example?

[gmoniava]

Maybe I am missing something, but @gaearon maybe we should add clarification to Option 1 that it prevents rerenders in some cases; for example here it doesn't help:

import React from 'react';
import './style.css';

const { useState, createContext, useContext, useEffect, useRef } = React;

const ViewContext = createContext();
const ActionsContext = createContext();

function MyContainer() {
  const [contextState, setContextState] = useState();

  return (
    <ViewContext.Provider value={contextState}>
      <ActionsContext.Provider value={setContextState}>
        <MySetCtxComponent />
        <MyViewCtxComponent />
      </ActionsContext.Provider>
    </ViewContext.Provider>
  );
}

function MySetCtxComponent() {
  const setContextState = useContext(ActionsContext);

  const counter = useRef(0);
  console.log('Set');
  useEffect(() => {
    console.log('=======>>>>>>>>>>>>  Use Effect run in MySetCtxComponent');
    const intervalID = setInterval(() => {
      setContextState('New Value ' + counter.current);
      counter.current++;
    }, 1000);

    return () => clearInterval(intervalID);
  }, [setContextState]);

  return <button onClick={() => (counter.current = 0)}>Reset</button>;
}

function MyViewCtxComponent() {
  const contextState = useContext(ViewContext);
  console.log('View');

  return <div>This is the value of the context: {contextState}</div>;
}

export default MyContainer;

One can see both "View" and "Set" are being logged, which means both components got rerendered.

Wondering the same, have you found any solution for this specific example?

@hrvojegolcic
There is one approach, some people do:

function Wrapper(props) {
  const [contextState, setContextState] = useState();
  return (
    <ActionsContext.Provider value={{ contextState, setContextState }}>
      {props.children}
    </ActionsContext.Provider>
  );
}

Now if you change contextState of Wrapper, it will not re-render the children which don't rely on the context. Because react does some optimization (more info): since props.children didn't change (it comes from parent which didn't re-render), it will skip re-rendering it even though Wrapper re-rendered. It will re-render only those children, that rely on context which changed.

[hrvojegolcic]

@gmoniava This is worth to take into consideration, I'll check thanks. But your example from the above is very specific, one component uses only the GET state and another only the SET state, but even with the SET state, it's still the same context and it will indeed behave like both need to re-render. React will understand SET state, as using the context, while it's not really a consumer that needs to re-render.

[gmoniava]

@gmoniava This is worth to take into consideration, I'll check thanks. But your example from the above is very specific, one component uses only the GET state and another only the SET state, but even with the SET state, it's still the same context and it will indeed behave like both need to re-render. React will understand SET state, as using the context, while it's not really a consumer that needs to re-render.

@hrvojegolcic
I think why both components re-render in my case is that we updated state at the top parent (contextState), this by default causes all children to re-render, this is standard react behavior and the context providers in between don't prevent any re-renders, that would be a job of memo, if it were in between somewhere.

[hrvojegolcic]

@gmoniava I think I got ya, and it helps. Taking that into consideration then it seems the useMemo/useCallback will do here. As in your example, from my understanding the line <ActionsContext.Provider value={setContextState}> could change to <ActionsContext.Provider value={useCallback(setContextState, [])}>. That way, the setContextState will not report as changed when contextState is changed.

[gmoniava]

@hrvojegolcic No, in my example the components MySetCtxComponent and MyViewCtxComponent re-rendered not because setContextState changed (it didn't). But because I re-rendered the parent MyContainer which caused its children to re-render by default (this is standard react behavior). So in this case, splitting the context doesn't save any re-renders, I suppose gaearon above was talking about different use case. Probably he was having something like this in mind:

let MySetCtxComponent = React.memo(() => {
  const setContextState = useContext(ActionsContext);

  const counter = useRef(0);
  console.log('MySetCtxComponent');
  useEffect(() => {
    const intervalID = setInterval(() => {
      setContextState('New Value ' + counter.current);
      counter.current++;
    }, 1000);

    return () => clearInterval(intervalID);
  }, [setContextState]);

  return <button onClick={() => (counter.current = 0)}>Reset</button>;
});

let MyViewCtxComponent = React.memo(() => {
  const contextState = useContext(ViewContext);
  console.log('MyViewCtxComponent');

  return <div>This is the value of the context: {contextState}</div>;
});

I just wrapped the two components in memo. Now, if you run the original code with these examples, you can see only MyViewCtxComponent re-renders, because the context value which it uses changed.

[roggc]

Take a look at the library react-context-slices. With this library you do it like this:

// slices.js
import getHookAndProviderFromSlices from "react-context-slices";

export const {useSlice, Provider} = getHookAndProviderFromSlices({
  count1: {initialArg: 0},
  count2: {initialArg: 0},
  // rest of slices
});

// app.js
import {useSlice} from "./slices"

const App = () => {
  const [count1, setCount1] = useSlice("count1");
  const [count2, setCount2] = useSlice("count2");

  return <>
    <div>
      <button onClick={()=>setCount1(c => c + 1)}>+</button>{count1}
    </div>
    <div>
      <button onClick={()=>setCount2(c => c + 1)}>+</button>{count2}
    </div>
  </>;
};

export default App;

As you can see it's very easy to use and optimal. The key point is to create as many slices of Context as you need, and this library makes it extremely easy and straightforward to do it.