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Constate

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Write local state using React Hooks and lift it up to React Context only when needed with minimum effort.


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import React, { useState, useContext } from "react";
import createContainer from "constate";

// 1️⃣ Create a custom hook as usual
function useCounter() {
  const [count, setCount] = useState(0);
  const increment = () => setCount(count + 1);
  return { count, increment };
}

// 2️⃣ Create container
const CounterContainer = createContainer(useCounter);

function Button() {
  // 3️⃣ Use container context instead of custom hook
  // const { increment } = useCounter();
  const { increment } = useContext(CounterContainer.Context);
  return <button onClick={increment}>+</button>;
}

function Count() {
  // 4️⃣ Use container context in other components
  // const { count } = useCounter();
  const { count } = useContext(CounterContainer.Context);
  return <span>{count}</span>;
}

function App() {
  // 5️⃣ Wrap your components with container provider
  return (
    <CounterContainer.Provider>
      <Count />
      <Button />
    </CounterContainer.Provider>
  );
}

Installation

npm:

npm i constate

Yarn:

yarn add constate

API

createContainer(useValue[, createMemoInputs])

Constate exports a single method called createContainer. It receives two arguments: useValue and createMemoInputs (optional). And returns { Context, Provider }.

useValue

It's a custom hook that returns the Context value:

import React, { useState } from "react";
import createContainer from "constate";

const CounterContainer = createContainer(() => {
  const [count] = useState(0);
  return count;
});

console.log(CounterContainer); // { Context, Provider }

You can receive arguments in the custom hook function. They will be populated with <Provider />:

const CounterContainer = createContainer(({ initialCount = 0 }) => {
  const [count] = useState(initialCount);
  return count;
});

function App() {
  return (
    <CounterContainer.Provider initialCount={10}>
      ...
    </CounterContainer.Provider>
  );
}

The value returned in useValue will be accessible when using useContext(CounterContainer.Context):

import React, { useContext } from "react";

function Counter() {
  const count = useContext(CounterContainer.Context);
  console.log(count); // 10
}

createMemoInputs

Optionally, you can pass in a function that receives the value returned by useValue and returns an array of inputs. When any input changes, value gets re-evaluated, triggering a re-render on all consumers (components calling useContext()).

If createMemoInputs is undefined, it'll be re-evaluated everytime Provider renders:

// re-render consumers only when value.count changes
const CounterContainer = createContainer(useCounter, value => [value.count]);

function useCounter() {
  const [count, setCount] = useState(0);
  const increment = () => setCount(count + 1);
  return { count, increment };
}

This works similarly to the inputs parameter in React.useEffect and other React built-in hooks. In fact, Constate passes it to React.useMemo inputs internally.

You can also achieve the same behavior within the custom hook. This is an equivalent implementation:

import { useMemo } from "react";

const CounterContainer = createContainer(() => {
  const [count, setCount] = useState(0);
  const increment = () => setCount(count + 1);
  // same as passing `value => [value.count]` to `createMemoInputs` parameter
  return useMemo(() => ({ count, increment }), [count]);
});

Contributing

If you find a bug, please create an issue providing instructions to reproduce it. It's always very appreciable if you find the time to fix it. In this case, please submit a PR.

If you're a beginner, it'll be a pleasure to help you contribute. You can start by reading the beginner's guide to contributing to a GitHub project.

When working on this codebase, please use yarn. Run yarn examples:start to run examples.

License

MIT © Diego Haz