6-global-context-pattern.md

Global Context Pattern

{% hint style="info" %} Follow along with code examples here! {% endhint %}

Table of Contents:

• Global Context Design Pattern
  • 1) Create a ContextProvider wrapper component
  • 2) Import and Render the ContextProvider component in main.jsx
  • 3) Refactor other components as necessary

Global Context Design Pattern

The illustration above shows how we can **wrap the **entire App component in our context, but in the example we only wrap the PostList component.

import Header from "./components/Header";
import PicturesList from "./components/PicturesList";
import { useState } from "react";
import InstagramContext from "./context/InstagramContext";

const App = () => {
  const [totalLikes, setTotalLikes] = useState(0);
  const incrementTotalLikes = () => setTotalLikes(totalLikes + 1);

  return (
    <>
      <Header likes={totalLikes} />
      <InstagramContext.Provider value={incrementTotalLikes}>
        <PicturesList />
      </InstagramContext.Provider>
    </>
  );
};

If we import the InstagramContext into main.jsx and render the InstagramContext.Provider around the App component...

import ReactDOM from "react-dom/client";
import App from "./App.jsx";
import "./index.css";
import InstagramContext from "./context/InstagramContext";

ReactDOM.createRoot(document.getElementById("root")).render(
  <InstagramContext.Provider value={"???"}>
    <App />
  </InstagramContext.Provider>
);

...we run into an issue - what value do we provide?

The state values that we want to provide (incrementTotalLikes) lives in the App component. We can't lift that state up to the main.jsx file because there isn't a component here!

Wrapping the App component is common, and there is a common solution:

1) Create a ContextProvider wrapper component

We start by creating a new "wrapper-style" component that imports the Context object and renders its .Provider.

// src/context/InstagramContextProvider.jsx
import { useState } from "react";
import InstagramContext from "./InstagramContext";

// children is whatever this component is wrapped around
const InstagramContextProvider = ({ children }) => {
  const [totalLikes, setTotalLikes] = useState(0);
  const incrementTotalLikes = () => setTotalLikes(totalLikes + 1);

  const contextValues = {
    totalLikes,
    incrementTotalLikes,
  };

  return (
    <InstagramContext.Provider value={contextValues}>
      {children}
    </InstagramContext.Provider>
  );
};

export default InstagramContextProvider;

• All of the global state for this application can be managed by this component.
• Note how the children prop is used. This will become clearer next.

2) Import and Render the ContextProvider component in main.jsx

It is easier to see how InstagramContextProvider is a wrapper by looking at how it is used:

import ReactDOM from "react-dom/client";
import App from "./App.jsx";
import "./index.css";
import InstagramContextProvider from "./context/InstagramContextProvider";

ReactDOM.createRoot(document.getElementById("root")).render(
  <InstagramContextProvider>
    <App /> {/* <--- This is the `children` */}
  </InstagramContextProvider>
);

It is literally wrapped around the App component which is automatically provided to InstagramContextProvider as its children prop.

3) Refactor other components as necessary

With this change, the rest of our components that previously needed to manage state become greatly simplified:

Our App now is just a container for Header and PicturesList

import Header from "./components/Header";
import PicturesList from "./components/PicturesList";

const App = () => {
  return (
    <>
      <Header />
      <PicturesList />
    </>
  );
};

Our Header previously needed to take in a totalLikes prop. Now, it retrieves it from InstagramContext.

import { useContext } from "react";
import InstagramContext from "../context/InstagramContext";

const Header = () => {
  const { totalLikes } = useContext(InstagramContext);

  return (
    <header>
      <h1>My Pet Pics</h1>
      <p>My pictures have been liked {totalLikes} times!</p>
    </header>
  );
};

However, there is still room for other pieces of state that can be managed more "locally":

import { useState, useContext } from "react";
import InstagramContext from "../context/InstagramContext";

const LikesButton = () => {
  // Only this one component needs this state
  const [likes, setLikes] = useState(0);
  const incrementTotalLikes = useContext(InstagramContext);

  const handleClick = () => {
    incrementTotalLikes();
    setLikes(likes + 1);
  };

  return (
    <div className="likes-container">
      <button onClick={handleClick}>Like</button>
      <p>Likes: {likes}</p>
    </div>
  );
};