React context with hooks prevent re render
I believe what is happening here is expected behavior. The reason it renders twice is because you are automatically grabbing a new book/user when you visit the book or user page respectively.
This happens because the page loads, then useEffect
kicks off and grabs a book or user, then the page needs to re-render in order to put the newly grabbed book or user into the DOM.
I have modified your CodePen in order to show that this is the case.. If you disable 'autoload' on the book or user page (I added a button for this), then browse off that page, then browse back to that page, you will see it only renders once.
I have also added a button which allows you to grab a new book or user on demand... this is to show how only the page which you are on gets re-rendered.
All in all, this is expected behavior, to my knowledge.
Books
and Users
currently re-render on every cycle - not only in case of store value changes.
1. Prop and state changes
React re-renders the whole sub component tree starting with the component as root, where a change in props or state has happened. You change parent state by getUsers
, so Books
and Users
re-render.
const App = () => {
const [state, dispatch] = React.useReducer(
state => ({
count: state.count + 1
}),
{ count: 0 }
);
return (
<div>
<Child />
<button onClick={dispatch}>Increment</button>
<p>
Click the button! Child will be re-rendered on every state change, while
not receiving any props (see console.log).
</p>
</div>
);
}
const Child = () => {
console.log("render Child");
return "Hello Child ";
};
ReactDOM.render(<App />, document.getElementById("root"));
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.13.0/umd/react.production.min.js" integrity="sha256-32Gmw5rBDXyMjg/73FgpukoTZdMrxuYW7tj8adbN8z4=" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.13.0/umd/react-dom.production.min.js" integrity="sha256-bjQ42ac3EN0GqK40pC9gGi/YixvKyZ24qMP/9HiGW7w=" crossorigin="anonymous"></script>
<div id="root"></div>
Optimization technique
Use React.memo
to prevent a re-render of a comp, if its own props haven't actually changed.
// prevents Child re-render, when the button in above snippet is clicked
const Child = React.memo(() => {
return "Hello Child ";
});
// equivalent to `PureComponent` or custom `shouldComponentUpdate` of class comps
Important: React.memo
only checks prop changes (useContext
value changes trigger re-render)!
2. Context changes
All context consumers (useContext
) are automatically re-rendered, when the context value changes.
// here object reference is always a new object literal = re-render every cycle
<ApiContext.Provider value={{ ...state, dispatch }}>
{children}
</ApiContext.Provider>
Optimization technique
Make sure to have stable object references for the context value, e.g. by useMemo
Hook.
const [state, dispatch] = useReducer(rootReducer, {});
const store = React.useMemo(() => ({ state, dispatch }), [state])
<ApiContext.Provider value={store}>
{children}
</ApiContext.Provider>
Other
Not sure, why you put all these constructs together in Books
, just use one useContext
:
const { dispatch, books } = useContext(ApiContext);
// drop these
const contextValue = useContext(ApiContext);
<ApiContext.Consumer> /* ... */ </ApiContext.Consumer>;
You also can have a look at this code example using both React.memo
and useContext
.