This project was created to investigate how the useSelector hook works in React.js.
useSelector selects only the part of the application state that a component needs. Especially when used with shallowEqual, is supposed to prevent the component from re-rendering if its part of the state hasn't changed.
In this application the components Apple and Mushroom each select a part of the application's state. If you open this application in Stackblitz (https://use-selector-explore.stackblitz.io/) and open Redux Dev Tools in your browser's Dev Tools, then click on either Apple or Mushroom buttons, you will see which components are being re-rendered.
(It is best to use Firefox, because in Firefox Redux Dev Tools a component is highlighted when it is being re-rendered. In Chrome, you can still see which components were re-rendered when, in the Profiler tab of Redux Dev Tools, but it is less convenient. Also, a lot of times the Stackblitz URL will hang while loading, so just try to reload it until it loads. It's an unfortunate bug in Stackblitz.)
Currently, if you click Apple or Mushroom, only that component will re-render, and the other will not. But if you change one line in this application, both will re-render. Try editing this appication in Stackblitz
https://stackblitz.com/edit/use-selector-explore
and uncomment line 69 in App.js:
const appState = useSelector((state) => state, shallowEqual);
After this, both Apple and Mushrooms components will re-render when you click either button. This is despite the fact that the console.log in reducers.js shows that the shallowEqual of the other component's part of the state is true, which is to say that for the other component its previous state is shallowEqual to the new state.
Why? That's the mystery I'm trying to figure out.
In this very simple application appState is not used, so there is no need to have that line. But there may be use cases where we would need this line, because we need some data bit from appState that exists at the application level and can't be farmed out to individual components. In that case I would like components not to re-render unnecessarily.
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In the project directory, you can run:
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Open http://localhost:3000 to view it in your browser.
The page will reload when you make changes.
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See the section about running tests for more information.
Builds the app for production to the build folder.
It correctly bundles React in production mode and optimizes the build for the best performance.
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Your app is ready to be deployed!
See the section about deployment for more information.
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Instead, it will copy all the configuration files and the transitive dependencies (webpack, Babel, ESLint, etc) right into your project so you have full control over them. All of the commands except eject will still work, but they will point to the copied scripts so you can tweak them. At this point you're on your own.
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