What is Zustand?

Zustand is a simple and easy-to-use state management library.

While using react-scan, I noticed that when I updated a state in the store in component A, it caused a re-render of component B, leading to significant performance issues.

So, I revisited the documentation and realized that my previous usage of Zustand was incorrect.

Mistake 1: Not using a single selector or shallow comparison to get state

Here’s the previous implementation and the corresponding component. useGlobalStore has more than just uploadToServerProgress and uploadStatus states.

const ComponentA = () => {
  const { uploadToServerProgress, uploadStatus } = useGlobalStore();
  return (
    <UploadStateContainer
      uploadToServerProgress={uploadToServerProgress}
      uploadStatus={uploadStatus}
    />
  );
};

When uploadToServerProgress or uploadStatus is updated, UploadStateContainer re-renders (as expected). But when other states in useGlobalStore are updated, UploadStateContainer also re-renders (which is not expected).

This is because const { uploadToServerProgress, uploadStatus } = useGlobalStore(); is shorthand for destructuring, which is equivalent to:

const globalStore = useGlobalStore();
const { uploadToServerProgress, uploadStatus } = globalStore;

In React, any update to globalStore triggers a re-render of UploadStateContainer. Therefore, we need to modify it as follows:

Solution 1: Use a single selector

const uploadToServerProgress = useGlobalStore(state => state.uploadToServerProgress);
const uploadStatus = useGlobalStore(state => state.uploadStatus);

Solution 2: Use useShallow

const { uploadToServerProgress, uploadStatus } = useGlobalStore(useShallow(state => ({
  uploadToServerProgress: state.uploadToServerProgress,
  uploadStatus: state.uploadStatus,
})));

Mistake 2: Not having sufficient granularity when using selectors

cameraData is obtained by subscribing to other services and contains data such as camera image and camera ID. The camera image is frequently updated data.

import React from 'react';

export interface CameraData {
  cameraBase64: string; // base64 encoded image
  cameraId: number;
}

const CurrentCamera: React.FC = () => {
  const cameraData = useGlobalStore(state => state.cameraData);

  return <CurrentCameraId cameraId={cameraData.cameraId} />;
};

Though this is using a single selector, when cameraBase64 updates, cameraData will also update, causing CurrentCameraId to re-render.

To solve this, follow the principle of minimal granularity and only fetch the required data:

import React from "react";

export interface CameraData {
  cameraBase64: string; // base64 encoded image
  cameraId: number;
}

const CurrentCamera: React.FC = () => {
  const cameraId = useGlobalStore(state => state.cameraData.cameraId);

  return <CurrentCameraId cameraId={cameraId} />;
};

Mistake 3: Using multiple stores to manage different states, instead of using slices to group states

In the official documentation Flux-inspired practice - Zustand, it is mentioned that you should use a single store, not multiple stores for different states.

Recommended patterns

Single store

Your application’s global state should be located in a single Zustand store.

If you have a large application, Zustand supports splitting the store into slices.

Mistake 4: Accessing UI-unrelated state via selectors

In a component, I had a handleSave method that called an API to save data.

Previously, I accessed the needed data through a selector within component A and then called the API to save it. But this was incorrect.

const ComponentA = () => {
  const { state1, state2 } = useGlobalStore(
    useShallow(state => ({
      state1: state.state1,
      state2: state.state2,
    }))
  );

  const handleSave = () => {
    save(state1, state2);
  };

  return <SaveButton onClick={handleSave} />;
};

In component B, I changed state1, but since I accessed state1 in component A, the change would cause component A to re-render.

The correct approach is to directly access the store inside the handleSave method instead of passing the state via selectors:

const ComponentA = () => {
  const handleSave = () => {
    const { state1, state2 } = useGlobalStore.getState();
    save(state1, state2);
  };

  return <SaveButton onClick={handleSave} />;
};

References

Zustand

master branch notes/zustand learning notes

How to efficiently manage React local state? This time I chose Zustand! | Bosh’s Technical Exploration Notes