Set state with same value using hooks will cause a rerender?

问题

Using hooks, If I call setState with the same value as the state, will it rerender the component?

If yes, how can I avoid that?

e.g.

const [state, setState] = useState(foo)

...
// any where in the code
setState(foo)

Considering that foo can be any thing, such as {}, true, props.bar or a variable from out side of the component (constant).

回答1:

It won't re-render the component if you call setState with the same value.

Try this out:

import React, { useState, useEffect } from "react";

const foo = { foo: 'bar' };

export default ({ name }) => {
  const [state, setState] = useState(foo);
  console.log("rendered!");
  useEffect(() => {
    setState(foo);
    console.log("state reset!");
  });

  const handleClick = () => {
    console.log("handleClick!");
    setState(foo);
    // setState({ ...foo, bar : 'baz' });
  }

  return (<div>
  <h1>Hello {name}!</h1>
  <button onClick={handleClick}>Click Me</button>
  </div>);
};

You'll notice that even when the button is clicked since the value has not changed. it's not re-rendering the Component. If you change the argument that you call setState with, it will re-render the component.

Here's a Code Sample for your ref.

Try commenting the first setState and un-commenting the second one inside the handleClick method to see the difference.

回答2:

It's a js syntax related question, Just like === operation.

let times = 0
const init = {name: 'Bob'}
function App() {
  const [state, setState] = useState(init)
  function modify() {
    setState({name: 'Bob'})
  }
  function modify2() {
    setState(init)
  }
  times ++
  return (
    <div className="App">
      <p>{ times }</p>
      <button onClick={modify}>Same Value Will Rerender</button>
      <button onClick={modify2}>Same Reference Never Rerender</button>
    </div>
  );
}

Here's a Code Sandbox

You can rewrite a wrapper method:

let times = 0
const init = {name: 'Bob'}
function App() {
  const [state, setState] = useState(init)
  function modify3() {
    setState2({ name: 'Bob' })
  }
  function setState2(value) {
    if (value.name === state.name) {
      return
    }
    setState(value)
  }
  times ++
  return (
    <div className="App">
      <p>{ times }</p>
      <button onClick={modify3}>Same Value Will Not Rerender Yet</button>
    </div>
  );
}

来源：https://stackoverflow.com/questions/59489959/set-state-with-same-value-using-hooks-will-cause-a-rerender

标签

reactjs

react-hooks