I have a React component that renders a moderately large list of inputs (100+ items). It renders okay on my computer, but there's noticeable input lag on my phone. The React DevTools shows that the entire parent object is rerendering on every keypress.
Is there a more efficient way to approach this?
https://codepen.io/anon/pen/YMvoyy?editors=0011
function MyInput({obj, onChange}) {
return (
<div>
<label>
{obj.label}
<input type="text" value={obj.value} onChange={onChange} />
</label>
</div>
);
}
// Passed in from a parent component
const startingObjects =
new Array(100).fill(null).map((_, i) => ({label: i, value: 'value'}));
function App() {
const [objs, setObjects] = React.useState(startingObjects);
function handleChange(obj) {
return (event) => setObjects(objs.map((o) => {
if (o === obj) return {...obj, value: event.target.value}
return o;
}));
}
return (
<div>
{objs.map((obj) => <MyInput obj={obj} onChange={handleChange(obj)} />)}
</div>
);
}
const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);
The issue is related to:
function handleChange(obj) {
return (event) => setObjects(objs.map((o) => {
if (o === obj) return {...obj, value: event.target.value}
return o;
}));
}
In this, you will update the objs array. This is obviously fine, but React doesn't know what has changed, so triggered Render on all the Children.
If your function component renders the same result given the same props, you can wrap it in a call to React.memo for a performance boost.
https://reactjs.org/docs/react-api.html#reactmemo
const MyInput = React.memo(({obj, onChange}) => {
console.log(`Rerendered: ${obj.label}`);
return <div style={{display: 'flex'}}>
<label>{obj.label}</label>
<input type="text" value={obj.value} onChange={onChange} />
</div>;
}, (prevProps, nextProps) => prevProps.obj.label === nextProps.obj.label && prevProps.obj.value === nextProps.obj.value);
However, React.Memo only does a shallow comparison when trying to figure out if it should render, so we can pass a custom comparison function as the second argument.
(prevProps, nextProps) => prevProps.obj.label === nextProps.obj.label && prevProps.obj.value === nextProps.obj.value);
Basically saying, if the label and the value, on the obj prop, are the same as the previous attributes on the previous obj prop, don't rerender.
Finally, setObjects much like setState, is also asynchronous, and will not immediately reflect and update. So to avoid the risk of objs being incorrect, and using the older values, you can change this to a callback like so:
function handleChange(obj) {
return (event) => {
const value = event.target.value;
setObjects(prevObjs => (prevObjs.map((o) => {
if (o === obj) return {...obj, value }
return o;
})))
};
}
https://codepen.io/anon/pen/QPBLwy?editors=0011 has all this, as well as console.logs, showing if something rerendered.
Is there a more efficient way to approach this?
You are storing all your values in an array, which means you don't know which element needs to be updated without iterating through the whole array, comparing if the object matches.
If you started with an Object:
const startingObjects =
new Array(100).fill(null).reduce((objects, _, index) => ({...objects, [index]: {value: 'value', label: index}}), {})
After some modifications, your handle function would change to
function handleChange(obj, index) {
return (event) => {
const value = event.target.value;
setObjects(prevObjs => ({...prevObjs, [index]: {...obj, value}}));
}
}
https://codepen.io/anon/pen/LvBPEB?editors=0011 as an example of this.
Related
React Hook "useState" cannot be called inside a callback. React Hooks must be called in a React function component or a custom React Hook function
I understand why I am getting that message, but at the same time I think what I want should be achievable. Here's the code:
import React, { useState } from "react";
import "./styles.css";
export default function App() {
const children = useState([]);
return (
<div>
{children.map((child, i) => (<Child child={child} />))}
<button
onClick={() => { children[1]([...children, useState(0)]); }}
>
Add
</button>
</div>
);
}
function Child(props) {
const [state, setState] = props.child;
return (
<div>
<input
type="range"
min="1"
max="255"
value={state}
onChange={(e) => setState(e.target.value)}
></input>
</div>
);
}
I want each <Child> to be in complete control of its state without having to declare a function of type updateChild(i : Index, data) then finding the right children in the list, etc. As this doesn't scale well with deep nested view hierarchies.
Is there a way to both:
Keep the state in the parent (single source of truth)
Allow children to mutate their own state
In a nutshell, I want to achieve this but with more than one child component.
To illustrate the problem with your current approach and why React won't let you do that, here's an ersatz implementation of useState that has approximately the same behaviour as the real version (I've left triggering a re-render as an exercise to the user and it doesn't support functional updates, but the important thing here was showing the underlying state of the Parent component).
// Approximation of useState
let parentStateIndex;
const parentState = [];
const useState = (defaultValue) => {
const i = parentStateIndex;
if (parentState.length === i) {
parentState.push(defaultValue);
}
parentStateIndex += 1;
return [parentState[i], (value) => parentState[i] = value];
}
const Parent = () => {
parentStateIndex = 0; // hack required to reset on each "render"
const [state, setState] = useState([]);
return (
<>
{state.map((child) => (
<Child child={child} />
)}
<button
onClick={() => setState([...state, useState(0)])}
>
Add
</button>
</>
);
};
const Child = ({ child }) => {
const [state, setState] = child;
return (
<input
type="range"
min="0"
max="255"
value={state}
onChange={({ target }) => setState([target.value, setState])}
/>
);
};
(Note a bit of knowledge about the parent state has already crept into the child here - if it only setState(target.value) then the [value, setter] pair would be replaced by just value and other things would start exploding. But I think this way around gives a better illustration of what happens further down.)
The first time the parent is rendered, the new array passed to useState is added to the state:
parentState = [
[
[],
(value) => parentState[0] = value
],
];
All good so far. Now imagine the Add button is clicked. useState is called again, and the state is updated to add a second item and add that new item to the first item:
parentState = [
[
[
[0, (value) => parentState[1] = value]
],
(value) => parentState[0] = value,
],
[
0,
(value) => parentState[1] = value,
],
];
This also seems to have worked, but now what happens when the child value updates to e.g. 1?
parentState = [
[
[
[0, /* this gets called: */ (value) => parentState[1] = value],
],
(value) => parentState[0] = value,
],
[
/* but this gets changed: */ 1,
(value) => parentState[1] = value,
],
];
The second part of the state is updated, which changes its type completely, but the first one still holds the old value.
When the parent re-renders, useState is is only called once, so the second item in the parent state is irrelevant; the child gets the old value parentState[0][0], which is still [0, () => ...].
When the Add button gets clicked again, because that's only the second time useState gets called on this render, now we get the new value that was intended for the first child, as the second child:
parentState = [
[
[
[0, (value) => parentState[1] = value],
[1, (value) => parentState[1] = value],
],
(value) => parentState[0] = value,
],
[
1,
(value) => parentState[1] = value,
],
];
And, as you can see, changes to either the first or second child both target the same value; they would not actually appear anywhere until the Add button was clicked again and they'd suddenly be the value of the third child.
For more on how hooks work and why the call order is so important, see e.g. "Why Do React Hooks Rely on Call Order?" by Dan Abramov.
So what would work? For the child to be able to correctly update its parent's state, it needs at least the setter and its own index:
const Parent = () => {
const [state, setState] = useState([]);
return (
<>
{state.map((child, index) => (
<Child child={child} index={index} setState={setState} />
)}
<button
onClick={() => setState([...state, 0])}
>
Add
</button>
</>
);
};
const Child = ({ child, index, setState }) => {
return (
<input
type="range"
min="0"
max="255"
value={child}
onChange={({ target }) => setState((oldState) => {
return oldState.map((oldValue, i) => i === index
? target.value
: oldValue);
})}
/>
);
};
But that means the parent no longer controls its own state, and the child has to know all about its parent's state - those two components are very closely coupled, so the boundary between them is clearly in the wrong place (and maybe shouldn't exist at all).
So what's the correct solution? It's the thing you didn't want to do, having the child "phone home" with an updated value and letting the parent update its state accordingly. This keeps the child decoupled from the details of the parent and its implementation correspondingly simple:
const Parent = () => {
const [state, setState] = useState([]);
const onChange = (newValue, index) => setState((oldState) => {
return oldState.map((oldValue, i) => i === index
? newValue
: oldValue);
};
return (
<>
{state.map((child, index) => (
<Child child={child} onChange={(value) => onChange(value, index)} />
)}
<button
onClick={() => setState([...state, 0])}
>
Add
</button>
</>
);
};
const Child = ({ child, onChange }) => {
return (
<input
type="range"
min="0"
max="255"
value={child}
onChange={({ target: { value } }) => onChange(value)}
/>
);
};
The Goal:
My React Native App shows a list of <Button /> based on the value from a list of Object someData. Once a user press a <Button />, the App should shows the the text that is associated with this <Button />. I am trying to achieve this using conditional rendering.
The Action:
So first, I use useEffect to load a list of Boolean to showItems. showItems and someData will have the same index so I can easily indicate whether a particular text associated with <Button /> should be displayed on the App using the index.
The Error:
The conditional rendering does not reflect the latest state of showItems.
The Code:
Here is my code example
import {someData} from '../data/data.js';
const App = () => {
const [showItems, setShowItems] = useState([]);
useEffect(() => {
const arr = [];
someData.map(obj => {
arr.push(false);
});
setShowItems(arr);
}, []);
const handlePressed = index => {
showItems[index] = true;
setShowItems(showItems);
//The list is changed.
//but the conditional rendering does not show the latest state
console.log(showItems);
};
return (
<View>
{someData.map((obj, index) => {
return (
<>
<Button
title={obj.title}
onPress={() => {
handlePressed(index);
}}
/>
{showItems[index] && <Text>{obj.item}</Text>}
</>
);
})}
</View>
);
};
This is because react is not identifying that your array has changed. Basically react will assign a reference to the array when you define it. But although you are changing the values inside the array, this reference won't be changed. Because of that component won't be re rendered.
And furthermore, you have to pass the key prop to the mapped button to get the best out of react, without re-rendering the whole button list. I just used trimmed string of your obj.title as the key. If you have any sort of unique id, you can use that in there.
So you have to notify react, that the array has updated.
import { someData } from "../data/data.js";
const App = () => {
const [showItems, setShowItems] = useState([]);
useEffect(() => {
const arr = [];
someData.map((obj) => {
arr.push(false);
});
setShowItems(arr);
}, []);
const handlePressed = (index) => {
setShowItems((prevState) => {
prevState[index] = true;
return [...prevState];
});
};
return (
<View>
{someData.map((obj, index) => {
return (
<>
<Button
key={obj.title.trim()}
title={obj.title}
onPress={() => {
handlePressed(index);
}}
/>
{showItems[index] && <Text>{obj.item}</Text>}
</>
);
})}
</View>
);
};
showItems[index] = true;
setShowItems(showItems);
React is designed with the assumption that state is immutable. When you call setShowItems, react does a === between the old state and the new, and sees that they are the same array. Therefore, it concludes that nothing has changed, and it does not rerender.
Instead of mutating the existing array, you need to make a new array:
const handlePressed = index => {
setShowItems(prev => {
const newState = [...prev];
newState[index] = true;
return newState;
});
}
I am using the React useState hook to update a list of items. I would like for only the added/updated components to be rendered but everytime the state of of the list changes all the items in list are re-rendered.
I have followed Preventing list re-renders. Hooks version. to solve the re-render issue but it doesn't work
Can someone help me understand, what's wrong with the below code or if this is actually not the right way to do it
function App() {
const [arr, setArr] = useState([])
useEffect(() => {
//getList here returns a list of elements of the form {id: number, name: string}
setArr(getList());
}, [])
const clickHandle = useCallback((e, id) => {
e.preventDefault()
setArr((arr) => {
return [...arr, {
id: id + 100,
name: `test${id+100}`
}]
})
}, [arr])
return (
<div className="App">
{
arr.map((item) => {
return (
<NewComp key={`${item.id}`} item={item} clickHandle={clickHandle} />
);
})
}
</div>
);
}
const NewComp = ({
item,
clickHandle
}) => {
return (
<div>
<button onClick={(e) => clickHandle(e, item.id)}>{item.name}</button>
</div>
);
}
The reason all your NewComp re-render is because your clickHandle function is being recreated whenever there is any change in the state arr.
This happens because you have added arr as a dependency to useCallback. This however is not required.
Once you fix it, you can wrap your NewComp with React.memo to optimize their re-renders. Also you must note that call the render function of a component is different from actually re-rendering it in the DOM.
const clickHandle = useCallback((e, id) => {
e.preventDefault()
setArr((arr) => {
return [...arr, {
id: id + 100,
name: `test${id+100}`
}]
})
}, []);
const NewComp = React.memo({
item,
clickHandle
}) => {
return (
<div>
<button onClick={(e) => clickHandle(e, item.id)}>{item.name}</button>
</div>
);
});
This is the render method, how i am calling the handler and setting the reactstrap checkbox.
this.state.dishes.map(
(dish, i) => (
<div key={i}>
<CustomInput
type="checkbox"
id={i}
label={<strong>Dish Ready</strong>}
value={dish.ready}
checked={dish.ready}
onClick={e => this.onDishReady(i, e)}
/>
</div>))
The handler for the onClick listener, I've tried with onchange as well but it apears that onchange doesnt do anything, idk why?
onDishReady = (id, e) => {
console.log(e.target.value)
var tempArray = this.state.dishes.map((dish, i) => {
if (i === id) {
var temp = dish;
temp.ready = !e.target.value
return temp
}
else {
return dish
}
})
console.log(tempArray)
this.setState({
dishes: tempArray
});
}
The event.target.value isn't the "toggled" value of an input checkbox, but rather event.target.checked is.
onDishReady = index => e => {
const { checked } = e.target;
this.setState(prevState => {
const newDishes = [...prevState.dishes]; // spread in previous state
newDishes[index].ready = checked; // update index
return { dishes: newDishes };
});
};
The rendered CustomInput reduces to
<CustomInput
checked={dish.ready}
id={i}
label={<strong>DishReady</strong>}
onChange={this.onDishReady(i)}
type="checkbox"
/>
No need to pass in a value prop since it never changes.
Note: Although an onClick handler does appear to work, semantically it isn't quite the correct event, you want the logic to respond to the checked value of the checkbox changing versus a user clicking on its element.
You can do it this way:
this.setState(function (state) {
const dishes = [...state.dishes];
dishes[id].ready = !e.target.value;
return dishes;
});
I've come accross a performance optimization issue that I feel could be fixed somehow but I'm not sure how.
Suppose I have a collection of objects that I want to be editable. The parent component contains all objects and renders a list with an editor component that shows the value and also allows to modify the objects.
A simplified example would be this :
import React, { useState } from 'react'
const Input = props => {
const { value, onChange } = props
handleChange = e => {
onChange && onChange(e.target.value)
}
return (
<input value={value} onChange={handleChange} />
)
}
const ObjectEditor = props => {
const { object, onChange } = props
return (
<li>
<Input value={object.name} onChange={onChange('name')} />
</li>
)
}
const Objects = props => {
const { initialObjects } = props
const [objects, setObjects] = useState(initialObjects)
const handleObjectChange = id => key => value => {
const newObjects = objects.map(obj => {
if (obj.id === id) {
return {
...obj,
[key]: value
}
}
return obj
})
setObjects(newObjects)
}
return (
<ul>
{
objects.map(obj => (
<ObjectEditor key={obj.id} object={obj} onChange={handleObjectChange(obj.id)} />
))
}
</ul>
)
}
export default Objects
So I could use React.memo so that when I edit the name of one object the others don't rerender. However, because of the onChange handler being recreated everytime in the parent component of ObjectEditor, all objects always render anyways.
I can't solve it by using useCallback on my handler since I would have to pass it my objects as a dependency, which is itself recreated everytime an object's name changes.
It seems to me like it is not necessary for all the objects that haven't changed to rerender anyway because the handler changed. And there should be a way to improve this.
Any ideas ?
I've seen in the React Sortly repo that they use debounce in combination with each object editor changing it's own state.
This allows only the edited component to change and rerender while someone is typing and updates the parent only once if no other change event comes up in a given delay.
handleChangeName = (e) => {
this.setState({ name: e.target.value }, () => this.change());
}
change = debounce(() => {
const { index, onChange } = this.props;
const { name } = this.state;
onChange(index, { name });
}, 300);
This is the best solution I can see right now but since they use the setState callback function I haven't been able to figure out a way to make this work with hooks.
You have to use the functional form of setState:
setState((prevState) => {
// ACCESS prevState
return someNewState;
});
You'll be able to access the current state value (prevState) while updating it.
Then way you can use the useCallback hook without the need of adding your state object to the dependency array. The setState function doesn't need to be in the dependency array, because it won't change accross renders.
Thus, you'll be able to use React.memo on the children, and only the ones that receive different props (shallow compare) will re-render.
EXAMPLE IN SNIPPET BELOW
const InputField = React.memo((props) => {
console.log('Rendering InputField '+ props.index + '...');
return(
<div>
<input
type='text'
value={props.value}
onChange={()=>
props.handleChange(event.target.value,props.index)
}
/>
</div>
);
});
function App() {
console.log('Rendering App...');
const [inputValues,setInputValues] = React.useState(
['0','1','2']
);
const handleChange = React.useCallback((newValue,index)=>{
setInputValues((prevState)=>{
const aux = Array.from(prevState);
aux[index] = newValue;
return aux;
});
},[]);
const inputItems = inputValues.map((item,index) =>
<InputField
value={item}
index={index}
handleChange={handleChange}
/>
);
return(
<div>
{inputItems}
</div>
);
}
ReactDOM.render(<App/>, document.getElementById('root'));
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.3/umd/react-dom.production.min.js"></script>
<div id="root"/>
Okay, so it seems that debounce works if it's wrapped in useCallback
Not sure why it doesn't seem to be necessary to pass newObject as a dependency in the updateParent function though.
So to make this work I had to make the following changes :
First, useCallback in the parent and change it to take the whole object instead of being responsible for updating the keys.
Then update the ObjectEditor to have its own state and handle the change to the keys.
And wrap the onChange handler that will update the parent in the debounce
import React, { useState, useEffect } from 'react'
import debounce from 'lodash.debounce'
const Input = props => {
const { value, onChange } = props
handleChange = e => {
onChange && onChange(e.target.value)
}
return (
<input value={value} onChange={handleChange} />
)
}
const ObjectEditor = React.memo(props => {
const { initialObject, onChange } = props
const [object, setObject] = useState(initialObject)
const updateParent = useCallback(debounce((newObject) => {
onChange(newObject)
}, 500), [onChange])
// synchronize the object if it's changed in the parent
useEffect(() => {
setObject(initialObject)
}, [initialObject])
const handleChange = key => value => {
const newObject = {
...object,
[key]: value
}
setObject(newObject)
updateParent(newObject)
}
return (
<li>
<Input value={object.name} onChange={handleChange('name')} />
</li>
)
})
const Objects = props => {
const { initialObjects } = props
const [objects, setObjects] = useState(initialObjects)
const handleObjectChange = useCallback(newObj => {
const newObjects = objects.map(obj => {
if (newObj.id === id) {
return newObj
}
return obj
})
setObjects(newObjects)
}, [objects])
return (
<ul>
{
objects.map(obj => (
<ObjectEditor key={obj.id} initialObject={obj} onChange={handleObjectChange} />
))
}
</ul>
)
}
export default Objects