In terms of writing components, which would be the preferred way to write below component? Assume that removeCard is outside of shown scope, ie. redux action.
My assumption would be that ComponentCardB would be, as it avoids passing an unnecessary argument which would be in the scope anyway. I imagine in terms of performance in the grand scheme of things, the difference is negligible, just more of a query in regards to best practise.
TIA
const ComponentCardA = (id) => {
const handleRemove = (cardId) => {
removeCard(cardId);
};
<div onClick={() => handleRemove(id)} />;
};
const ComponentCardB = (id) => {
const handleRemove = () => {
removeCard(id);
};
<div onClick={handleRemove} />;
};
With functional components like that, yes, there's no reason for the extra layer of indirection in ComponentCardA vs ComponentCardB.
Slightly tangential, but related: Depending on what you're passing handleRemove to and whether your component has other props or state, you may want to memoize handleRemove via useCallback or useMemo. The reason is that if other props or state change, your component function will get called again and (with your existing code) will create a new handleRemove function and pass that to the child. That means that the child has to be updated or re-rendered. If the change was unrelated to id, that update/rerender is unnecessary.
But if the component just has id and no other props, there's no point, and if it's just passing it to an HTML element (as opposed to React component), there's also probably no point as updating that element's click handler is a very efficient operation.
The second option is better way because using an arrow function in render creates a new function each time the component renders, which may break optimizations based on strict identity comparison.
Also if you don't want to use syntax with props.id you rather create function component with object as parameter:
const Component = ({id}) => { /* ... */ }
Of course using arrow function is also allowed but remember, when you don't have to use them then don't.
Related
I have a pretty decent understanding of how useCallback works. Figuring out when to use it though seems to be subjective amongst me and my colleagues though. I'm curious what everyone else thinks about our current dilemma.
Imagine we have a component that is dispatching an action to redux as a result of something being selected:
const SelectionComponent = props => {
const dispatch = useDispatch()
const handleSelect = (selection) => {
dispatch(actions.updateSelection(selection))
}
return <Select onSelect={handleSelect} ... />
}
My colleague believes we should wrap handleSelect in a useCallback to make sure the function has a stable identity since it's being passed as a callback to a child component:
const SelectionComponent = props => {
const dispatch = useDispatch()
const handleSelect = useCallback((selection) => {
dispatch(actions.updateSelection(selection))
}, [dispatch])
return <Select onSelect={handleSelect} ... />
}
So my question is, which is the better solution, and why?
Some notes:
useDispatch returns a dispatch function with a stable identity
No rerenders or performance issues occur without the useCallback
EDIT
Just to clarify, this is a question on whether or not we should memoize a function on the basis of maintaining a stable identity when passed to a child component, even if the component tree is not expected to rerender for any reason.
Memoizing handleSelect only makes a difference if Select is memoized as well. Remember - when a component re-renders it also re-renders all of it's children by default (regardless of if their props change or not).
Therefore without knowing how Select is implemented, we can't really say if the useCallback actually has any affect much less if it is "better".
Usually, this type of optimization is unnecessary though. Unless your Select is complex or expensive in some way, you probably do not need to memoize either.
same my question,
I just find the answer https://github.com/reduxjs/react-redux/issues/1468
"dispatch will be the same function reference the entire time. (In fact, in earlier versions of React-Redux, passing a new store reference was forbidden. We do now support changing it at runtime, but realistically you probably won't do that.)
However, the ESLint rule doesn't know that - it just knows that dispatch isn't a built-in React hook return value, so it might change, and therefore it tells you it should be added to the dependencies array just in case it ever does change."
I have the following component where within the useEffect, I am calling some data reading related
functions meant to happen once on load.
The problem is, some of the prop data are not available at this stage (still undefined) like the prodData and index.
They are only available when I get into the Nested components like <NestedComponent1 />.
I wish to move this logic into the nested components which will resolve this issue.
But I do not want to repeat these code inside the useEffect for each component. Instead looking to write these 7 lines once maybe in a function
and just call it with the 3 NestedComponents.
Issue is that there is a higher order function wrapping here plus all the values like prodData and index is coming from Redux store.
I can't just move all these logic inside useEffect into a normal JS function and instead need a functional component for this.
And if I make a functional component to perform these operations, I can't call it in the useEffect for each of the NestedComponents.
Cos this is not valid syntax.
React.useEffect(() => {
<NewlyCreatedComponentWithReadingFunctionality />
}, []);
Thus my query is, is there a way I could write a functional component which has the data reading logic inside its useEffect.
And then extend this functional component for each of the functional components so that the useEffect would just fire
when each of these NestedComponents are called?
Doesn't seem to be possible to do this thus looking for alternatives.
This is the existing component where some of these prop values are undefined at this stage.
const MyComponent = ({
prodData,
index,
country,
highOrder: {
AHigherOrderComponent,
},
}) => {
// this is the logic which I am looking to write once and be
// repeatable for all the NestedComponent{1,2,3}s below.
React.useEffect(() => {
const [, code] = country.split('-');
const sampleData = prodData[index].sampleData = sampleData;
const period = prodData[index].period = period;
const indication = prodData[index].indication = indication;
AHigherOrderComponent(someReadDataFunction(code, sampleData));
AHigherOrderComponent(someReadDataFunction(code, period);
AHigherOrderComponent(someReadDataFunction(code, indication);
}, []);
return (
{/* other logics not relevant */}
<div>
<div>
<NestedComponent1 />
<NestedComponent2 />
<NestedComponent3 />
</div>
</div>
);
};
export default connect( // redux connect
({
country,
prodData,
index,
}) => ({
country,
prodData,
index,
})
)(withHighOrder(MyComponent));
React components implement a pattern called composition. There are a few ways to share state between parts of your React application but whenever you have to remember some global state and offer some shared functionality, I would try and manage that logic inside a context provider.
I would try the following:
Wrap all your mentioned components inside a context provider component
Offer the someReadDataFunction as a callback function as part of the context
Within your provider, manage react state, e.g. functionHasBeenCalled that remembers if someReadDataFunction has been called already
Set functionHasBeenCalled to true inside someReadDataFunction
Call someReadDataFunction inside your components within a useEffect based on the props data
This way, your application globally remembers if the function has been executed already but you can still use the latest data within your useEffect within your components to call someReadDataFunction.
Say we got a Page-component that delegates the rendering of notifications to a Notification-component. The Page-component's render method contains the following ...
{this.state.notifications &&
<Notifications
notifications={this.state.notifications}
removeNotifAt={index => this.setState(prevState => {
const copy = [...prevState.notifications]
copy.splice(index, 1)
return { notifications: copy }
})}
removeNotifyBy={id => this.setState(prevState => {
const copy = [...prevState.notifications]
const index = copy.findIndex((notif, _) => { return notif.id === id })
copy.splice(index, 1)
return { notifications: copy }
})}
/>
}
... as you might notice, Notifications require some rather large function to alter the state of its parent. Since they access this.state, these functions have to be defined in the parent of Notifications, in this case Page.
Now, one can imagine that multiple pages have notifications that needs rendering and so they all have to code-duplicate the code snippet above. As we all know, code-duplication is bad, so how can we best avoid it?
It's impossible to extract the functions removeNotifyAt and removeNotifyBy out into functions defined in, say, Notifications.js since they need to access this.state.
So, what's the react-way of dealing with such duplicate functions that you can't extract away because it needs to access this.state? I suppose I am not the first one stumpling upon this, giving how trivial of a case this is.
You can move the code from removeNotifyBy and removeNotifyBy into functions, and place them in the parent component. Then you can pass them into the child components as props.
You could extract those functions into Notifications.js. (That is in case you don't wish to write those functions inside parent component).
Both the parent's state as will as the function that sets the state can be passed as props to Notifications component.
Example:
<Notifications parentState={this.state} parentStateHandler={this.setState} />
//Note: Ideally props shouldn't be named like this and one must avoid passing entire state object as prop, rather you should split it into props that the child component requires. However this is just to give you the clarity regarding how to access parent's state in child
Now Notifications component has entire parents state which can be accessed by props.parentState and you can also set Parent state by using props.parentStateHandler instead of this.setState
Is useState's setter able to change during a component life ?
For instance, let's say we've got a useCallback which will update the state.
If the setter is able to change, it must be set as a dependency for the callback since the callback use it.
const [state, setState] = useState(false);
const callback = useCallback(
() => setState(true),
[setState] // <--
);
The setter function won't change during component life.
From Hooks FAQ:
(The identity of the setCount function is guaranteed to be stable so it’s safe to omit.)
The setter function (setState) returned from useState changes on component re-mount, but either way, the callback will get a new instance.
It's a good practice to add state setter in the dependency array ([setState]) when using custom-hooks. For example, useDispatch of react-redux gets new instance on every render, you may get undesired behavior without:
// Custom hook
import { useDispatch } from "react-redux";
export const CounterComponent = ({ value }) => {
// Always new instance
const dispatch = useDispatch();
// Should be in a callback
const incrementCounter = useCallback(
() => dispatch({ type: "increment-counter" }),
[dispatch]
);
return (
<div>
<span>{value}</span>
// May render unnecessarily due to the changed reference
<MyIncrementButton onIncrement={dispatch} />
// In callback, all fine
<MyIncrementButton onIncrement={incrementCounter} />
</div>
);
};
The short answer is, no, the setter of useState() is not able change, and the React docs explicitly guarantee this and even provide examples proving that the setter can be omitted.
I would suggest that you do not add anything to the dependencies list of your useCallback() unless you know its value can change. Just like you wouldn't add any functions imported from modules or module-level functions, constant expressions defined outside the component, etc. adding those things is just superfluous and makes it harder to read your handlers.
All that being said, this is all very specific to the function that is returned by useState() and there is no reason to extend that line of reasoning to every possible custom hook that may return a function. The reason is that the React docs explicitly guarantee the stable behavior of useState() and its setters, but it does not say that the same must be true for any custom hook.
React hooks are still kind of a new and experimental concept and we need to make sure we encourage each other to make them as readable as possible, and more importantly, to understand what they actually do and why. If we don't it will be seen as evidence that hooks are a "bad idea," which will prohibit adoption and wider understanding of them. That would be bad; in my experience they tend to produce much cleaner alternatives to the class-based components that React is usually associated with, not to mention the fact that they can allow organizational techniques that simply aren't possible with classes.
When creating a React app, if I use the hook useSelector, I need to adhere to the hooks invoking rules (Only call it from the top level of a functional component). If I use the mapStateToProps, I get the state in the props and I can use it anywhere without any issues... Same issue for useDispatch
What are the benefits of using the hook besides saving lines of code compared to mapStateToProps?
Redux store state can be read and changed from anywhere in the component, including callbacks. Whenever the store state is changed the component rerenders. When the component rerenders, useSelector runs again, and gives you the updated data, later to be used wherever you want. Here is an example of that and a usage of useDispatch inside a callback (after an assignment in the root level):
function Modal({ children }) {
const isOpen = useSelector(state => state.isOpen);
const dispatch = useDispatch();
function handleModalToggeled() {
// using updated data from store state in a callback
if(isOpen) {
// writing to state, leading to a rerender
dispatch({type: "CLOSE_MODAL"});
return;
}
// writing to state, leading to a rerender
dispatch({type: "OPEN_MODAL"});
}
// using updated data from store state in render
return (isOpen ? (
<div>
{children}
<button onClick={handleModalToggeled}>close modal</button>
</div>
) : (
<button onClick={handleModalToggeled}>open modal</button>
);
);
}
There is nothing you can do with mapStateToProps/mapDispatchToProps that you can't do with the useSelector and useDispatch hooks as well.
With that said, there are a couple of differences between the two methods that are worth considering:
Decoupling: with mapStateToProps, container logic (the way store data is injected into the component) is separate from the view logic (component rendering).
useSelector represents a new and different way of thinking about connected components, arguing that the decoupling is more important between components and that components are self contained. Which is better? Verdict: no clear winner. source
DX (Developer experience): using the connect function usually means there should be another additional container component for each connected component, where using the useSelector and useDispatch hooks is quite straightforward. Verdict: hooks have better DX.
"Stale props" and "Zombie child": there are some weird edge cases with useSelector, if it depends on props, where useSelector can run before the newest updated props come in. These are mostly rare and avoidable edge cases, but they had been already worked out in the older connect version. verdict: connect is slightly more stable than hooks. source
Performance optimizations: both support performance optimizations in different ways: connect has some advanced techniques, using merge props and other options hidden in the connect function. useSelector accepts a second argument - an equality function to determine if the state has changed. verdict: both are great for performance in advanced situations.
Types: using typescript with connect is a nightmare. I remember myself feverishly writing three props interfaces for each connected component (OwnProps, StateProps, DispatchProps). Redux hooks support types in a rather straightforward way. verdict: types are significantly easier to work with using hooks.
The future of React: Hooks are the future of react. This may seam like an odd argument, but change to the ecosystem is right around the corner with "Concurrent mode" and "Server components". While class components will still be supported in future React versions, new features may rely solely on hooks. This change will of course also affect third party libraries in the eco system, such as React-Redux. verdict: hooks are more future proof.
TL;DR - Final verdict: each method has its merits. connect is more mature, has less potential for weird bugs and edge cases, and has better separation of concerns. Hooks are easier to read and write, as they are collocated near the place where they are used (all in one self contained component). Also, they are easier to use with TypeScript. Finally, they will easily be upgradable for future react versions.
I think you misunderstand what "top level" is. It merely means that, inside a functional component, useSelector() cannot be placed inside loops, conditions and nested functions. It doesn't have anything to do with root component or components structure
// bad
const MyComponent = () => {
if (condition) {
// can't do this
const data = useSelector(mySelector);
console.log(data);
}
return null;
}
---
// good
const MyComponent = () => {
const data = useSelector(mySelector);
if (condition) {
console.log(data); // using data in condition
}
return null;
}
If anything, mapStateToPtops is located at even higher level than a hook call
the rules of hooks make it very hard to use that specific hook. You still need to somehow access a changing value from the state inside callbacks
To be fair you almost never have to access changing value inside a callback. I can't remember last time I needed that. Usually if your callback needs the latest state, you are better off just dispatching an action and then handler for that action (redux-thunk, redux-saga, redux-observable etc) will itself access the latest state
This is just specifics of hooks in general (not just useSelector) and there are tons of ways to go around it if you really want to, for example
const MyComponent = () => {
const data = useSelector(mySelector);
const latestData = useRef()
latestData.current = data
return (
<button
onClick={() => {
setTimeout(() => {
console.log(latestData.current) // always refers to latest data
}, 5000)
}}
/>
)
}
What are the benefits of using the hook besides saving lines of code compared to mapStateToProps?
You save time by not writing connect function any time you need to access store, and removing it when you no longer need to access store. No endless wrappers in react devtools
You have clear distinction and no conflicts between props coming from connect, props coming from parent and props injected by wrappers from 3rd party libraries
Sometimes you (or fellow developers you work with) would choose unclear names for props in mapStateToProps and you will have to scroll all the way to mapStateToProps in the file to find out which selector is used for this specific prop. This is not the case with hooks where selectors and variables with data they return are coupled on the same line
By using hooks you get general advantages of hooks, the biggest of which is being able couple together and reuse related stateful logic in multiple components
With mapStateToProps you usually have to deal with mapDispatchToProps which is even more cumbersome and easier to get lost in, especially reading someone else's code (object form? function form? bindActionCreators?). Prop coming from mapDispatchToProps can have same name as it's action creator but different signature because it was overridden in mapDispatchToprops. If you use one action creator in a number of components and then rename that action creator, these components will keep using old name coming from props. Object form easily breaks if you have a dependency cycle and also you have to deal with shadowing variable names
.
import { getUsers } from 'actions/user'
class MyComponent extends Component {
render() {
// shadowed variable getUsers, now you either rename it
// or call it like this.props.getUsers
// or change import to asterisk, and neither option is good
const { getUsers } = this.props
// ...
}
}
const mapDispatchToProps = {
getUsers,
}
export default connect(null, mapDispatchToProps)(MyComponent)
See EDIT 2 at the end for the final answer
Since no one knows how to answer, it seems like the best answer is that you should NOT be using useselector when you need information in other places other than the root level of your component. Since you don't know if the component will change in the future, just don't use useselector at all.
If someone has a better answer than this, I'll change the accepted answer.
Edit: Some answers were added, but they just emphasize why you shouldn't be using useselector at all, until the day when the rules of hooks will change, and you'll be able to use it in a callback as well. That being said, if you don't want to use it in a callback, it could be a good solution for you.
EDIT 2: An answer with examples of all that I wanted was added and showed how useSelector and useDispatch are easier to use.
The redux state returned from the useSelector hook can be passed around anywhere else just like its done for mapStateToProps. Example: It can be passed to another function too. Only constraint being that the hook rules has to be followed during its declaration:
It has to be declared only within a functional component.
During declaration, it can not be inside any conditional block . Sample code below
function test(displayText) {
return (<div>{displayText}</div>);
}
export function App(props) {
const displayReady = useSelector(state => {
return state.readyFlag;
});
const displayText = useSelector(state => {
return state.displayText;
});
if(displayReady) {
return
(<div>
Outer
{test(displayText)}
</div>);
}
else {
return null;
}
}
EDIT: Since OP has asked a specific question - which is about using it within a callback, I would like to add a specific code.In summary, I do not see anything that stops us from using useSelector hook output in a callback. Please see the sample code below, its a snippet from my own code that demonstrates this particular use case.
export default function CustomPaginationActionsTable(props) {
//Read state with useSelector.
const searchCriteria = useSelector(state => {
return state && state.selectedFacets;
});
//use the read state in a callback invoked from useEffect hook.
useEffect( ()=>{
const postParams = constructParticipantListQueryParams(searchCriteria);
const options = {
headers: {
'Content-Type': 'application/json'
},
validateStatus: () => true
};
var request = axios.post(PORTAL_SEARCH_LIST_ALL_PARTICIPANTS_URI, postParams, options)
.then(function(response)
{
if(response.status === HTTP_STATUS_CODE_SUCCESS) {
console.log('Accessing useSelector hook output in axios callback. Printing it '+JSON.stringify(searchCriteria));
}
})
.catch(function(error) {
});
}, []);
}
For callback functions you can use the value returned from useSelector the same way you would use the value from useState.
const ExampleComponent = () => {
// use hook to get data from redux state.
const stateData = useSelector(state => state.data);
// use hook to get dispatch for redux store.
// this allows actions to be dispatched.
const dispatch = useDispatch();
// Create a non-memoized callback function using stateData.
// This function is recreated every rerender, a change in
// state.data in the redux store will cause a rerender.
const callbackWithoutMemo = (event) => {
// use state values.
if (stateData.condition) {
doSomething();
}
else {
doSomethingElse();
}
// dispatch some action to the store
// can pass data if needed.
dispatch(someActionCreator());
};
// Create a memoized callback function using stateData.
// This function is recreated whenever a value in the
// dependency array changes (reference comparison).
const callbackWithMemo = useCallback((event) => {
// use state values.
if (stateData.condition) {
doSomething();
}
else {
doSomethingElse();
}
// dispatch some action to the store
// can pass data if needed.
dispatch(someActionCreator());
}, [stateData, doSomething, doSomethingElse]);
// Use the callbacks.
return (
<>
<div onClick={callbackWithoutMemo}>
Click me
</div>
<div onClick={callbackWithMemo}>
Click me
</div>
</>
)
};
Rules of hooks says you must use it at the root of your component, meaning you CANT use it anywhere.
As Max stated in his answer just means that the hook statement itself must not be dynamic / conditional. This is because the order of the base hooks (react's internal hooks: useState, etc) is used by the backing framework to populate the stored data each render.
The values from hooks can be used where ever you like.
While I doubt this will be close to answering your complete question, callbacks keep coming up and no examples had been posted.
not the answer but this hook can be very helpful if you want to get decoupled nature of mapDispatchToProps while keeping simplicity and dev experience of hooks:
https://gist.github.com/ErAz7/1bffea05743440d6d7559afc9ed12ddc
the reason I don't mention one for mapStatesToProps is that useSelector itself is more store-logic-decoupling than mapStatesToProps so don't see any advantage for mapStatesToProps. Of course I dont mean using useSelector directly but instead create a wrapper on it in your store files (e.g. in reducer file) and import from there, like this:
// e.g. userReducer.js
export const useUserProfile = () => useSelector(state => state.user.profile)