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Should I use useselector/useDispatch instead of mapStateToProps

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)

Correct way to use immutablejs (toJS and fromJS) with redux

I wonder if this is a correct way to use immutable.js with redux and reselect (also redux-saga). Specifically I wonder about toJS() and from fromJS() and where to use them.
My idea is that:
I use toJS() when sending data to a saga.
I do not use fromJS() in reducer because I think that it is done anyway by the fact that I use fromJS() for initialState. Or am I wrong about that?
I use toJS() in selector from reselect so I can use js data in react component.
Example:
1) In my react component I do:
// mapDispatchToProps
function mapDispatchToProps(dispatch) {
return {
loginRequest: values => dispatch(loginRequest(values)),
};
}
// Sending values.toJS() to my redux-saga.
submit = values => {
this.props.loginRequest(values.toJS());
};
2) In reducer I do (should one use fromJS() here or not? According to redux docs you should):
const { fromJS } = require('immutable');
const state = fromJS({
pages: {
usersPage: {
loading: false,
isFetched: false,
list: [],
}
}
});
function reducer(state, action) {
switch(action.type) {
case 'USERS_LOADED':
return state
.setIn(['usersPage', 'list'], action.payload) // fromJS() here or not?
.setIn(['usersPage', 'isFetched'], true)
.setIn(['usersPage', 'loading'], false)
;
default:
return state;
}
}
export default reducer;
3) In my selector I do toJS() again:
const selectUser = state => state.get('user', initialState);
const makeSelectList= () =>
createSelector(selectUser, userState => userState.getIn(['usersPage',
'list']).toJS());
// Which I then use in my react component:
const mapStateToProps = createStructuredSelector({
list: makeSelectList(),
});
So basically I wonder if that is a correct flow of convertion between js and immutable. Or can it be optimized in some way (less convertion steps)? Maybe the above is a non-optimal way of logic?
Best Regards

The saga--being redux middleware--can handle Immutable types directly, no need to use an expensive toJS call here
Any point you're converting (e.g. set, setIn, update, etc) a plain JS non-simple type into the Immutable redux state tree, use fromJS to ensure a fully Immutable type Make entire state tree immutable
IMHO, selectors (e.g. reselect)--by providing memoization after the initial retrieval--can be the most ideal place to utilize the expensive toJS calls, as in your example #3. I guess it really depends upon how much one dislikes using Immutable retrieval methods in their "container/smart" components, and/or creating a whole bunch of selectors to retrieve simple JS types from the redux state tree Use Immutable everywhere
To me there's the question of where to actually use the fromJS call, e.g. action creators, in the "container/smart" components dispatch or, in the reducer, e.g. react-boilerplate uses the fromJS call in the reducer.

How is state passed to selectors in a react-redux app?

I came across an example, where the mapStateToProps function is using memoization. I was just wondering how the "state" parameter is passed onto memoized selectors. After looking at the docs for reselect as well as redux, it seems that the mapStateToProps can return a function which accepts state as its argument, and the connect decorator might be the one passing the state to it but am not sure. Can someone please shed some light?
views/tracklist/index.js
const mapStateToProps = createSelector(
getBrowserMedia,
getPlayerIsPlaying,
getPlayerTrackId,
getCurrentTracklist,
getTracksForCurrentTracklist,
(media, isPlaying, playerTrackId, tracklist, tracks) => ({
displayLoadingIndicator: tracklist.isPending || tracklist.hasNextPage,
isMediaLarge: !!media.large,
isPlaying,
pause: audio.pause,
pauseInfiniteScroll: tracklist.isPending || !tracklist.hasNextPage,
play: audio.play,
selectedTrackId: playerTrackId,
tracklistId: tracklist.id,
tracks
})
);
export default connect(
mapStateToProps,
mapDispatchToProps
)(Tracklist);
core/tracklists/selectors.js
export function getCurrentTracklist(state) {
// console.log(state);
let tracklists = getTracklists(state);
return tracklists.get(tracklists.get('currentTracklistId'));
}
export const getTracksForCurrentTracklist = createSelector(
getCurrentPage,
getCurrentTrackIds,
getTracks,
(currentPage, trackIds, tracks) => {
return trackIds
.slice(0, currentPage * TRACKS_PER_PAGE)
.map(id => tracks.get(id));
}
);

Overview of how state is passed down to a selector when we use the Connect component from react-redux
What is a selector?
A selector extracts a subset of data from a source.
Let us think of the Redux store as our 'front end database'. For the purposeIn a database if you want to extract a subset of the total data you execute a query. In a similar fashion selectors are our queries to the Redux store.
In the simplest case, a selector could just return the state of the entire store.
The reselect docs give us three great reasons to use selectors
Selectors can compute derived data, allowing Redux to store the
minimal possible state.
Selectors are efficient. A selector is not
recomputed unless one of its arguments change.
Selectors are
composable. They can be used as input to other selectors.
What is a higher order component?
A higher-order component is a function that takes an existing component and returns a new component.
Connect is a higher order component that be given a selector
Taken from this brilliant gist which gives a good explanation of connect.
connect() is a function that injects Redux-related props into your
component.
Connect is a higher order component that makes our React component know about the Redux store. When we call connect we can pass mapStateToProps and mapDispatchToProps. These functions define the way in which our new component will be connected to the redux store.
We can give it access to state by passing a mapStateToProps function as an argument.
We can also bind action creators to store.dispatch through mapDispatchToProps. The advantage of this is that we don't need to pass down the entire store in order for a component to have access to store.dispatch so that the component can dispatch its own Redux actions.
The mapStateToProps function we pass to Connect is a selector
From the react-redux docs
The mapStateToProps function takes a single argument of the entire
Redux store’s state and returns an object to be passed as props. It is
often called a selector.
Think of the object that is returned by mapStateToProps as the result of our query to the Redux store. The resulting
The mapStateToProps function should normally return a plain object.
The result of calling mapStateToProps will normally be a plain object representing the data we extracted from the redux store.
The higher order Connect component allows us to extend the functionality of an existing component by merging in the data from this new object with the component's existing props.
Since selectors are just functions we can connect them to the Redux store using the connect component as well.
However in some cases we can return a function. Why would we do this?
By returing a function in mapStateToProps we can hijack the rendering cycle of components and optimise performance
In advanced scenarios where you need more control over the rendering
performance, mapStateToProps() can also return a function. In this
case, that function will be used as mapStateToProps() for a particular
component instance. This allows you to do per-instance memoization.
By passing the mapStateToProps function as an argument to our higher order component our connected component will be updated anytime the some state has changed in the Redux store.
If these updates happen very frequently or the state tree is large then the reselect library is useful as it allows us to use memoized selectors.
This fancy word means that results of selector calls are stored in case they need to be retrieved again.
So if mapStatesToProps returned a plain object instead of a function then whenever our store state changed then we would have new props for our component.???
Connecting selectors to the store
If you are using React Redux, you can call selectors as regular functions inside mapStateToProps():
import { getVisibleTodos } from '../selectors'
const mapStateToProps = (state) => {
return {
todos: getVisibleTodos(state)
}
}
Sharing Selectors Across Multiple Components
We can give reselect selectors props just like components when using the reselect library. This allows us to share selectors across multiple components.
Say we have multiple toDo lists each with their own Id. We would still use the same getVisibleTodos selector for each toDo list instance but just pass a different id as a prop.
However the issue with this is that createSelector only returns the cached value when its set of arguments is the same as its previous set of arguments.
The reselect docs point out that we can overcome this limitation by returning a function inside mapStateToProps:
In order to share a selector across multiple components and retain
memoization, each instance of the component
needs its own private copy of the selector.
If the mapStateToProps argument supplied to connect returns a function
instead of an object, it will be used to create an individual
mapStateToProps function for each instance of the container.
By returning a function inside mapStateToProps we can overcome this limitation and memoization will work correctly.
For a more detailed explanation see this

Is so simple: let's give you an example, I have a mapStateToProps like this:
function mapStateToProps(state) {
return {
categoryHistory: getCategoryHistory(state,'extended')
}
}
then I've create a selector like this:
export const getCategoryHistory = (state, type) => createSelector([getTaxonomy, selectedCategoryID], (categories, categoryID) => categories.getIn([type, categoryID]) || [])(state)
The solution is to call createSelector() passing the state as parameters:
createSelector()(state)
inside the selector you can use all the parameter you want to pass through.

In the cases you mentioned, mapStateToProps is a function which takes in state and returning object. When you passed mapStateToProps to connect, you passed a function which accepts state provided by connect as its argument.
createSelector creates a function which can take in state and returning object as well. Therefore you can assign it to mapStateToProps and pass it into connect.
In documentation, you'll normally find the following:
const mapStateToProps = (state) => {
whatever code
}
and
export default connect(mapStateToProps, mapDispatchToProps)(Component)
where mapStateToProps takes in state argument which is provided by connect.
However, one can let mapStateToProps to be a selector as followed:
const mapStateToProps = createSelector(
whatever code
)
This is because createSelector can take in a state as followed:
createSelector(whatever code)(state)
and return an object, just like what you find a mapStateToProps does in documentation.

Understanding React-Redux and mapStateToProps()

I'm trying to understand the connect method of react-redux, and the functions it takes as parameters. In particular mapStateToProps().
The way I understand it, the return value of mapStateToProps will be an object derived from state (as it lives in the store), whose keys will be passed to your target component (the component connect is applied to) as props.
This means that the state as consumed by your target component can have a wildly different structure from the state as it is stored on your store.
Q: Is this OK?
Q: Is this expected?
Q: Is this an anti-pattern?

Yes, it is correct. Its just a helper function to have a simpler way to access your state properties
Imagine you have a posts key in your App state.posts
state.posts //
/*
{
currentPostId: "",
isFetching: false,
allPosts: {}
}
*/
And component Posts
By default connect()(Posts) will make all state props available for the connected Component
const Posts = ({posts}) => (
<div>
{/* access posts.isFetching, access posts.allPosts */}
</div>
)
Now when you map the state.posts to your component it gets a bit nicer
const Posts = ({isFetching, allPosts}) => (
<div>
{/* access isFetching, allPosts directly */}
</div>
)
connect(
state => state.posts
)(Posts)
mapDispatchToProps
normally you have to write dispatch(anActionCreator())
with bindActionCreators you can do it also more easily like
connect(
state => state.posts,
dispatch => bindActionCreators({fetchPosts, deletePost}, dispatch)
)(Posts)
Now you can use it in your Component
const Posts = ({isFetching, allPosts, fetchPosts, deletePost }) => (
<div>
<button onClick={() => fetchPosts()} />Fetch posts</button>
{/* access isFetching, allPosts directly */}
</div>
)
Update on actionCreators..
An example of an actionCreator: deletePost
const deletePostAction = (id) => ({
action: 'DELETE_POST',
payload: { id },
})
So, bindActionCreators will just take your actions, wrap them into dispatch call. (I didn't read the source code of redux, but the implementation might look something like this:
const bindActionCreators = (actions, dispatch) => {
return Object.keys(actions).reduce(actionsMap, actionNameInProps => {
actionsMap[actionNameInProps] = (...args) => dispatch(actions[actionNameInProps].call(null, ...args))
return actionsMap;
}, {})
}

Q: Is this ok?
A: yes
Q: Is this expected?
Yes, this is expected (if you are using react-redux).
Q: Is this an anti-pattern?
A: No, this is not an anti-pattern.
It's called "connecting" your component or "making it smart". It's by design.
It allows you to decouple your component from your state an additional time which increases the modularity of your code. It also allows you to simplify your component state as a subset of your application state which, in fact, helps you comply with the Redux pattern.
Think about it this way: a store is supposed to contain the entire state of your application.
For large applications, this could contain dozens of properties nested many layers deep.
You don't want to haul all that around on each call (expensive).
Without mapStateToProps or some analog thereof, you would be tempted to carve up your state another way to improve performance/simplify.

You got the first part right:
Yes mapStateToProps has the Store state as an argument/param (provided by react-redux::connect) and its used to link the component with certain part of the store state.
By linking I mean the object returned by mapStateToProps will be provided at construction time as props and any subsequent change will be available through componentWillReceiveProps.
If you know the Observer design pattern it's exactly that or small variation of it.
An example would help make things clearer:
import React, {
Component,
} from 'react-native';
class ItemsContainer extends Component {
constructor(props) {
super(props);
this.state = {
items: props.items, //provided by connect#mapStateToProps
filteredItems: this.filterItems(props.items, props.filters),
};
}
componentWillReceiveProps(nextProps) {
this.setState({
filteredItems: this.filterItems(this.state.items, nextProps.filters),
});
}
filterItems = (items, filters) => { /* return filtered list */ }
render() {
return (
<View>
// display the filtered items
</View>
);
}
}
module.exports = connect(
//mapStateToProps,
(state) => ({
items: state.App.Items.List,
filters: state.App.Items.Filters,
//the State.App & state.App.Items.List/Filters are reducers used as an example.
})
// mapDispatchToProps, that's another subject
)(ItemsContainer);
There can be another react component called itemsFilters that handle the display and persisting the filter state into Redux Store state, the Demo component is "listening" or "subscribed" to Redux Store state filters so whenever filters store state changes (with the help of filtersComponent) react-redux detect that there was a change and notify or "publish" all the listening/subscribed components by sending the changes to their componentWillReceiveProps which in this example will trigger a refilter of the items and refresh the display due to the fact that react state has changed.
Let me know if the example is confusing or not clear enough to provide a better explanation.
As for: This means that the state as consumed by your target component can have a wildly different structure from the state as it is stored on your store.
I didn't get the question, but just know that the react state (this.setState) is totally different from the Redux Store state!
The react state is used to handle the redraw and behavior of the react component. The react state is contained to the component exclusively.
The Redux Store state is a combination of Redux reducers states, each is responsible of managing a small portion app logic. Those reducers attributes can be accessed with the help of react-redux::connect#mapStateToProps by any component! Which make the Redux store state accessible app wide while component state is exclusive to itself.

This react & redux example is based off Mohamed Mellouki's example.
But validates using prettify and linting rules. Note that we define our props
and dispatch methods using PropTypes so that our compiler doesn't scream at us.
This example also included some lines of code that had been missing in Mohamed's
example. To use connect you will need to import it from react-redux. This
example also binds the method filterItems this will prevent scope problems in
the component. This source code has been auto formatted using JavaScript Prettify.
import React, { Component } from 'react-native';
import { connect } from 'react-redux';
import PropTypes from 'prop-types';
class ItemsContainer extends Component {
constructor(props) {
super(props);
const { items, filters } = props;
this.state = {
items,
filteredItems: filterItems(items, filters),
};
this.filterItems = this.filterItems.bind(this);
}
componentWillReceiveProps(nextProps) {
const { itmes } = this.state;
const { filters } = nextProps;
this.setState({ filteredItems: filterItems(items, filters) });
}
filterItems = (items, filters) => {
/* return filtered list */
};
render() {
return <View>/*display the filtered items */</View>;
}
}
/*
define dispatch methods in propTypes so that they are validated.
*/
ItemsContainer.propTypes = {
items: PropTypes.array.isRequired,
filters: PropTypes.array.isRequired,
onMyAction: PropTypes.func.isRequired,
};
/*
map state to props
*/
const mapStateToProps = state => ({
items: state.App.Items.List,
filters: state.App.Items.Filters,
});
/*
connect dispatch to props so that you can call the methods from the active props scope.
The defined method `onMyAction` can be called in the scope of the componets props.
*/
const mapDispatchToProps = dispatch => ({
onMyAction: value => {
dispatch(() => console.log(`${value}`));
},
});
/* clean way of setting up the connect. */
export default connect(mapStateToProps, mapDispatchToProps)(ItemsContainer);
This example code is a good template for a starting place for your component.

React-Redux connect is used to update store for every actions.
import { connect } from 'react-redux';
const AppContainer = connect(
mapStateToProps,
mapDispatchToProps
)(App);
export default AppContainer;
It's very simply and clearly explained in this blog.
You can clone github project or copy paste the code from that blog to understand the Redux connect.

It's a simple concept. Redux creates a ubiquitous state object (a store) from the actions in the reducers. Like a React component, this state doesn't have to be explicitly coded anywhere, but it helps developers to see a default state object in the reducer file to visualise what is happening. You import the reducer in the component to access the file. Then mapStateToProps selects only the key/value pairs in the store that its component needs. Think of it like Redux creating a global version of a React component's
this.state = ({
cats = [],
dogs = []
})
It is impossible to change the structure of the state by using mapStateToProps(). What you are doing is choosing only the store's key/value pairs that the component needs and passing in the values (from a list of key/values in the store) to the props (local keys) in your component. You do this one value at a time in a list. No structure changes can occur in the process.
P.S. The store is local state. Reducers usually also pass state along to the database with Action Creators getting into the mix, but understand this simple concept first for this specific posting.
P.P.S. It is good practice to separate the reducers into separate files for each one and only import the reducer that the component needs.

Here's an outline/boilerplate for describing the behavior of mapStateToProps:
(This is a vastly simplified implementation of what a Redux container does.)
class MyComponentContainer extends Component {
mapStateToProps(state) {
// this function is specific to this particular container
return state.foo.bar;
}
render() {
// This is how you get the current state from Redux,
// and would be identical, no mater what mapStateToProps does
const { state } = this.context.store.getState();
const props = this.mapStateToProps(state);
return <MyComponent {...this.props} {...props} />;
}
}
and next
function buildReduxContainer(ChildComponentClass, mapStateToProps) {
return class Container extends Component {
render() {
const { state } = this.context.store.getState();
const props = mapStateToProps(state);
return <ChildComponentClass {...this.props} {...props} />;
}
}
}

Yes, you can do this. You can also even process the state and return the object.
function mapStateToProps(state){
let completed = someFunction (state);
return {
completed : completed,
}
}
This would be useful if you want to shift the logic related to state from render function to outside of it.

I would like to re-structure the statement that you mentioned which is:
This means that the state as consumed by your target component can
have a wildly different structure from the state as it is stored on
your store
You can say that the state consumed by your target component has a small portion of the state that is stored on the redux store. In other words, the state consumed by your component would be the sub-set of the state of the redux store.
As far as understanding the connect() method is concerned, it's fairly simple! connect() method has the power to add new props to your component and even override existing props. It is through this connect method that we can access the state of the redux store as well which is thrown to us by the Provider. A combination of which works in your favor and you get to add the state of your redux store to the props of your component.
Above is some theory and I would suggest you look at this video once to understand the syntax better.

import React from 'react';
import {connect} from 'react-redux';
import Userlist from './Userlist';
class Userdetails extends React.Component{
render(){
return(
<div>
<p>Name : <span>{this.props.user.name}</span></p>
<p>ID : <span>{this.props.user.id}</span></p>
<p>Working : <span>{this.props.user.Working}</span></p>
<p>Age : <span>{this.props.user.age}</span></p>
</div>
);
}
}
function mapStateToProps(state){
return {
user:state.activeUser
}
}
export default connect(mapStateToProps, null)(Userdetails);

How to optimize small updates to props of nested component in React + Redux?

Example code: https://github.com/d6u/example-redux-update-nested-props/blob/master/one-connect/index.js
View live demo: http://d6u.github.io/example-redux-update-nested-props/one-connect.html
How to optimize small updates to props of nested component?
I have above components, Repo and RepoList. I want to update the tag of the first repo (Line 14). So I dispatched an UPDATE_TAG action. Before I implemented shouldComponentUpdate, the dispatch takes about 200ms, which is expected since we are wasting lots of time diffing <Repo/>s that haven't changed.
After added shouldComponentUpdate, dispatch takes about 30ms. After production build React.js, the updates only cost at about 17ms. This is much better, but timeline view in Chrome dev console still indicate jank frame (longer than than 16.6ms).
Imagine if we have many updates like this, or <Repo/> is more complicated than current one, we won't be able to maintain 60fps.
My question is, for such small updates to a nested component's props, is there a more efficient and canonical way to update the content? Can I still use Redux?
I got a solution by replacing every tags with an observable inside reducer. Something like
// inside reducer when handling UPDATE_TAG action
// repos[0].tags of state is already replaced with a Rx.BehaviorSubject
get('repos[0].tags', state).onNext([{
id: 213,
text: 'Node.js'
}]);
Then I subscribe to their values inside Repo component using https://github.com/jayphelps/react-observable-subscribe. This worked great. Every dispatch only costs 5ms even with development build of React.js. But I feel like this is an anti-pattern in Redux.
Update 1
I followed the recommendation in Dan Abramov's answer and normalized my state and updated connect components
The new state shape is:
{
repoIds: ['1', '2', '3', ...],
reposById: {
'1': {...},
'2': {...}
}
}
I added console.time around ReactDOM.render to time the initial rendering.
However, the performance is worse than before (both initial rendering and updating). (Source: https://github.com/d6u/example-redux-update-nested-props/blob/master/repo-connect/index.js, Live demo: http://d6u.github.io/example-redux-update-nested-props/repo-connect.html)
// With dev build
INITIAL: 520.208ms
DISPATCH: 40.782ms
// With prod build
INITIAL: 138.872ms
DISPATCH: 23.054ms
I think connect on every <Repo/> has lots of overhead.
Update 2
Based on Dan's updated answer, we have to return connect's mapStateToProps arguments return an function instead. You can check out Dan's answer. I also updated the demos.
Below, the performance is much better on my computer. And just for fun, I also added the side effect in reducer approach I talked (source, demo) (seriously don't use it, it's for experiment only).
// in prod build (not average, very small sample)
// one connect at root
INITIAL: 83.789ms
DISPATCH: 17.332ms
// connect at every <Repo/>
INITIAL: 126.557ms
DISPATCH: 22.573ms
// connect at every <Repo/> with memorization
INITIAL: 125.115ms
DISPATCH: 9.784ms
// observables + side effect in reducers (don't use!)
INITIAL: 163.923ms
DISPATCH: 4.383ms
Update 3
Just added react-virtualized example based on "connect at every with memorization"
INITIAL: 31.878ms
DISPATCH: 4.549ms

I’m not sure where const App = connect((state) => state)(RepoList) comes from.
The corresponding example in React Redux docs has a notice:
Don’t do this! It kills any performance optimizations because TodoApp will rerender after every action.
It’s better to have more granular connect() on several components in your view hierarchy that each only
listen to a relevant slice of the state.
We don’t suggest using this pattern. Rather, each connect <Repo> specifically so it reads its own data in its mapStateToProps. The “tree-view” example shows how to do it.
If you make the state shape more normalized (right now it’s all nested), you can separate repoIds from reposById, and then only have your RepoList re-render if repoIds change. This way changes to individual repos won’t affect the list itself, and only the corresponding Repo will get re-rendered. This pull request might give you an idea of how that could work. The “real-world” example shows how you can write reducers that deal with normalized data.
Note that in order to really benefit from the performance offered by normalizing the tree you need to do exactly like this pull request does and pass a mapStateToProps() factory to connect():
const makeMapStateToProps = (initialState, initialOwnProps) => {
const { id } = initialOwnProps
const mapStateToProps = (state) => {
const { todos } = state
const todo = todos.byId[id]
return {
todo
}
}
return mapStateToProps
}
export default connect(
makeMapStateToProps
)(TodoItem)
The reason this is important is because we know IDs never change. Using ownProps comes with a performance penalty: the inner props have to be recalculate any time the outer props change. However using initialOwnProps does not incur this penalty because it is only used once.
A fast version of your example would look like this:
import React from 'react';
import ReactDOM from 'react-dom';
import {createStore} from 'redux';
import {Provider, connect} from 'react-redux';
import set from 'lodash/fp/set';
import pipe from 'lodash/fp/pipe';
import groupBy from 'lodash/fp/groupBy';
import mapValues from 'lodash/fp/mapValues';
const UPDATE_TAG = 'UPDATE_TAG';
const reposById = pipe(
groupBy('id'),
mapValues(repos => repos[0])
)(require('json!../repos.json'));
const repoIds = Object.keys(reposById);
const store = createStore((state = {repoIds, reposById}, action) => {
switch (action.type) {
case UPDATE_TAG:
return set('reposById.1.tags[0]', {id: 213, text: 'Node.js'}, state);
default:
return state;
}
});
const Repo = ({repo}) => {
const [authorName, repoName] = repo.full_name.split('/');
return (
<li className="repo-item">
<div className="repo-full-name">
<span className="repo-name">{repoName}</span>
<span className="repo-author-name"> / {authorName}</span>
</div>
<ol className="repo-tags">
{repo.tags.map((tag) => <li className="repo-tag-item" key={tag.id}>{tag.text}</li>)}
</ol>
<div className="repo-desc">{repo.description}</div>
</li>
);
}
const ConnectedRepo = connect(
(initialState, initialOwnProps) => (state) => ({
repo: state.reposById[initialOwnProps.repoId]
})
)(Repo);
const RepoList = ({repoIds}) => {
return <ol className="repos">{repoIds.map((id) => <ConnectedRepo repoId={id} key={id}/>)}</ol>;
};
const App = connect(
(state) => ({repoIds: state.repoIds})
)(RepoList);
console.time('INITIAL');
ReactDOM.render(
<Provider store={store}>
<App/>
</Provider>,
document.getElementById('app')
);
console.timeEnd('INITIAL');
setTimeout(() => {
console.time('DISPATCH');
store.dispatch({
type: UPDATE_TAG
});
console.timeEnd('DISPATCH');
}, 1000);
Note that I changed connect() in ConnectedRepo to use a factory with initialOwnProps rather than ownProps. This lets React Redux skip all the prop re-evaluation.
I also removed the unnecessary shouldComponentUpdate() on the <Repo> because React Redux takes care of implementing it in connect().
This approach beats both previous approaches in my testing:
one-connect.js: 43.272ms
repo-connect.js before changes: 61.781ms
repo-connect.js after changes: 19.954ms
Finally, if you need to display such a ton of data, it can’t fit in the screen anyway. In this case a better solution is to use a virtualized table so you can render thousands of rows without the performance overhead of actually displaying them.
I got a solution by replacing every tags with an observable inside reducer.
If it has side effects, it’s not a Redux reducer. It may work, but I suggest to put code like this outside Redux to avoid confusion. Redux reducers must be pure functions, and they may not call onNext on subjects.