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What is mapDispatchToProps?
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Closed 4 years ago.
What's the difference between mapStateToProps and mapDispatchToProps arguments to the connect function in react-redux?
mapStateToProps is a function that you would use to provide the store data to your component, whereas mapDispatchToProps is something that you will use to provide the action creators as props to your component.
According to the docs:
If mapStateToProps argument is specified, the new component will
subscribe to Redux store updates. This means that any time the store
is updated, mapStateToProps will be called. The results of
mapStateToProps must be a plain object, which will be merged into
the component’s props.
With mapDispatchToProps every action creator wrapped into a dispatch
call so they may be invoked directly, will be merged into the
component’s props.
A simple example would be
function mapStateToProps(state) {
return { todos: state.todos }
}
function mapDispatchToProps(dispatch) {
return { addTodo: bindActionCreators(addTodo, dispatch) }
}
export default connect(mapStateToProps, mapDispatchToProps)(Todos);
In a very simple term,
mapStateToProps: It connects redux state to props of react component.
mapDispatchToProps: It connects redux actions to react props.
A really light example: ( I hope, you know what I mean)
// state
const mapStateToProps = state => {
return { lists: state.lists };
};
// props
const mapDispatchToProps = ({ lists }) => (
<ul>
{
lists.map(el => (
<li key={ el.id }>
{ el.heading }
</li>
)
}
</ul>
);
// Now, connect state to prop
const List = connect(mapStateToProps)(mapDispatchToProps);
In very simple terms:
mapStateToProps is called when you want to get the value of the global state from your component
function mapStateToProps(state) {
return {
message: state.message
};
}
The value of the global state is only changed with the help of an action. So if you want to change the value of global state you need an action. mapDispatchToProps is used to bind action in your component.
Related
I've an application when most of the data are stored in the store but the selected item is provided thought the usage of a React.Context.
React-Redux provide the connect api that accept a mapStateToProps function with state and props as a component.
What I would like, if it didn't break the hooks, is something like:
function mapStateToProps(state){
const selectedItemId = useContext(MySelectedItemContext)
return {
item: state.items[selectedItemId]
}
}
but of course it is not possible since I'm outside of the component and cannot invoke the useContext.
So, I tried to use the old API context:
function mapStateToProps(state, props){
return {
item: state.items[props.id]
}
}
export default connect(mapStateToProps)((props) =>
<MySelectedItemContext.Consumer>
{ selectedItemId => <Component id={selectedItemId} {...props}/> }
</MySelectedItemContext.Consumer>)
but this still not works because the connect returns a new component that has the consumer inside instead of outside and id prop is not defined yet in mapStateToProps.
Any idea?
The best way is to remove mapStateToProps and use useSelector hooks and Redux selectors. But if you need mapStateToProps, then you can wrap your component that must be connected to Redux into another component that will get value from context and will pass it to a component that uses Redux.
// Use this component
export function ContextConsumerComponent() {
const selectedItemId = useContext(SelectedItemIdContext);
return <ReduxConsumerComponent id={selectedItemId} />;
}
function mapStateToProps(state, props) {
return {
item: state.items[props.id]
}
}
const ReduxConsumerComponent = connect(mapStateToProps)((props) => {
// props.item will be here
});
i have below snippet.
AnotherFile.js
import MyAccount from './MyAccount';
<MyAccount name='peace and love' />
MyAccount.js
const MyAccount = (props) => {
// I can access props.isLoggedIn and props.setLogout,
// and I can access props.name too, wow
}
const mapStateToProps = (state) => ({
isLoggedIn: state.isLoggedIn
})
const mapDispatchToProps = dispatch => ({
setLogout: () => dispatch(setLogout())
})
export default connect(mapStateToProps, mapDispatchToProps)(MyAccount)
So I pass some props to the connected components, and the presentational component MyAccount can access these props too.
Previously, if I intend to achieve that, I would add a second argument in mapStateToProps, like below. But now it seems it is unnecessary to pass ownProps? Can someone confirm please?
const mapStateToProps = (state, ownProps) => ({
isLoggedIn: state.isLoggedIn,
...ownProps
})
From the react-redux docs:
ownProps (optional)
[...]
You do not need to include values from ownProps in the object returned from mapStateToProps. connect will automatically merge those different prop sources into a final set of props.
So yes, it is unnecessary to explicitly spread ownProps into the props returned by mapStateToProps.
You usually only need ownProps if your component needs the data from its own props to retrieve data from the store, e.g. using an id prop to select a certain item from a list of items.
I'm looking at the async redux example here:
https://github.com/reactjs/redux/blob/master/examples/async/src/containers/App.js
If I add the following:
function mapDispatchToProps(dispatch) {
return {};
}
export default connect(mapStateToProps,mapDispatchToProps)(App)
Then dispatch is no longer defined in componentDidMount. Why is this? How can I access dispatch here while using mapDispatchToProps?
The connect function automatically passes dispatch to your props if you do not specify a mapDispatchToProps function. With that in mind:
export default connect(mapStateToProps)(App)
And it should work. You can read more about the react-redux bindings in the docs.
If I understand you correctly, you want to call dispatch from the componentDidMount as a function in the this.props (which I don't recommend, check how redux works and create the action creators that you need). Even so, to achieve that you can do that:
function mapDispatchToProps(dispatch) {
return { dispatch };
}
That creates a dispatch property that is the dispatch function itself.
Still, don't do that on a real use case and keep reading about redux action.creators.
Following the given example, you pass mapDispatchToProps to connect like this
function mapDispatchToProps(dispatch) {
return {
onClick: nextReddit => {
dispatch(selectReddit(nextReddit));
}
};
}
export default connect(mapStateToProps, mapDispatchToProps)(App)
Component App will then be empowered with a prop onClick which is used as below.
handleChange = nextReddit => {
this.props.onClick(nextReddit)
}
render() {
...
return (
...
<Picker value={selectedReddit}
onChange={this.handleChange}
options={[ 'reactjs', 'frontend' ]} />
...
);
}
I was reading the documentation for the Redux library and it has this example:
In addition to reading the state, container components can dispatch actions. In a similar fashion, you can define a function called mapDispatchToProps() that receives the dispatch() method and returns callback props that you want to inject into the presentational component.
This actually makes no sense. Why do you need mapDispatchToProps when you already have mapStateToProps?
They also provide this handy code sample:
const mapDispatchToProps = (dispatch) => {
return {
onTodoClick: (id) => {
dispatch(toggleTodo(id))
}
}
}
What is this function and why it is useful?
I feel like none of the answers have crystallized why mapDispatchToProps is useful.
This can really only be answered in the context of the container-component pattern, which I found best understood by first reading:Container Components then Usage with React.
In a nutshell, your components are supposed to be concerned only with displaying stuff. The only place they are supposed to get information from is their props.
Separated from "displaying stuff" (components) is:
how you get the stuff to display,
and how you handle events.
That is what containers are for.
Therefore, a "well designed" component in the pattern look like this:
class FancyAlerter extends Component {
sendAlert = () => {
this.props.sendTheAlert()
}
render() {
<div>
<h1>Today's Fancy Alert is {this.props.fancyInfo}</h1>
<Button onClick={sendAlert}/>
</div>
}
}
See how this component gets the info it displays from props (which came from the redux store via mapStateToProps) and it also gets its action function from its props: sendTheAlert().
That's where mapDispatchToProps comes in: in the corresponding container
// FancyButtonContainer.js
function mapDispatchToProps(dispatch) {
return({
sendTheAlert: () => {dispatch(ALERT_ACTION)}
})
}
function mapStateToProps(state) {
return({fancyInfo: "Fancy this:" + state.currentFunnyString})
}
export const FancyButtonContainer = connect(
mapStateToProps, mapDispatchToProps)(
FancyAlerter
)
I wonder if you can see, now that it's the container 1 that knows about redux and dispatch and store and state and ... stuff.
The component in the pattern, FancyAlerter, which does the rendering doesn't need to know about any of that stuff: it gets its method to call at onClick of the button, via its props.
And ... mapDispatchToProps was the useful means that redux provides to let the container easily pass that function into the wrapped component on its props.
All this looks very like the todo example in docs, and another answer here, but I have tried to cast it in the light of the pattern to emphasize why.
(Note: you can't use mapStateToProps for the same purpose as mapDispatchToProps for the basic reason that you don't have access to dispatch inside mapStateToProp. So you couldn't use mapStateToProps to give the wrapped component a method that uses dispatch.
I don't know why they chose to break it into two mapping functions - it might have been tidier to have mapToProps(state, dispatch, props) IE one function to do both!
1 Note that I deliberately explicitly named the container FancyButtonContainer, to highlight that it is a "thing" - the identity (and hence existence!) of the container as "a thing" is sometimes lost in the shorthand
export default connect(...)
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
syntax that is shown in most examples
It's basically a shorthand. So instead of having to write:
this.props.dispatch(toggleTodo(id));
You would use mapDispatchToProps as shown in your example code, and then elsewhere write:
this.props.onTodoClick(id);
or more likely in this case, you'd have that as the event handler:
<MyComponent onClick={this.props.onTodoClick} />
There's a helpful video by Dan Abramov on this here:
Redux: Generating Containers with connect() from React Redux (VisibleTodoList)
mapStateToProps() is a utility which helps your component get updated state(which is updated by some other components),
mapDispatchToProps() is a utility which will help your component to fire an action event (dispatching action which may cause change of application state)
mapStateToProps, mapDispatchToProps and connect from react-redux library provides a convenient way to access your state and dispatch function of your store. So basically connect is a higher order component, you can also think as a wrapper if this make sense for you. So every time your state is changed mapStateToProps will be called with your new state and subsequently as you props update component will run render function to render your component in browser. mapDispatchToProps also stores key-values on the props of your component, usually they take a form of a function. In such way you can trigger state change from your component onClick, onChange events.
From docs:
const TodoListComponent = ({ todos, onTodoClick }) => (
<ul>
{todos.map(todo =>
<Todo
key={todo.id}
{...todo}
onClick={() => onTodoClick(todo.id)}
/>
)}
</ul>
)
const mapStateToProps = (state) => {
return {
todos: getVisibleTodos(state.todos, state.visibilityFilter)
}
}
const mapDispatchToProps = (dispatch) => {
return {
onTodoClick: (id) => {
dispatch(toggleTodo(id))
}
}
}
function toggleTodo(index) {
return { type: TOGGLE_TODO, index }
}
const TodoList = connect(
mapStateToProps,
mapDispatchToProps
)(TodoList)
Also make sure that you are familiar with React stateless functions and Higher-Order Components
Now suppose there is an action for redux as:
export function addTodo(text) {
return {
type: ADD_TODO,
text
}
}
When you do import it,
import {addTodo} from './actions';
class Greeting extends React.Component {
handleOnClick = () => {
this.props.onTodoClick(); // This prop acts as key to callback prop for mapDispatchToProps
}
render() {
return <button onClick={this.handleOnClick}>Hello Redux</button>;
}
}
const mapDispatchToProps = dispatch => {
return {
onTodoClick: () => { // handles onTodoClick prop's call here
dispatch(addTodo())
}
}
}
export default connect(
null,
mapDispatchToProps
)(Greeting);
As function name says mapDispatchToProps(), map dispatch action to props(our component's props)
So prop onTodoClick is a key to mapDispatchToProps function which delegates furthere to dispatch action addTodo.
Also if you want to trim the code and bypass manual implementation, then you can do this,
import {addTodo} from './actions';
class Greeting extends React.Component {
handleOnClick = () => {
this.props.addTodo();
}
render() {
return <button onClick={this.handleOnClick}>Hello Redux</button>;
}
}
export default connect(
null,
{addTodo}
)(Greeting);
Which exactly means
const mapDispatchToProps = dispatch => {
return {
addTodo: () => {
dispatch(addTodo())
}
}
}
mapStateToProps receives the state and props and allows you to extract props from the state to pass to the component.
mapDispatchToProps receives dispatch and props and is meant for you to bind action creators to dispatch so when you execute the resulting function the action gets dispatched.
I find this only saves you from having to do dispatch(actionCreator()) within your component thus making it a bit easier to read.
React redux: connect: Arguments
I have a react-redux container component that is created within a React Native Navigator component. I want to be able to pass the navigator as a prop to this container component so that after a button is pressed inside its presentational component, it can push an object onto the navigator stack.
I want to do this without needing to hand write all the boilerplate code that the react-redux container component gives me (and also not miss out on all the optimisations that react-redux would give me here too).
Example container component code:
const mapStateToProps = (state) => {
return {
prop1: state.prop1,
prop2: state.prop2
}
}
const mapDispatchToProps = (dispatch) => {
return {
onSearchPressed: (e) => {
dispatch(submitSearch(navigator)) // This is where I want to use the injected navigator
}
}
}
const SearchViewContainer = connect(
mapStateToProps,
mapDispatchToProps
)(SearchView)
export default SearchViewContainer
And I'd want to be able to call the component like this from within my navigator renderScene function:
<SearchViewContainer navigator={navigator}/>
In the container code above, I'd need to be able to access this passed prop from within the mapDispatchToProps function.
I don't fancy storing the navigator on the redux state object and don't want to pass the prop down to the presentational component.
Is there a way I can pass in a prop to this container component? Alternatively, are there any alternative approaches that I'm overlooking?
Thanks.
mapStateToProps and mapDispatchToProps both take ownProps as the second argument.
[mapStateToProps(state, [ownProps]): stateProps] (Function):
[mapDispatchToProps(dispatch, [ownProps]): dispatchProps] (Object or Function):
For reference
You can pass in a second argument to mapStateToProps(state, ownProps) which will give you access to the props passed into the component in mapStateToProps
There's a few gotchas when doing this with typescript, so here's an example.
One gotcha was when you are only using dispatchToProps (and not mapping any state props), it's important to not omit the state param, (it can be named with an underscore prefix).
Another gotcha was that the ownProps param had to be typed using an interface containing only the passed props - this can be achieved by splitting your props interface into two interfaces, e.g.
interface MyComponentOwnProps {
value: number;
}
interface MyComponentConnectedProps {
someAction: (x: number) => void;
}
export class MyComponent extends React.Component<
MyComponentOwnProps & MyComponentConnectedProps
> {
....// component logic
}
const mapStateToProps = (
_state: AppState,
ownProps: MyComponentOwnProps,
) => ({
value: ownProps.value,
});
const mapDispatchToProps = {
someAction,
};
export default connect(mapStateToProps, mapDispatchToProps)(MyComponent);
The component can be declared by passing the single parameter:
<MyComponent value={event} />
Using Decorators (#)
If you are using decorators, the code below give an example in the case you want to use decorators for your redux connect.
#connect(
(state, ownProps) => {
return {
Foo: ownProps.Foo,
}
}
)
export default class Bar extends React.Component {
If you now check this.props.Foo you will see the prop that was added from where the Bar component was used.
<Bar Foo={'Baz'} />
In this case this.props.Foo will be the string 'Baz'
Hope this clarifies some things.