Should mapStateToProps be called every time an action is dispatched?

[dara76]

I was under the impression that mapStateToProps is called every time an action is dispatched which causes the store to change.

In my simplified example below, this doesn't seem to be the case.

reducers.js

const foo = (state = true, action)  =>{
  switch (action.type) {
  case "foo" :
    return false;
  default :
    return state;
  }
}

const bar = (state = 0, action) => {
  switch (action.type) {
  case "bar" :
    return state + 1;
  default :
    return state;
  }
}


const rootReducer = combineReducers({
  foo,
  bar 
});

Container

import React, { Component } from 'react';
import { connect } from 'react-redux';

class Container extends Component {
  constructor(props) {
    super(props);
    this.onBarClick = this.onBarClick.bind(this);
  }

  onBarClick () {
    this.props.dispatch({
      type : "bar"
    }); 

    // I expect mapStateToProps to be called here 
    // which would allow me to check the value
    // of the updated props, and dispatch foo if the
    // condition is met

    if (this.props.isBarGreaterThanEqual3) {
      this.props.dispatch({
        type : "foo"
      }); 
    }   
  }

  render () {
    return (
      <div>
        <button onClick={this.onBarClick}>Click bar</button>
      </div>
    )   
  }
}

const mapStateToProps = (state) => {
  console.log("map state to props");
  return Object.assign({}, state, {
    isBarGreaterThanEqual3 : state.bar >= 3
  });
};

export default connect(mapStateToProps)(Container);

If it is wrong of me to expect mapStateToProps to be called after every dispatch, what is the recommended way to conditionally dispatch an action when the condition depends on the state of the store that has just been updated by another action?

[gaearon]

It should be called. Please provide a runnable project reproducing this (on Github or something like http://jsbin.com). I’m confident we don’t have bugs related to this because the library is covered by tests and used in a ton of projects. So something is off with the way you set it up but it’s hard for me to say without running the code and reproducing the issue.

[dara76]

I've put an example up at https://github.com/dara76/react-redux-issue.git

The app can be run with npm run dev-server which will make the app accessible at http://localhost:9999.

(I've changed the foo reducer to now to have initial state=true)

const foo = (state = true, action)  =>{
  switch (action.type) {
  case "foo" :
    return false;
  default :
    return state;
  }
}

In the app, I expect the "Waiting for foo" message to change after 3 clicks.

[gaearon]

I think I (probably) see your point now. I haven’t run the app yet but from the source it looks like you expect this.props.isBarGreaterThanEqual3 to update instantaneously after you dispatch in the same event handler.

This is not how React works. In React, state changes (and Reacf Redux uses setState internally) are potentially asynchronous. This is because React batches update that happen during the same event handler. So calling dispatch will update the store state immediately but your components will be updated a bit later during the same tick, together.

Instead of assuming dispatch updates props synchronously, you can use componentWillReceiveProps to react to changes in the props when they happen.

  onBarClick() {
    this.props.dispatch({
      type : "bar"
    });
  }

  componentWillReceiveProps(nextProps) {
    if (
      !this.props.isBarGreaterThanEqual3 &&
      nextProps.isBarGreaterThanEqual3
    ) {
      this.props.dispatch({
        type : "foo"
      });
    }
  }

Note that also, in general, dispatching actions in response to store-dependant prop changes is an anti-pattern. If you want to react to an action, it is best to do so in reducer. If you want to calculate some state that depends on the store state, it is best to do this in a selector.

[dara76]

Thanks for looking at this.

Yeah, I was expecting the following flow

start handling click event->dispatch bar action->state is updated->mapStateToProps called->render called->check if this.props.isBarGreaterThanEqual3->dispatch foo action if it is->state is updated->mapStateToProps called->render called

but you explain that

React batches updates that happen during the same event handler.

which means the flow is actually more like

start handling click event->dispatch bar action->state is updated->check if this.props.isBarGreaterThanEqual3->dispatch foo action if it is->state is updated->mapStateToProps called->render called

So is it fair to say that mapStateToProps isn't necessarily called after every dispatch, but it is only called once per batch of dispatched actions?

I like the simplicity of this advice

If you want to react to an action, it is best to do so in reducer.
If you want to calculate some state that depends on the store state, it is best to do this in a selector.

Having said that, it's not clear to me how I would update foo via a reducer when bar reaches 3?

const bar = (state = 0, action) => {
  switch (action.type) {
  case "bar" :
    const newState = state + 1;

    if (newState >= 3) {
      // set foo somehow
    }   

    return newState;
  default :
    return state;
  }
};

const foo = (state = false, action)  =>{ 
  switch (action.type) {
  case "foo" :
    return true;
  default :
    return state;
  }
};

[gaearon]

So is it fair to say that mapStateToProps isn't necessarily called after every dispatch, but it is only called once per batch of dispatched actions?

It is fair to say mapStateToProps is called after but not necessarily immediately after the dispatch. It is called when the component is about to re-render, which depends on whether React batches the updates or not. By default, React batches updates from event handlers.

Having said that, it's not clear to me how I would update foo via a reducer when bar reaches 3?

Since one depends on the state of the other, maybe they should just be a single reducer.

const reducer = (state = { counter: 0, foo: false }, action) {
  switch (action.type) {
  case "bar":
    return Object.assign({}, state, {
      counter: state.counter + 1,
      foo: state.counter === 2
       // depending on your use case, it could also be:
       // foo: !state.foo && state.counter === 2 
       // foo: !state.hasEverBeenFooBefore && state.counter === 2 
       // etc
    });
  case "foo" :
    return Object.assign({}, state, {
      foo: true
    });
  default:
    return state
  }
}

Another, simpler, option is to keep them separate, and compute the result as derived data:

const bar = (state = 0, action) => {
  switch (action.type) {
  case "bar" :
    return state + 1;
  default :
    return state;
  }
};

const foo = (state = false, action)  => { 
  switch (action.type) {
  case "foo" :
    return true;
  default :
    return state;
  }
};

const reducer = combineReducers({
  foo,
  bar
});

const isFooishEnough = (state) => {
  return state.foo || state.bar >= 3;
}

In this case, we defined a selector called isFooishEnough. You can call it from your mapStateToProps and use that value instead. In general, this is the preferred pattern: whatever can be computed from Redux store state, should not be there.

[dara76]

Thanks for the explanations and insights, they're very helpful.

[harry-dev98]

very similar doubt, i guess which got clear now reading comments. Please someone correct me if i am wrong. So basically if not using redux, and using some event handler function i call more than one setState (class or function with hook component), then react collects all the batch of changes at the very end of the event handler and then re-renders the component.

[markerikson]

@harry-dev98 Two things:

• Please don't wake up threads that have been closed for multiple years
• See my extensive post A (Mostly) Complete Guide to React Rendering Behavior for all the details on how React batching and timing works.