Mapping Dispatch to Props

Objectives

Write functions that connect Redux actions to component events

Introduction

In the last lessons, we learned that mapStateToProps() separates concerns. We no longer have to reference the store inside our component when retrieving the state. We are moving towards having state management in one part of our code, and display logic in a different part.

In other words, we're moving knowledge of Redux outside our components.

What prevented us from fully removing a reference to Redux inside our components was that we did not know how to dispatch actions without calling store.dispatch() from our component. Well, in this lesson we'll learn how to do just that. We'll remove knowledge of the store from our components by using a function similar to mapStateToProps(), which is called mapDispatchToProps().

Identifying the Problem

To begin, take a look at the starting code provided in src/App.js:

// ./src/App.js
import React, { Component } from 'react';
import './App.css';
import { connect } from 'react-redux';
import { addItem } from  './actions/items';

class App extends Component {

  handleOnClick() {
    this.props.store.dispatch(addItem());
  }

  render() {
    return (
      <div className="App">
        <button onClick={(event) => this.handleOnClick(event)}>
          Click
          </button>
        <p>{this.props.items.length}</p>
      </div>
    );
  }
};

const mapStateToProps = (state) => {
  return {
    items: state.items
  };
};

export default connect(mapStateToProps)(App);

We can see that mapStateToProps() is already implemented and is making state.items available to App as this.props.items. We also see that the button in render() calls handleOnClick() when clicked. The handleOnClick() does one thing - it dispatches an action to the store.

In the earlier mapStateToProps() Readme, we changed our code such that we no longer reference the store to get an updated state of the items, but here we still reference the store in handleOnClick() to dispatch an action:

// ./src/app.js
...

handleOnClick(){
  this.props.store.dispatch(addItem())
}

...

Okay, so this may seem small, but it introduces our old problem. Our component is no longer indifferent about its state management system. Instead, this line of code makes the component reliant on Redux.

Well we can fix this problem with our connect() function. Just like we can write code like connect(mapStateToProps)(App) to add new props to our App component, we can pass connect() a second argument, and add our action creator as props. Then we can reference this action creator as a prop to call it from our component. We'll spend the rest of this lesson unpacking the previous sentence. Okay, let's see how this works.

Using `mapDispatchToProps`

To quickly review: The first argument passed into connect() is a function. That function is written to accept the Redux store's state as an argument and returns an object created using all or some of that state. Key/value pairs in this returned object will become values we can access in the component we've wrapped with connect(). The below example, for instance, would make the entire state available as a prop:

const mapStateToProps = state => {
  return state
}

We call this function mapStateToProps because that is what it does. This function is passed in as the first argument to connect(). When connect() executes, it calls the function passed in as its first argument, passing in the current state to the function.

Just like the first argument, connect() accepts a function for the second argument. This time, again, when connect() executes, it calls the second function passed in. However, instead of passing state in, it passes in the dispatch function. This means we can write a function assuming we have access to dispatch(). We call it mapDispatchToProps because that is what it does. Updating our ./src/App.js file, it looks like the following:

// src/App.js

import React, { Component } from 'react';
import './App.css';
import { connect } from 'react-redux';
import { addItem } from  './actions/items';

class App extends Component {

  handleOnClick = event => {
    this.props.addItem() // Code change: this.props.dispatch.store is no longer being called
  }

  render() {
    return (
      <div className="App">
        <button onClick={this.handleOnClick}>
          Click
          </button>
        <p>{this.props.items.length}</p>
      </div>
    );
  }
};

const mapStateToProps = (state) => {
  return {
    items: state.items
  };
};

// Code change: this new function takes in dispatch as an argument
// It then returns an object that contains a function as a value!
// Notice above in handleOnClick() that this function, addItem(),
// is what is called, NOT the addItem action creator itself.
const mapDispatchToProps = dispatch => {
  return {
    addItem: () => {
      dispatch(addItem())
    }
  };
};

export default connect(mapStateToProps, mapDispatchToProps)(App);

Okay, so let's see what adding our mapDispatchToProps() function, and passing it through as a second argument accomplished. We'll place in another debugger in our component, right at the beginning of render(), just before the return statement.

// src/App.js
...
render() {
    debugger
    return (
      <div className="App">
        <button onClick={this.handleOnClick}>
          Click
          </button>
        <p>{this.props.items.length}</p>
      </div>
    );
  }
...

Now, boot up the app, open up your console and when you hit the debugger statement, type in this.props.addItem. You'll see that it returns a function with dispatch inside. So, just like with mapStateToProps(), we added a prop that pointed to a value, here we add a prop addItem that points to the value, a function. The dispatch function is available as an argument to mapDispatchToProps. By defining the function addItem inside mapDispatchToProps, we're able to include dispatch in the definition; we've bundled everything we need into a single prop value.

With dispatch integrated into this.props.addItem, we can change our code such that when the handleOnClick() function gets called, we execute our action creator by referencing it as a prop:

// ./src/App.js

...

handleOnClick = event => {
  this.props.addItem()
}

...

This code calls the handleOnClick() function after the button is clicked. The handleOnClick() references and then executes the addItem() function by calling this.props.addItem().

Alternative Method

There is an even simpler way to approach bundling our actions and dispatch into props. The second argument of connect will accept a function (as we've seen) or an object. If we pass in a function, mapDispatchToProps(), we must incorporate dispatch as with the previous example. If we pass in an object, connect handles this step for us! The object just needs to contain key/value pairs for each action creator we want to become props. In our example, we've using the addItem action creator, so the object would look like this:

{
  addItem: addItem
}

As of JavaScript ES6, when we have an object with a key and value with the same name, we can use the shorthand syntax and write:

{
  addItem
}

This is all we need to pass in as a second argument for connect().

App then changes to look like the following:

import React, { Component } from 'react';
import './App.css';
import { connect } from 'react-redux';
import { addItem } from  './actions/items';

class App extends Component {

  handleOnClick = event => {
    this.props.addItem()
  }

  render() {
    debugger
    return (
      <div className="App">
        <button onClick={this.handleOnClick}>
          Click
          </button>
        <p>{this.props.items.length}</p>
      </div>
    );
  }
};

const mapStateToProps = (state) => {
  return {
    items: state.items
  };
};

export default connect(mapStateToProps, { addItem })(App); // Code change: no mapDispatchToProps function required!

Aside: We could go further and get rid of mapStateToProps() as well. We still need to pass in a function as the first argument, but it can be an anonymous arrow function that handles everything in one line:

export default connect(state => ({ items: state.items }), { addItem })(App);

This is equivalent to writing:

const mapStateToProps = state => {
  return {
    items: state.items
  }
}

const mapDispatchToProps = dispatch => {
  return {
    addItem: () => { dispatch(addItem()) }
  }
}

export default connect(mapStateToProps, mapDispatchToProps)(App);

Default Dispatch Behavior

In addition to this, as per Dan Abramov, the creator of Redux:

By default mapDispatchToProps is just dispatch => ({ dispatch }). So if you don't specify the second argument to connect(), you'll get dispatch injected as a prop in your component.

This means that if we were to simply write:

export default connect(state => ({ items: state.items }))(App);

...we would still have this.props.dispatch() available to us in App. If you would rather write this.props.dispatch({ type: 'INCREASE_COUNT' }) in App, or pass dispatch down to children, you can!

Resources

Dan Abramov Stack Overflow Response about mapDispatchToProps

Summary

In this lesson, we saw that we can remove all references to our store from our component via the mapDispatchToProps() function. We saw that mapDispatchToProps() allows us to bring in actions, combine them with dispatch and connect events on our page to actions in our store.

Trejon/map-dispatch-to-props-readme-online-web-pt-061019