presentational-and-container

React in patterns / Presentational and container components

• Source code

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When we start using React we very soon also start asking questions. Where I'm supposed to put my data, how to communicate changes or how to manage state? The answers of these questions are very often a matter of context and sometimes just practice and experience with the library. However, there is a pattern which is used widely and helps organizing React based applications - splitting the components into presentational and containers.

Let's start with a simple example, illustrate the problem and then split the component into container and presentation. We will use a Clock component. It accepts a Date object as a prop and displays the time which is changing in real time.

class Clock extends React.Component {
  constructor(props) {
    super(props);
    this.state = { time: this.props.time };
    this._update = this._updateTime.bind(this);
  }
  render() {
    var time = this._formatTime(this.state.time);
    return (
      <h1>{ time.hours } : { time.minutes } : { time.seconds }</h1>
    );
  }
  componentDidMount() {
    this._interval = setInterval(this._update, 1000);
  }
  componentWillUnmount() {
    clearInterval(this._interval);
  }
  _formatTime(time) {
    var [ hours, minutes, seconds ] = [
      time.getHours(),
      time.getMinutes(),
      time.getSeconds()
    ].map(num => num < 10 ? '0' + num : num);

    return { hours, minutes, seconds };
  }
  _updateTime() {
    this.setState({ time: new Date(this.state.time.getTime() + 1000) });
  }
};

ReactDOM.render(<Clock time={ new Date() }/>, ...);

In the constructor of the component we store the passed time object to the internal state. By using setInterval we update the state every second and the component is rerendered. To make it looks like a real clock we use two helper methods - _formatTime and _updateTime. The first one is all about extracting hours, minutes and seconds and making sure that they are following the two-digits format. _updateTime is mutating the current time object by one second.

The problems

There are couple of things happening in our component. It looks like it has a little bit too many responsibilities.

• It mutates the state by itself. Changing the time inside the component may not be a good idea because then only Clock knows the current value. If there is another part of the system that depends on this data it will be difficult to share it.
• _formatTime is actually doing two things - it extracts the needed information from the date object and makes sure that the values are always with two digits. That's fine but it will be nice if the extracting is not part of this component because then Clock is bound to the type of the time object (coming as a prop). I.e. knows specifics about the shape of the data.

Solution

So, let's split the component into two parts - container and presentation.

Container

Containers know about data, it's shape and where it comes from. They know details about how the things work or the so called business logic. They receive information and format it so it is easy to use by the presentational component. Very often we use higher-order components to create containers. Their render method contains only the presentational component. In the context of the flux architecture that's the bit which is bound to the stores' changes and calls action creators.

Here's how our ClockContainer looks like:

// Clock/index.js
import Clock from './Clock.jsx'; // <-- that's the presentational component

export default class ClockContainer extends React.Component {
  constructor(props) {
    super(props);
    this.state = { time: props.time };
    this._update = this._updateTime.bind(this);
  }
  render() {
    return <Clock { ...this._extract(this.state.time) }/>;
  }
  componentDidMount() {
    this._interval = setInterval(this._update, 1000);
  }
  componentWillUnmount() {
    clearInterval(this._interval);
  }
  _extract(time) {
    return {
      hours: time.getHours(),
      minutes: time.getMinutes(),
      seconds: time.getSeconds()
    };
  }
  _updateTime() {
    this.setState({ time: new Date(this.state.time.getTime() + 1000) });
  }
};

It still accepts time (a date object), does the setInterval loop and knows details about the data (getHours, getMinutes and getSeconds). In the end renders the presentational component and passes three numbers for hours, minutes and seconds.

Presentational component

Presentational components are concert with how the things look. They have the additional markup needed for making the page pretty. Such components are not bound to anything and have no dependencies. Very often implemented as a stateless functional components they don't have internal state.

In our case the presentational component contains only the two-digits check and returns the <h1> tag:

// Clock/Clock.jsx
export default function Clock(props) {
  var [ hours, minutes, seconds ] = [
    props.hours,
    props.minutes,
    props.seconds
  ].map(num => num < 10 ? '0' + num : num);

  return <h1>{ hours } : { minutes } : { seconds }</h1>;
};

Benefits

Splitting the components in containers and presentation increases the reusability of the components. Our Clock function/component may exist in application that doesn't change the time or it's not working with JavaScript's Date objects. That's because it's pretty dummy. No details about the data, it's initial shape and where it comes from.

The nice things about containers is that they encapsulate logic and may inject data into different renderers. Very often a file that exports a container is not sending out a class directly but a function. For example, instead of using

import Clock from './Clock.jsx';

export default class ClockContainer extends React.Component {
  render() {
    return <Clock />;
  }
}

we may export a function that accepts the presentational component:

export default function(Component) {
  return class Container extends React.Component {
    render() {
      return <Component />;
    }
  }
}

Using this technique our container is really flexible in rendering its result. It will be really helpful if we want to switch from digital to analog clock representation.

Even the testing becomes easier because we have to think less for our components. Containers are not concern with UI stuff and very often the actions that trigger logic are controlled with callbacks. Presentational components are just about rendering the incoming props and their unit tests are more or less checking if the right callback is called if something is clicked/filled in.

Other resources

• Presentational and Container Components by Dan Abramov
• Container Components by "Learn React with chantastic"

A side note

Nothing is set in stone. Presentational components do have internal state sometimes. Containers may have additional markup. The concept described here has no strict rules and what exactly to put where depends on the concrete context.