Lesson2.md

React Lesson 2

In this lesson we will be learning to use React components more effectively with deeper understanding of Props and State.

A React app is basically just a pile of big components (parents) that call smaller (child) components.

Props

React components are best understood in terms of parents and children.

Parent or container components may have access to the data that they need in order to pass on to multiple child components. These larger components often contain the actions (e.g. API calls) or data processing functions that their child components require and thus are referred to as smart components.

Example:

import React, { Component } from 'react';
import Organisation from './Organisation';

class App extends Component {
  render() {
    return (
      <div className="App">
        <Organisation
            name="Refugee Support Network"
            website="http://www.refugeesupportnetwork.org"
        />
      </div>
    );
  }
}

export default App;

Child or presentational components are essentially smaller customisable to the data that they receive but usually do not have the data in themselves. These are often referred to as dumb components - because they do not process the data in themselves:

import React, { Component } from 'react';

class Organisation extends Component {
  render() {
    const name = this.props.name;
    const website = this.props.website;
    return (
        <div>
            Name: {name} <br />
            Website: {website}
        </div>
    );
  }
}

export default Organisation;

• Props exercise

JSX: Map Arrays to Components

When working with APIs we often have collections of things in Arrays which we want to map to components. This means we can loop through that data and display the customisable dumb component for each one.

renderOrganisations() {
    const orgData = [{ name: 'Organisation 1', borough: 'Camden'}, {name: 'Organisation 2'}];
    return orgData.map(function renderData(organisation) {
        return (
            <Organisation
                name= {organisation.name}
                borough={organisation.borough ? organisation.borough : 'None'}
                website= {organisation.website}
            />
        )
    });
}
class App extends Component {
  render() {
      return (
        <div className="App">
          {this.renderOrganisations()}
        </div>
      );
  }
}

• Mapping Exercise

State

Sometimes a component needs to react to data that doesn't come from a parent component. This is where the component's State comes in.

Props are a Compenent's configuration, they are received from above and they don't usually change by the component itself. State starts with a default value (set in the constructor typically) then it gets changed (mutated) by events of the application (user interactions, form input, API calls etc..)

Example:

import React, { Component } from 'react';
import Organisation from './Organisation';

class App extends Component {
  constructor(props) {
    super(props);
    this.state = {
        website: null
    }
  }

  handleChange(event) {
    this.setState({website: event.target.value});
  }
  render() {
    return (
      <div className="App">
        <Organisation
            name="Refugee Support Network"
            website={this.state.website}
        />
        <input type="text" value={this.state.website} onChange={this.handleChange.bind(this)} />
      </div>
    );
  }
}

export default App;

• State Exercise

Component Life Cycle

Components have life cycle events which are methods that run code at particular times in the process. According to the time that the methods are called, some of them are often prefixed with will and did.

The diagram below shows the full picture of what happens during the rendering of a component. Even though you have all these methods available to use at your disposal, the only required method is render().

Mounting

These are called when a component is being mounted on the DOM such as:

• Constructor()
• componentWillMount()
• render()
• etc

Updating

When there are changes to the props or state:

• componentWillReceiveProps()
• componentDidUpdate()
• render()
• etc

Unmonting

When a component is being removed from the DOM:

• componentWillUnmount()

Other APIs

These include setState() and forceUpdate(). You cannot access the state in some lifecycle methods, refer to the diagram above to find out which.

The complete list of these methods with their descriptions can be found in the documentation here.

API calls

In order to make an API called you can call the functions same

callAPI = () => {
    const APIAddress = 'https://code-your-future.github.io/api-demo/area/All/index.json';
    fetch(APIAddress)
    .then(function(response) {
      return response.json();
    })
    .then(function(jsonData) {
      this.setState({ orgData: jsonData });
    });
}

In order to use this function when the component first loads we will call it in the componentDidMount() lifecycle method.

componentDidMount() {
    this.callAPI();
}

Once the state is updated with the data, your component will re-render.

Homework

• Lesson 2 Homework