Description

React applications are organized into components. This lesson will show how components can be built up out of smaller components and then reused in new contexts.



React Components

  • Isolated pieces of a website or app
  • Can be rendered manually or programatically
  • Dynamic data is passed in via Props
  • Generate child compoents with render()
class Profile extends React.Component {
  userData = () => {
    return data || fetchDataFromDatabase();
  }

  render() {
    <Header>
      <Profile
        name={userData.name}
        interests={userData.interests}
        description={userData.description}
      />
      <Activity />
      <Contact />
    </Header>
  }
}

Using Components

  • Typically the index.js initiates the components tree from the top level
  • Child components are rendered after their parents
  • Parents pass Props and State to children
<html>
  <body>
    <div id="root"/>
  </body>
  <script>
    ReactDom.render(<Profile />,
      document.getElementById('root')
    )
  </script>
</html>

Using Arrays of Components

  • Many components can be rendered at once
  • Wrap the components in an array
  • React will iterate over and render each
class ProfileList extends React.Component {
  render() {
    <Header>
      [
        <Profile userId="One"/>,
        <Profile userId="Two"/>,
        <Profile userId="Three"/>
      ]
    </Header>
  }
}

Building components with Iteration

  • Iteration and Loops can be used just like in plain JavaScript
  • Many components can be built based on collections of data
  • Components can be conditionally rendered
class Comment extends React.Component {
  render () {
    return (
      <li>
        <p>User: {this.props.user}</p>
        <p>Comment: {this.props.content}</p>
      </li>
    )
  }
}

class CommentList extends React.Component {
  render() {
    const comments = [
      { user: 'Joshua', content: 'Components are my fave!' },
      { user: 'Ada', content: 'Yes they make life easy' },
      { user: 'Alex', content: 'Loved them for years' }
    ];
    const commentComponents = forms.map((comment, index) => {
      return <Comment user={ comment.user } content={ comment.content }/>
    });

    return (
    <div className="comments">
      <h2>Comment List</h2>
      <ul>
      { commentComponents }
      </ul>
    </div>
    )
  }
}

Immutable Components

  • Once components are rendered they cannot be updated
  • Re-rendering the component is how to update
<html>
  <body>
    <div id="root"/>
  </body>
  <script>
    function tick() {
      const element = (
        <div>
          <h1>Hello, world!</h1>
          <h2>It is {new Date().toLocaleTimeString()}.</h2>
        </div>
      );
      // highlight-next-line
      ReactDOM.render(element, document.getElementById('root'));
    }

    setInterval(tick, 1000);
  </script>
</html>

Ticking Clock Live

See the Pen React Clock Example by Joshua Burke (@Dangeranger) on CodePen.

https://codepen.io/Dangeranger/pen/gjdNEB

Granular Render Updates

render performance

Component Lifecycle

Mounting

  • constructor()
  • static getDerivedStateFromProps()
  • render()
  • componentDidMount()

Updating

  • static getDerivedStateFromProps()
  • shouldComponentUpdate()
  • render()
  • getSnapshotBeforeUpdate()
  • componentDidUpdate()

Unmounting

  • componentWillUnmount()

Errors

  • componentDidCatch()

Lifecycle Methods Diagram

react-component-lifecycle

http://projects.wojtekmaj.pl/react-lifecycle-methods-diagram/