Composition Vs Inheritance Slides

Composition vs Inheritance

Inheritance can be overused
Composition of behavior can be simpler and easier
React supports using composition over deep inheritance

Composition - Using props.children

Any Components within <FancyBorder> become props.children
The parent component can wrap children in a <div> for layout/style

Composing Behavior with Components

/* FancyBorder will componse with children*/
function FancyBorder(props) {
  return (
    <div className={'FancyBorder FancyBorder-' + props.color}>
      {props.children}
    </div>
  );
}

/* WelcomeDialog uses FancyBorder with children*/
function WelcomeDialog() {
  return (
    <FancyBorder color="blue">
      <h1 className="Dialog-title">
        Welcome
      </h1>
      <p className="Dialog-message">
        Thank you for visiting our spacecraft!
      </p>
    </FancyBorder>
  );
}

Composition - Child Groups

Sometimes components have children in several places
Assigning the child components to prop names can help organize

function SplitPane(props) {
  return (
    <div className="SplitPane">
      <div className="SplitPane-left">
        {props.left}
      </div>
      <div className="SplitPane-right">
        {props.right}
      </div>
    </div>
  );
}

function App() {
  return (
    <SplitPane
      left={
        <Contacts />
      }
      right={
        <Chat />
      } />
  );
}

Composition - Classes Work Too

Both Stateless Functions and Classes compose
Specific Components can configure General Components
Props can be used to configure

function Dialog(props) {
  return (
    <FancyBorder color="blue">
      <h1 className="Dialog-title">
        {props.title}
      </h1>
      <p className="Dialog-message">
        {props.message}
      </p>
      {props.children}
    </FancyBorder>
  );
}

class SignUpDialog extends React.Component {
  constructor(props) {
    super(props);
    this.handleChange = this.handleChange.bind(this);
    this.handleSignUp = this.handleSignUp.bind(this);
    this.state = {login: ''};
  }

  handleChange(e) {
    this.setState({login: e.target.value});
  }

  handleSignUp() {
    alert(`Welcome aboard, ${this.state.login}!`);
  }

  render() {
    return (
      <Dialog title="Mars Exploration Program"
              message="How should we refer to you?">
        <input value={this.state.login}
               onChange={this.handleChange} />

        <button onClick={this.handleSignUp}>
          Sign Me Up!
        </button>
      </Dialog>
    );
  }
}

Composition - When Inheritance?

Facebook doesn't use inheritance beyond initial Component Classes
Frontends can be built using a mixture of 'general' and 'specific' components
Use Props to pass specific attributes and children to render specific behavior
Behavior shared within Components such as logic or utilities can be shared as JavaScript libraries or modules
Using import is more useful and less restricting then extends