Lifting State to Parents

React components can manage their own state
Often components need to communicate state to others
Siblings do not pass state to each other directly
State should pass through a parent, then trickle down

Boiling Point Calculator

function BoilingVerdict(props) {
  if (props.celsius >= 100) {
    return (
      <p>The water would boil.</p>;
    )
  }
  return (
    <p>The water would not boil.</p>
  )
}

Lifting State - Calculator Input

An element to collect input is needed
The input is passed to the BoilingVerdict Component

function Calculator (props) {

  const [temperature, setTemperature] = useState('')

  return (
    <fieldset>
      <legend>Enter temperature in Celsius:</legend>
      <input
        value={temperature}
        onChange={(evt) => {
          setTemperature(evt.target.value)
        }} />

      <BoilingVerdict
        celsius={parseFloat(temperature)} />

    </fieldset>
  );
}

CodePen

Lifting State - Another Input

The calculator should work for Celsius & Fahrenheit

What we want

function Calculator (props) {

  return (
    <div>
      <TemperatureInput scale="c" />
      <TemperatureInput scale="f" />
    </div>
  );
}

Extract TemperatureInput

function TemperatureInput (props) {
  const [temperature, setTemperature] = useState('')

  const scale = props.scale;
  return (
    <fieldset>
      <legend>Enter temperature in {scaleNames[scale]}:</legend>
      <input value={temperature}
              onChange={(evt) => {
                  setTemperature(evt.target.value)
                }
              }
      />
    </fieldset>
    );
  }
}

Lifting State - Passing State

The components must be kept in sync
Each TemperatureInput holds it's own state
Sibling components can not communicate directly
State can be moved to Calculator to achieve this

Remove State from TemperatureInput

function TemperatureInput (props) {


  /* Send changes to the parent */
  handleChange(evt) {
    props.setTemperature(evt.target.value);
    props.setScale(props.scale)
  }

  /* Use READ-ONLY Props instead of State */

  return (
    <fieldset>
      <legend>Enter temperature in {scaleNames[scale]}:</legend>
      <input value={props.temperature}
             onChange={this.handleChange} />
    </fieldset>
  );
}

Lifting State - Parent State

Children call setTemperature from props with new state
Parent updates state with setTemperature
Children re-render with new props

Parent Calculator Passes State to Children

function Calculator (props) {

    const [scale, setScale] = useState('c')
    const [temperature, setTemperature] = useState('')

    const celsius = scale === 'f' ? tryConvert(temperature, toCelsius) : temperature;

    const fahrenheit = scale === 'c' ? tryConvert(temperature, toFahrenheit) : temperature;

    return (
      <div>
        <TemperatureInput
          scale="c"
          temperature={celsius}
          setScale={setScale}
          setTemperature={setTemperature}
        />

        <TemperatureInput
          scale="f"
          temperature={fahrenheit}
          setScale={setScale}
          setTemperature={setTemperature}
        />

        <BoilingVerdict
          celsius={parseFloat(celsius)} />
      </div>
    );
  }
}

CodePen

Lifting State - Live Example

See the Pen djrKLj by Joshua Burke (@Dangeranger) on CodePen.

Lifting State - Conclusions

A single "point of truth" holds the state
Components communicate state by lifting it up to a Parent
Child components use a Parent updater function to lift state up
State flowing down makes state changes simpler to debug
Props should be derived from State

Lab: Post Selector

Let's create a React component that consists of two main components; a list of posts, and a display box. These should be two seperate components. You will also need a parent component that renders both the list, and the display box.

Using state and props set up the page so that when you click on a post the cotent is rendered in the display box.

Hint: If you want to store your posts locally it might be easiest to create JSON files to represent the posts. Or you could just use JSONPlaceholder