Rails for Front-end Development: Essential Tools/

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Sharing State - Row & Seat Components

Sharing State - Row & Seat Components

Read about how to share state in the row component in this lesson.

We'll cover the following...

Row Component

The rows in the venue body are the same as before, except we are passing the number of tickets to buy as a new prop in the object.

The Row component has gotten a lot more functionality, some of which has been taken from the Seat component. In this version of the code, the status of a seat depends on the status of the seats next to it—if two tickets are bought, then a set with an already sold seat to its right is not available. Because an individual seat can no longer contain all the data needed to ascertain its status, the status for the entire row needs to be stored in the Row component.

Here’s the Row code:

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//START:P1
import * as React from "react"
import Seat from "components/seat"
interface RowProps {
  rowNumber: number
  seatsPerRow: number
  ticketsToBuyCount: number
}
const Row = (props: RowProps): React.ReactElement => {
  const [seatStatuses, setSeatStatuses] = React.useState(
    Array.from(Array(props.seatsPerRow).keys()).map(() => "unsold")
  )
  function isSeatValid(seatNumber): boolean {
    if (seatNumber + props.ticketsToBuyCount > props.seatsPerRow) {
      return false
    }
    for (let i = 1; i < props.ticketsToBuyCount; i++) {
      if (seatStatuses[seatNumber + i] === "held") {
        return false
      }
    }
    return true
  }
  function validSeatStatus(seatNumber): string {
    if (seatStatuses[seatNumber] === "held") {
      return "held"
    } else {
      return isSeatValid(seatNumber) ? "unsold" : "invalid"
    }
  }
  function newState(oldStatus: string): string {
    if (oldStatus === "unsold") {
      return "held"
    } else if (oldStatus === "held") {
      return "unsold"
    } else {
      //END:P1
      //START:P2
      return "invalid"
    }
  }
  function onSeatChange(seatNumber: number): void {
    if (validSeatStatus(seatNumber) === "invalid") {
      return
    }
    setSeatStatuses(
      seatStatuses.map((status, index) => {
        if (
          index >= seatNumber &&
          index < seatNumber + props.ticketsToBuyCount
        ) {
          return newState(seatStatuses[seatNumber])
        } else {
          return status
        }
      })
    )
  }
  const seatItems = Array.from(Array(props.seatsPerRow).keys()).map(
    (seatNumber) => {
      return (
        <Seat
          key={seatNumber}
          seatNumber={seatNumber}
          status={validSeatStatus(seatNumber)}
          clickHandler={onSeatChange}
        />
      )
    }
  )
  return <tr className="h-20">{seatItems}</tr>
}
export default Row
//END:P2

This component keeps track of the seat statuses in an array and defines a click handler to be executed when a seat is clicked. To make that work, the Row component passes that handler function as a prop to the Seat component, namely clickHandler.

The first thing that we need to do is call the useState hook again, this time to create an array of seat statuses. The initial value, which is the argument to the useState call, is an array of unsold values, one for each seat in the row.

Using an array or an object as the value of the state in a useState call, rather than an individual value, works differently than we might expect. It is important to keep in mind that the argument to the useState method is only applied to the state value the first time the component is drawn. When the component re-renders, that value is ignored.

An implication of that rule is that we generally don’t want the argument to the useState method to be dynamic. In our case, it’s a static array of unsold values, since we don’t expect the number of seats in the row to change. Later, in the chapter ...