第一次单击时反应状态未更新，并且未渲染组件

Question

retrieveInTransit() is the click handler. An API call retrieves data and returns an array of objects into the new state object in the setState() function. The first click logs an empty array for the pumps property. If I click it again, I get the populated array of pumps . Why do I have to click twice? The Equipment component does not render even though the log in the render() function returns the correct array of pumps which is passed in as a prop for the Equipment component . I am at a loss on this and any help would be appreciated.

import React, { Component } from 'react';
import {ListGroup} from 'reactstrap';
import Equipment from './Equipment';

class Transit extends Component {
  constructor(props) {
    super(props);
    this.state = {
      pending: false,
      pumps: []
    };
  }

  handleCancelClick = (index) => {
    this.setState((prevState) =>
      prevState.pumps[index].isCancelled = !prevState.pumps[index].isCancelled
    );

    this.setState((prevState) => {
        prevState.pumps.every((equipment) => equipment.isCancelled === false)
          ? prevState.pending = false: prevState.pending = true
      }
    )};


  populate_transit = () => {
    let pumpArray = [];
    fetch('http://192.168.86.26:8000/api/v1/transit_list', {mode: 'cors'})
      .then(response => response.json())
      .then(MyJson => MyJson.map((entry) =>{
        if (document.getElementsByClassName('title')[0].innerText.toLowerCase().includes(entry.transferfrom)) {
          pumpArray.push(
            {
              unitnumber: entry.unitnumber,
              id: entry.id,
              message: entry.unitnumber + ' was moved to ' + entry.transferto,
              isCancelled: false
            });
        }}));

    return pumpArray
  };


  cancelTransit = () => {
    let cancelled = this.state.pumps.filter((movement) => movement.isCancelled);
    let cancelledId = cancelled.map((object) => object.id);
    fetch('http://192.168.86.26:8000/api/v1/transit_list/', {
      method:'POST',
      mode: 'cors',
      body: JSON.stringify(cancelledId),
      headers:{
        'Content-Type': 'application/json'
      }
        }
      );
    this.populate_transit()
  };

  retrieveInTransit = () => {
    if (this.state.pending) {
      this.setState({
        pending: false,
        pumps: this.cancelTransit()
      }, console.log(this.state))} else {
      this.setState({
        pending: false,
        pumps: this.populate_transit()
      }, console.log(this.state))
    }

  };


  render() {
    console.log(this.state.pumps);
    return (
      <ListGroup>
        <Equipment transitequipment={this.state.pumps} cancelClick={this.handleCancelClick}/>
        <button className='btn btn-dark' onClick={this.retrieveInTransit}>
          {this.state.pending ? 'Submit Cancellations': 'Refresh'}
        </button>
      </ListGroup>
    );
  }
}

export default  Transit;

Answer 1

 populate_transit = () => {
let pumpArray = [];
fetch('http://192.168.86.26:8000/api/v1/transit_list', {mode: 'cors'})
  .then(response => response.json())
  .then(MyJson => MyJson.map((entry) =>{
    if (document.getElementsByClassName('title')[0].innerText.toLowerCase().includes(entry.transferfrom)) {
      pumpArray.push(
        {
          unitnumber: entry.unitnumber,
          id: entry.id,
          message: entry.unitnumber + ' was moved to ' + entry.transferto,
          isCancelled: false
        });
    }}));

return pumpArray};

So, In the code above, you are returning pumpArray outside of the fetch call. Since fetch takes time to call the api and resolve the promise, the current value of your pumpArray = [] that's why you get the empty array at first. On the next click promise is already resolved so you get your pumpedArray . In order to fix this, move the pumpArray inside then .