反应Redux派遣

Question

What is the best way to call a dispatch to get initial data on a React component. My understanding is that ComponentWillMount is called before render. So in theory if I call dispatch on ComponentWillMount, by the time I hit render and then ComponentDidMount I should have my data in the component's props, right? I'm not seeing that.

I'm seeing that render gets called twice and that on the first go when the component is being initialized, I cannot access the data in props. It also seems like dispatch does not actually get called until the second render. I'm basically looking to have some light shed on the best way to call a dispatch when initially setting up a component. I'm essentially trying to do something like the following where I use a container component to get my data from dispatch and then pass it to a child component as props. But I also want to initialize some state variables in the ContainerComponent and then pass them to the ChildComponent as props. The thing is that the state variables I want to initialize depend on the data returned from dispatch and ideally I would do the initialization in ComponentWillMount or ComponentDidMount.

import React from 'react';
import axios from 'axios';

import { connect } from 'react-redux';
import ChildComponent from './ChildComponent.js';

import { getTransactionsAll } from '../actions/actions.js';

class ContainerComponent extends React.Component {
  constructor() {
    super();
    this.state = {
      acctList:[],
      acctChecked:[],
      categoryList:[]
    }
}
  componentWillMount() {

    console.log("componentWillMount entered");

    this.props.get_data();
    console.log(this.props.searchProps.transactions_all);//this is undefined meaning the dispatch has not assigned the data yet...??

  }

  componentDidMount() {
    console.log("componentDidMount entered");
    console.log(this.props.searchProps.transactions_all);//this is undefined meaning the dispatch has not assigned the data yet...??
}

  render() {

    console.log("TransactionManagerContainer render entered");
    console.log(this.props.searchProps.transactions_all);//this is undefined the first time around meaning the dispatch has not assigned the data yet...??, but is defined on the second call to render after the dispatch has actually occurred...

return <ChildComponent
             data={this.props.searchProps.data}/>;
}

const mapStateToProps = (state) => ({
  searchProps: state.searchProps
});

export default connect(mapStateToProps, {getTransactionsAll})(TransactionManagerContainer);

Here is my reducer that assigns the state:

import { combineReducers } from 'redux'

import {GET_TRANSACTIONS } from '../actions/actions.js'
import {GET_TRANSACTIONS_ALL } from '../actions/actions.js'

const INITIAL_STATE = { defaultYear: 2016, transactions: []};

function get_transactions(state = INITIAL_STATE, action) {
  // console.log("this is in the reducer: get_transactions");
  // console.log(action);
  switch(action.type) {
    case GET_TRANSACTIONS:
      // return { ...state, transactions: action.payload };
      return Object.assign({}, state, {
        transactions: action.payload,
        selectedYear: action.selectedYear
      })
    default:
      return state;
  }
}

function get_transactions_all(state = INITIAL_STATE, action) {
  console.log("this is the value of action in the reducer: get_transactions_all");
  console.log(action);
  switch(action.type) {
    case GET_TRANSACTIONS_ALL:
      // return { ...state, transactions: action.payload };
      return Object.assign({}, state, {
        transactions_all: action.payload
      })
      console.log("this is the value of state in the reducer after being set");
      console.log(state);
    default:
      return state;
  }
}

const rootReducer = combineReducers({
  //stateProps: get_transactions,
  searchProps: get_transactions_all
})

export default rootReducer

Here are my actions:

import axios from 'axios';

export const GET_TRANSACTIONS = 'GET_TRANSACTIONS';

export function getTransactions(year) {
 return function(dispatch) {
  axios.get(`http://localhost:3001/api/transfilter?year=${year}&grouping=2`)
   .then(response => {
     dispatch({
       type: GET_TRANSACTIONS,
       payload: response.data,
       selectedYear: year
     });
   })
   .catch((error) => {
     console.log(error);
   })
 }
}

export const GET_TRANSACTIONS_ALL = 'GET_TRANSACTIONS_ALL';

export function getTransactionsAll(year) {
 return function(dispatch) {
  axios.get(`http://localhost:3001/api/trans?limit=20`)
   .then(response => {

     dispatch({
       type: GET_TRANSACTIONS_ALL,
       payload: response.data
     });
   })
   .catch((error) => {
     console.log(error);
   })
 }
}

Answer 1

I believe your main question is:

What is the best way to call a dispatch to get initial data on a React component?

Getting initial data requests (or any AJAX requests in general) should go in the componentDidMount lifecycle event.

There are a few reasons for this, here are two important:

1. Fiber, the next implementation of React's reconciliation algorithm, will have the ability to start and stop rendering as needed for performance benefits. One of the trade-offs of this is that componentWillMount, the other lifecycle event where it might make sense to make an AJAX request, will be “non-deterministic”. What this means is that React may start calling componentWillMount at various times whenever it feels like it needs to. This would obviously be a bad formula for AJAX requests.

2. You can't guarantee the AJAX request won't resolve before the component mounts. If it did, that would mean that you'd be trying to setState on an unmounted component, which not only won't work, but React will yell at you for. Doing AJAX in componentDidMount will guarantee that there's a component to update.

Credits: I learned that from here , there is also a discussion here .

Then, there are a lot of smaller question you've raised and it will be hard for me to answer all, but I'll try to cover most:

• After reading the above, you now should understand why your data is undefined in componentWillMount and componentDidMount . That's simply because the data has not arrived yet;
• It's normal that your data is undefined during the first render of the component. Initial render happens before data arrival;
• It's normal that the data is defined during the second render. The dispatch triggers asynchronous data fetch. Right after data comes, a reducer is hit and component gets re-rendered (that's the second re-render).

• If the child components in your main component require the data - check in the parent render method if data exists pass internal components conditionally, only if data is present. Like so:

   class ContainerComponent extends React.Component { // ... omitted for brevity render() { return ( { this.props.searchProps.data ? <ChildComponent data={this.props.searchProps.data} /> : <p>Loading</p> } ); } }