React-Native / Redux-无法在挂载组件之前获取数据

Question

I'm having issues with fetching data from my web-service and updating this.props before the component is rendered.

Homepage.js

import React, { Component } from 'react';
import {View, Text, StyleSheet, Image} from 'react-native';
import { Header, Footer , CarouselSlide} from './common';
import { connect } from 'react-redux';
import { getQuests} from '../actions';
import Carousel from 'react-native-snap-carousel';

const SLIDER_1_FIRST_ITEM = 1;

class HomePage extends Component {

  constructor (props) {
    super(props);
    this.state = {
      slider1ActiveSlide: SLIDER_1_FIRST_ITEM,
      slider1Ref: null
    };
  }
  componentWillMount() {
    this.props.getAllQuests();
  }

  _renderItemWithParallax({item, index}, parallaxProps) {
    return(
      <CarouselSlide 
        data={item}
        parallaxProps={parallaxProps}
      />
    );
  }

  render() {
    const {containerStyle, questHeaderStyle, questHeaderTextStyle} = styles;
    const {slider1ActiveSlide, slider1Ref} = this.state;

    return(
      <View style={containerStyle}>
        <Header />

        <View style={questHeaderStyle}>
          <Text style={questHeaderTextStyle}>Quests</Text>
        </View>

        <Carousel 
          ref={(c) => { if (!this.state.slider1Ref) { this.setState({ slider1Ref: c}); } }}
          data={this.props.questObject}
          renderItem={this._renderItemWithParallax}
          sliderWidth={300}
          itemWidth={300}  
          hasParallaxImages={true}
          firstItem={1}
          inactiveSlideScale={0.94}
          inactiveSlideOpacity={0.7}
          enableMomentum={false}
          loop={true}
          loopClonesPerSide={2}
          autoplay={true}
          autoplayDelay={500}
          autoplayInterval={3000}
          onSnapToItem={(index) => this.setState({ slider1ActiveSlide: index})}
        />

        <Footer />
      </View>
    );
  }
}

const styles = StyleSheet.create({
  containerStyle: {
    flex: 1
  },
  questHeaderStyle: {
    left: 17.5,
    top: 5
  },
  questHeaderTextStyle: {
    color: '#EF6D69',
    fontSize: 17.5,
    fontWeight: '800'
  }
})

const mapStateToProps = ({quest}) => {
  const { error, loading, questObject } = quest;

  return { error, loading, questObject};
};

const mapDispatchToProps = (dispatch) => {
  return {
    getAllQuests: () => {
      dispatch(getQuests());
    }
  };
};

export default connect(mapStateToProps, mapDispatchToProps)(HomePage);

it seems to only dispatch the action after the component has rendered, how do I dispatch the action before the component is mounted?

Answer 1

Since you are fetching the data from a web-service, it can take an unknown amount of time and you can't hold the rendering of the component till that time. What you need here is to maintain a state which tells if the quests has been retrieved from the server. If not, then you render a message saying Fetching quests or something and once you have some quests to render, you start rendering those. Pseudocode would like something like

class HomePage extends Component {
  state = {
    hasSomeQuests: false
  }

  updateHasSomeQuestsToRender = (newValue) => {
    this.setState({
      hasSomeQuests: newValue
    })
  }

  render() {
    if(!this.state.hasSomeQuests) {
      return <Fetching quests>
    }

    return JSX with the quests
  }
}

You update the hasSomeQuests state when you retrieve the quests and have at least one that you can render on the screen.