引发错误时，React-Native提取无法设置为setState

Question

Background

I am trying to handle a bad HTTP response with the fetch() function in my React Native Component. I have used the code from here which suggests creating a module to handle response errors.

// ApiUtils.js

var ApiUtils = {  
  checkStatus: function(response) {
    if (response.status >= 200 && response.status < 300) {
      return response;
    } else {
      let error = new Error(response.statusText);
      error.response = response;
      throw error;
    }
  }
};
export { ApiUtils as default };

This is the code for my component:

import React, { Component } from 'react';
import {View, Text, StyleSheet, Slider, ListView} from 'react-native';
import GLOBAL from "../../Globals.js"
import ApiUtils from "../utils/ApiUtils.js"

class FetchedList extends Component {
    constructor(props) {
    super(props);
    this.state = {
      dataSource: new ListView.DataSource({
          rowHasChanged: (row1, row2) => row1 != row2,
      }),
      loaded: false,
      load_failed: false,
    };
  }
  componentDidMount(){
    this.fetchData();
  }
  fetchData(){
    fetch(GLOBAL.BASE_URL + "/" + this.props.url_section + "/" + String(this.props.weekNo) + "/")
      .then(ApiUtils.checkStatus)
      .then((response) => {
        return response.json()
      })
      .then((responseData) => {
          if(responseData===[] || responseData.length === 0){
            this.setState({
              loaded: false,
              load_failed: true,
            });
          }
          else{
            this.setState({
              dataSource: this.state.dataSource.cloneWithRows(responseData),
              loaded: true,
            });
          }
      })
      .catch(function(error){
        console.log("Error:" + error.message);
        this.setState({load_failed: true});
      })
      .done();
  }
  render() {
    if (!this.state.loaded) {
      if (this.state.load_failed){
        return(
          <View></View>
        );
      }
      return this.renderLoadingView();
    }
    else{
        return (
          <ListView
            dataSource={this.state.dataSource}
            renderRow={this.renderComment}
          /***//>
        );
    }
  }
  renderLoadingView() {
     return (
       <View>
        <Text>Loading . . .</Text>
       </View>
     );
   }

  renderComment(comment){
    return(
      <Text style={styles.row}>{comment.content}</Text>
    )
  }
}

const styles = StyleSheet.create({
  row: {
    // backgroundColor: "antiquewhite",
    flexDirection: "row",
    justifyContent: "flex-start",
    alignItems: "center",
    height: 50
  },
});

module.exports = FetchedList

I have made sure that the test server is currently giving 502 Gateway errors.

The behaviour I expect is that when an error is thrown by the line .then(ApiUtils.checkStatus) it should be caught by the .catch function and state should be updated by this.setState({load_failed: true}); . However, I get the error message ExceptionsManager.js:55 this.setState is not a function .

I find this odd because the following works within the .then( . . .) function above it:

this.setState({
  dataSource: this.state.dataSource.cloneWithRows(responseData),
    loaded: true,
});

Question

Why does the .catch lambda not have access to this.setState where the previous function does? Can I use .bind() somehow?

Follow-On Question

If it is not possible to access this.setState within the catch function, how can I change state.load_failed to true if I get a poor HTTP response?

Attempted Fixes

I attempted to pass the exception to the calling function and then change the state from the parent function, like so:

I changed the .catch() function to this:

fetchData(){
    fetch(GLOBAL.BASE_URL + "/" + this.props.url_section + "/" + String(this.props.weekNo) + "/")
      .then(ApiUtils.checkStatus)
      .then((response) => {
        return response.json()
      })
      .then((responseData) => {
          . . .
      })
      .catch(function(error){
        console.log("Error!");
        throw error;
      })
      .done();
  }

and then changed the calling function like so:

componentDidMount(){
try{
  this.fetchData();
}
catch(error){
  this.setState({load_failed: true});
}
console.log(this.state.load_failed);

}

However, I then get a simple ExceptionsManager.js:55 Error .

I tried removing .done() , but the catch block fails to handle the exception, state does not change and I get a warning: Possible Unhandled Promise Rejection (id: 0): . I realise that this may have something to do with async functions in javascript and what the error is passed to, but I'm not 100% sure.

Environment: OSX 10.10, Android 4.1.2, React-native 0.29.2

Answer 1

Your function is not running in the same context ( this value) as you expect. To solve this, either use an arrow function which keeps the same this :

.catch(error => {
  console.log("Error:" + error.message);
  this.setState({load_failed: true});
})

or explicitly bind to the current this :

.catch(function(error){
  console.log("Error:" + error.message);
  this.setState({load_failed: true});
}.bind(this))