React Native Fetch API not returning my calls

问题

Sorry for the joke in the title.

I am currently exploring the fetch API in react native, but I have bumped in to some issues which I cannot wrap my head around.

So, I am trying to get a message from a server, which I am calling with the fetch API in the following manner:

var serverCommunicator = {
    test: function() {
        fetch(baseUrl  , {
          method: 'GET',
          headers: {
            'Accept': 'application/json',
            'Content-Type': 'application/json',
          }
        })
        .then((response) => response.text())
        .then((responseText) => {
             return (JSON.stringify(responseText));
        })
        .catch((error) => {
          console.warn(error);
        }).done();
    },
module.exports = serverCommunicator;

When I tested using only console.log(responseText) my log gave me the correct message. However, now when I wanna try to put the content in the code as a message in a View it does not return as expected. Calling it in the following manner:

import Method from '../Services/Methods';
.
.
.
<View style={styles.card}>
   <CardView
      message={Method.test()} 
    />

I can see how the function test is called properly when calling it like this, but for some reason, it does not write the message.

回答1:

This is a classic async issue, where your then function returns after render has already been called, with nowhere to return to.

Most common solution: display empty state message / loading indicator and fetch your server info when your component mounts. When your promise returns and then callback is fired, set the component state which will trigger a re-render, with your expected value.

class extends React Component

class YourComponent extends Component {
  constructor() {
    super()
    this.state.text = 'Loading, please wait!' // default text
  }

  componentDidMount() {
    fetch(baseUrl, options)
      .then((response) => response.text())
      .then((responseText) => {
         this.setState({ text: responseText }) // this triggers a re-render!
      })
  }

  render() {
    return (
      <View style={styles.card}>
        <CardView
          message={this.state.text} // <-- will change when fetch call returns
        />
      </View>
    )
  }

}

React.createClass

var YourComponent = React.createClass({
  getInitialState() {
    return { text: 'Loading, please wait!' }
  }, // <-- comma between functions, because object keys

  componentDidMount() {
    fetch(baseUrl, options)
      .then((response) => response.text())
      .then((responseText) => {
         this.setState({ text: responseText }) // this triggers a re-render!
      })
  },

  render() { /* ..same as above.. */ }
})

If you want to keep your current architecture where the fetch call is in the service, you need to return the initial call to fetch, which will return a promise. Then you can hook up then in your constructor:

var serverCommunicator = {
  test: function() {
    return fetch(baseUrl, options) // return a promise! ..important!
      .then((response) => response.text())
      .then((responseText) => {
         return responseText
      })
  }
}

Then your imported function will return a promise..

import Method from '../Services/Methods'

...

componentDidMount() {
  Method.test().then(responseText => {
    this.setState({ text: responseText })
  })
]

  ....

Hope that clears things up a bit about how promises work, and how to capture async data using state in React!

来源：https://stackoverflow.com/questions/37035423/react-native-fetch-api-not-returning-my-calls

标签

javascript

react-native

fetch-api