在组件内部未定义“ this”

Question

当我尝试从uiConfig内部调用this.setState（）时，“ this”出现为未定义状态（它指的是我想的uiConfig对象？）

我尝试创建一个内部调用this.setState的新函数，但是遇到了同样的“ this is undefined”错误。

那么调用this.setState并更新组件的正确方法是什么？

import React from 'react';
import firebase from 'firebase';
import StyledFirebaseAuth from 'react-firebaseui/StyledFirebaseAuth';

export default class Signin extends React.Component {
  constructor(props) {
    super(props);

    this.state = {
      isNewUser: 'false'
    };
  }

    uiConfig = {
        signInFlow: "popup",
        signInOptions: [
          firebase.auth.GoogleAuthProvider.PROVIDER_ID,
          firebase.auth.FacebookAuthProvider.PROVIDER_ID,
          firebase.auth.TwitterAuthProvider.PROVIDER_ID,
          firebase.auth.EmailAuthProvider.PROVIDER_ID
        ],
        callbacks: {
          signInSuccessWithAuthResult: function(authResult, redirectUrl){
            // Check if its a new user
            const user = authResult.user;
            const isNewUser = authResult.additionalUserInfo.isNewUser;
            if (isNewUser) {
              this.setState({ // this is undefined
                isNewUser: true
              });
              const db = firebase.firestore();
              const usersRef = db.collection("users");
              usersRef.doc(user.uid).set({
                username: null,
                email: user.email,
                photoURL: user.photoURL,
                emailVerified: user.emailVerified
              })
            }
          }
        }
      }

    componentDidMount = () => {
      firebase.auth().onAuthStateChanged(user => {
        this.props.changeSignedInStatus(user);
      })
    }

    render() {
      let promptUsername = <></>;
      if(this.state.isNewUser) {
        console.log('new user')
      }

      return (
          <div className='login'>
              <form action="" className='signin-form'>
                  <h1>Sign in</h1>
                  <StyledFirebaseAuth
                      uiConfig={this.uiConfig}
                      firebaseAuth={firebase.auth()}
                  />
                  {promptUsername}
              </form>
          </div>
      );
    }
}