取消订阅Firestore的ComponentWillUnmount

import React, { Component } from 'react';
import firebase from 'firebase/app';
import 'firebase/firestore';

class MyComponent extends Component {
  constructor(props) {
    super(props);
    this.unsubscribe = null;
    this.state = {
      data: []
    };
  }

  componentDidMount() {
    const db = firebase.firestore();
    this.unsubscribe = db.collection('myCollection').onSnapshot(snapshot => {
      const data = snapshot.docs.map(doc => doc.data());
      this.setState({ data });
    });
  }

  componentWillUnmount() {
    if (this.unsubscribe) {
      this.unsubscribe();
    }
  }

  render() {
    // 渲染组件
  }
}