获取数据后，React setState不会重新呈现

Question

This code works if data already fetched.
But doesn't work if I refresh the page and doesn't rerender element.
I'm also using Next JS if it's worth mentioning.

class Books extends Component {
    constructor(props) {
        super(props);
        this.state = {
            book: []
        }
        this.renderBooks= this.renderBooks.bind(this);
    }

    renderBooks() {
        let item;
        let items = [];
        return new Promise((resolve, reject) => {
            this.props.ids.forEach(address => {
                firebase.database().ref(`/books/${address}`)
                    .on('value', snap => {
                        item = snap.val();
                    });
                items.push(item);
            })
            resolve(items);
        });
    }

    async componentDidMount() {
        try {
            let res = [];
            res = await this.renderBooks();
            console.log(res);
            this.setState({ book: res });
        } catch (error) {
            console.log(error);
            this.setState(prevState => {
                return { book: 'err' }
              });
        }
    }

    render() {
        return (
            <div>
                { <List grid={{ gutter: 16 }}
                        dataSource={ this.state.book }
                        renderItem={ item => (
                            <List.Item>
                                <Card title={ !!item && item.title }>
                                    ...Book content...
                                </Card>
                            </List.Item>
                        )} />
                }
            </div>
        );
    }
}

export default Books; 

Is there anything to know about setState and fetching data that I missed here?
PS. Edited constructor to book: [].

Answer 1

You are initializing this.state.book with a promise. Try setting it to null instead:

this.state = {
    book: null
}

Answer 2

You cannot initialize book with a promise. Instead you can have a solution like below.

Add a conditional rendering to you render method so it will know when to render book . Also you don't need to return new Promise in this case.

class Books extends Component {
    constructor(props) {
        super(props);
        this.state = { books: null }
    }

    componentDidMount() {
        this.renderBooks()
    }

    renderBooks() {
        this.props.ids.forEach(address => {
            firebase.database().ref(`/books/${address}`)
             .on('value', snap => {
                this.setState({books: [...(this.state.books || []), snap.val()] });
            });
        });
    }

    render() {
        return (
           this.state.books ? 
            <div>
                { <List grid={{ gutter: 16 }}
                        dataSource={ this.state.books }
                        renderItem={ item => (
                            <List.Item>
                                <Card title={ !!item && item.title }>
                                    ...Book content...
                                </Card>
                            </List.Item>
                        )} />
                }
            </div> 
           : 'Initializing'
        );
    }
}

export default Books;

Promises are basically async functions that are resolved when it's time.

So when you do

 var item, items = []; // <---- Step 1
 this.props.ids.forEach(address => {
     firebase.database().ref(`/books/${address}`)
         .on('value', snap => {
             item = snap.val(); // <--- Step 3
         });
     });
     items.push(item); // <----- Step 2
 });

The steps are like this. So you were doing items.push(item) before item was assigned a new value which is snap.val() . And that makes item undefined .

I guess the second result you have is thanks to caching. If the internet connection is SOOOO FAST Step 3 might be earlier than Step 2 , but that's a bad assumption. That's why the second time you get the result correct.

In this answer's case, instead of having an items array, the snap.val() is added to this.state.books. But this makes it a bit heavyweight. Because every time a query on('value') is called, the setState method will be triggered and the component will be rendered again. If there were 1000 ids the state would change 1000 times.

That's why instead of getting the data one by one I would suggest you to get all the data at once. Try to google something like 'retrieve multiple data from firebase javascript' . Unfortunately I don't know much about firebase so cannot help there.