FlatList calling twice

Question

I have this code

class Home extends Component {
  constructor(props) {
    super(props);
    this.state = {
        dataSource: []
    }
    this._handleRenderItem = this._handleRenderItem.bind(this);
    this._keyExtractor = this._keyExtractor.bind(this);
  }

  componentDidMount() {

    let success = (response) => {
        this.setState({ dataSource: response.data });
    };

    let error = (err) => {
        console.log(err.response);
    };

    listarProdutos(success, error);
  }

  _keyExtractor = (item, index) => item._id;

  _handleRenderItem = (produto) => {
    return (
        <ItemAtualizado item={produto.item} />
    );
  }

  render() {
    return (
        <Container style={styles.container}>
            <Content>
                <Card>
                    <CardItem style={{ flexDirection: 'column' }}>
                        <Text style={{ color: '#323232' }}>Produtos atualizados recentemente</Text>
                        <View style={{ width: '100%' }}>
                            <FlatList
                                showsVerticalScrollIndicator={false}
                                data={this.state.dataSource}
                                keyExtractor={this._keyExtractor}
                                renderItem={this._handleRenderItem}
                            />
                        </View>
                    </CardItem>
                </Card>
            </Content>
        </Container>
    );
  }
}

export default Home;

The function _handleRenderItem() is being called twice and I can't find the reason. The first time the values inside my <ItemAtualizado /> are empty, but the second was an object.

Answer 1

This is normal RN behavior. At first, when the component is created you have an empty DataSource ([]) so the FlatList is rendered with that.

After that, componentDidMount triggers and loads the updated data, which updates the DataSource.

Then, you update the state with the setState which triggers a re render to update the FlatList.

All normal here. If you want to try, load the datasource in the constructor and remove the loading in the componentDidMount. It should only trigger once.

Answer 2

If you want to control render actions, you can use shouldComponentUpdate method.

For example:

shouldComponentUpdate(nextProps, nextState){ if(this.state.friends.length === nextState.friends.lenght) return false; }

it will break render if friends count not change.