Q:

React-Native FlatList not re-rendering with custom renderItem

I have a FlatList that works as expected when using a plain old <Text> tag, but when using a custom Component inside renderItem, the FlatList will not re-render when changing this.state.dayOfYear. On app load, when I set this.state.dayOfYear, it loads properly. But when I change state again, it will not change the FlatList. 
FlatList Code
<FlatList
    style={{flex: 1}}
    extraData={this.state}
    data={reading_data[this.state.dayOfYear]}
    renderItem={({item}) => <DayRow content={item}/>} //does not work
    // renderItem={({item}) => <Text>{item.ref}</Text>} //Works perfectly
    keyExtractor={(item, index) => index}
/>

Custom renderItem (DayView.js)
import {StyleSheet, Text, View} from 'react-native'
import React, {Component} from 'react';

export default class DayRow extends React.Component {

    constructor(props) {
        super(props)
        console.log(props)
        this.state = {
            content: props.content,
        }
    }

    render() {
        return (
            <View style={styles.row}>
                <Text style={styles.text}>{this.state.content.ref}</Text>
                <View style={{height: 2, backgroundColor:'#abb0ab'}}/>
            </View>
        );
    }
}

const styles = StyleSheet.create({
    row: {
        backgroundColor: '#fff'
    },
    text: {
        fontSize: 16,
        padding: 10,
        fontWeight: 'bold',
        color: '#000',
    },
});

module.exports = DayRow;

A:

I'm pretty sure that your DayRow items are being constructed before props.content is being set, you need to grab the props when the component is mounting. Try adding this:
componentWillMount() {
  const { content } = this.props;
  this.setState({content: content});
}

EDIT
I missed the part about "re-rendering"...
Basically you need a block of code that updates your components state when its props change, react components have another function similar to componentWillMount called componentWillReceiveProps, try:
componentWillReceiveProps(nextProps) {
  const { content } = nextProps;
  this.setState({content: content});
}

