ComponentWillMount only trigger for first time?

Question

MainComponent:

<Tabs 
  initialPage={this.props.day}
  tabBarUnderlineStyle={{ backgroundColor: '#5AF158' }} 
  renderTabBar={() => <ScrollableTab />}>
  {this.renderTabHeader()}
</Tabs>

renderTabHeader() {
  return (
    this.props.dateArray.map((date, i) => 
      <Tab 
        key={i}
        heading={date.format('DD/MM')} 
        tabStyle={styles.tabStyling} 
        activeTabStyle={styles.activeTabStyle} 
        textStyle={styles.tabTextStyle} 
        activeTextStyle={styles.activeTabTextStyle} 
      >
        <View style={{ backgroundColor: '#EEEEEE', flex: 1 }}>
          <Content contentDate={date.format('YYYY-MM-DD')} />
        </View>
      </Tab>
    )
  );
}

Content Component:

class Content extends Component {
  componentWillMount() {
    console.log('Component Will Mount() ?');
    this.props.loadTransactionByDate({ date: this.props.contentDate });
  }

render() {
  return (
    <View><Text>{this.props.contentDate}</Text></View>
  );
  }

Basically, in MainComponent there is a collection of tabs. I've noticed something rather weird which Content will be mounted on the first time their tab being click or active?

Meaning for the first time, we can click on Tab index 2 and seeing the console log in componentWillMount, then we switch to another tab and if coming back to Tab index 2 again, componentWillMount will not be triggered anymore?

Answer 1

First I would like to point out you should not use componentWillMount life cycle method since it has been deprecated on last minor update of React 16.3

Heres list of deprecated life cycle methods, (componentWillMount, componentWillReceiveProps, and componentWillUpdate). You can read more about deprecated life cycle methods here.

Secondary in your example life cycle works as expected. componentWillMount triggers only once since your component will be initial rendered/mounted only one time and that's how React works.

I would work this out with following method.

Add getDerivedStateFromProps life cycle to Content component, which will trigger when component receives new props and as well on initial mount.

static getDerivedStateFromProps(nextProps, prevState) {
  console.log('will log on props change');
  if( nextProps.contentDate !== prevState.contentDate ) {
    return { contentDate: nextProps.contentDate };
    // Notice we return plain object here to update state
  }
  return null;
  // return null when changes are not needed
}

This example checks that contentDate has changed and if so pushes it into component -state. And on render method you get it by this.state.contentDate.

render() {
  return (
    <View><Text>{this.state.contentDate}</Text></View>
  );
}

You could achieve similar behaviour with implementing this in componentDidUpdate but then you have much bigger risk to end up with infinite loops and much worse performance. But it's possible just have strong checks that data you have expected has really changed as you would expect. Then you can do setState and component re-renders.