Calling a function when opening a react-native screen

use componentWillMount() method. This will execute automatically before render() method gets triggered.

class Sample extends Component{
    state = {data : []};
    componentWillMount(){
        this.setState({data : inputObject});
    }
    render(){
        return(
            <View>
            //you can render the data here
            </View>
        );
    }
}

Reference: https://facebook.github.io/react/docs/react-component.html#componentwillmount