#ask react navigation

component.js

componentWillMount() {
        BackHandler.addEventListener('hardwareBackPress', this._handleBackButton);
    }

    componentWillUnmount() {
        this._navListener.remove();
        BackHandler.removeEventListener('hardwareBackPress', this._handleBackButton);
    }

    _handleBackButton = () => {
        const { click } = this.state;

        if (click == 1) {
            BackHandler.exitApp()
        } else {
            ToastAndroid.show('Tap one more to exit', ToastAndroid.SHORT)
            this.setState({click:1})
        }

        setTimeout(() => {
            this.setState({click:0})
        }, 1500)

        return true
    }