_onBackPress = (event) => {
    let nav = this.refs.navigator;
    let routers = nav.getCurrentRoutes();
    if (routers.length > 1) {
        nav.pop();
        return true;
    } else {
        if (this.lastBackPressed && this.lastBackPressed + 2000 >= Date.now()) {
            //两秒内连续点back退出
            return false;
        }
        this.lastBackPressed = Date.now();
        ToastAndroid.show(Strings.backAgainToExit, ToastAndroid.SHORT);
        return true;
    }
}

componentWillMount() {
    if (Platform.OS === 'android') {
        BackAndroid.addEventListener('hardwareBackPress', this._onBackPress);
    }
}

componetWillUnmount() {
    if (Platform.OS === 'android') {
        BackAndroid.removeEventListener('hardwareBackPress', this._onBackPress);
    }
}