React - 不在文件之间传递道具

Question

我有这三个文件：App，Main和Login。我正在尝试将用户名从登录传递到Main（然后传递给其他一些文件），但我未定义。 在应用程序中我有这些导航器：

const mainOptions = SwitchNavigator({
  main : Main,
  favPlaces : favPlaces,

})
const introStack = StackNavigator({
  loggingIn : LoggingIn,
  register : Register
})

const SwNavigator = SwitchNavigator({
  login : introStack,
  main : mainOptions

})

登录：

constructor(props){
        super(props);
        this.state={
          credentials: {
            username: "",
            password:"",

          }
        }
        this.navigate = this.props.navigation.navigate;
    }
      signIn(){ //when a button is clicked
          {....}
          this.navigate("main", {
                username: this.state.username,
              });
        }
  }

主要：

constructor(props) {
        super(props);
    this.navigate = this.props.navigation.navigate;
    this.params = this.props.navigation.state.params;
}
{...}
    render() {
    console.log(this.props.navigation.state.params.username); //UNDEFINED