title: set dynamic 'initialRouteName' in react-native
body: <p>question is simple I know there are lots of answers available for the same question but I have differently set my own <code>App.js</code> file i tried almost all but no success.</p>
  
  <p>so my <code>App.js</code> looks like this </p>
  
  <pre><code>import React, { Component } from 'react';
  import {
    Platform,
    StyleSheet,
    Text,
    View,
    StatusBar
  } from 'react-native';
  import { createStackNavigator  } from 'react-navigation';
  import { Header, Button, Spinner } from './src/components/comman';
  import SplashScreen from 'react-native-splash-screen';
  import LoginForm from './src/components/LoginForm';
  import SignupForm from './src/components/SignupForm';
  import Home from './src/components/Home';
  import ForgetPassword from './src/components/ForgetPassword';
  
  
  import { YellowBox } from 'react-native';
  YellowBox.ignoreWarnings(['Warning: isMounted(...) is deprecated', 'Module RCTImageLoader']);
  const App = createStackNavigator ({
    Login: { screen: LoginForm },
    Home: { screen: Home },
    Signup: { screen: SignupForm },
    ForgetPassword: { screen : ForgetPassword },
    },
    { 
    initialRouteName: 'Login',
    });
  
  export default App;
  </code></pre>
  
  <p>Now I store login state in <code>AsyncStorage</code> in my <code>LoginForm.js</code> like this </p>
  
  <pre><code>.
  .
  AsyncStorage.setItem('isLoggedIn', 'true').then((a) =&gt; {
      this.onLoginSuccess(responseData.user_id);
  });
  .
  .
  </code></pre>
  
  <p>so now i want to set <code>initialRouteName</code> based on my local storage element.
  Simple if <code>isLoggedIn</code> is <code>true</code> then <code>initialRouteName : Home</code> otherwise <code>initialRouteName : Login</code></p>
  
  <p>Simple but don't how to use this in <code>my</code> <code>App.js</code> </p>
  
  <p>Please help :)</p>