title: React Native StackNavigator default props for screens
body: <p>I am using 2 seperate StackNavigators in my application and render one them in an if condition structure. 
  For Example : </p>
  
  <pre><code>if(this.props.authToken)//authToken is a state object setting in one of my reducers
  return &lt;AppNavigator /&gt;
  else
  return &lt;AuthNavigator /&gt;
  </code></pre>
  
  <p>See i will use that authToken object almost in every screen of AppNavigator.</p>
  
  <p>One way i am using currently is that getting that on my "mapStateToProps" function in every screens export statement.</p>
  
  <pre><code>class SomeScreen extends React.Component {
  
      render() {
          return (
             &lt;View&gt;&lt;Text&gt;Some UI {this.props.authToken.userName} &lt;/Text&gt;&lt;/View&gt;
          );
      }
  }
  
  const mapStateToProps = state =&gt; {
      return {
  
  //This line that i have to write in every screen
         authToken: state.app.authToken
      };
  };
  
  const mapDispatchToProps = dispatch =&gt; {
      return {...}
  };
  
  
  export default connect(mapStateToProps, mapDispatchToProps)(SomeScreen);
  </code></pre>
  
  <p>But it i didn't like that. So if there is an object that i might use in most of my navigator screens I desire that passing it from my root component to rendering navigator component like : </p>
  
  <pre><code> if(this.props.authToken)//authToken is a state object setting in one of my reducers
      return &lt;AppNavigator authToken={this.props.authToken} someOtherCommonProp={{....}}/&gt;
      else
      return &lt;AuthNavigator /&gt;
  </code></pre>
  
  <p>So my question is that what would be the best practise for that issue.</p>