react-native-router-flux warning: Key is already defined

Question

The following is the Android log output when the React Native app is started for a while before disconnecting it from the Meteor (DDP) server by killing the Meteor server.

Using console.log("<App /> render") to indicate when the render function of App component is called, it appears that whenever the render function is called again after createContainer passes it some new props, the Key is already defined error is triggered.

What's causing this error/warning and how can we fix it? The app still runs fine, but something is definitely wrong here.

12-23 02:27:01.875 31197 19338 I ReactNativeJS: Running application "RNapp" with appParams: {"initialProps":{},"rootTag"
:1}. __DEV__ === true, development-level warning are ON, performance optimizations are OFF
12-23 02:27:01.891 31197 19338 I ReactNativeJS: render
12-23 02:27:01.995 31197 19338 I ReactNativeJS: Connected to DDP server.
12-23 02:27:01.999 31197 19338 I ReactNativeJS: Connected to DDP server.
12-23 02:27:02.012 31197 19338 I ReactNativeJS: render
12-23 02:27:02.013 31197 19338 I ReactNativeJS: Key home is already defined!
12-23 02:27:02.013 31197 19338 I ReactNativeJS: Key welcome is already defined!
12-23 02:27:02.013 31197 19338 I ReactNativeJS: Key loading is already defined!
12-23 02:27:02.013 31197 19338 I ReactNativeJS: Key root is already defined!
12-23 02:27:34.592 31197 19338 I ReactNativeJS: Disconnected from DDP server.
12-23 02:27:34.593 31197 19338 I ReactNativeJS: Disconnected from DDP server.
12-23 02:27:34.599 31197 19338 I ReactNativeJS: <App /> render
12-23 02:27:34.599 31197 19338 I ReactNativeJS: Key home is already defined!
12-23 02:27:34.599 31197 19338 I ReactNativeJS: Key welcome is already defined!
12-23 02:27:34.599 31197 19338 I ReactNativeJS: Key loading is already defined!
12-23 02:27:34.599 31197 19338 I ReactNativeJS: Key root is already defined!
12-23 02:27:34.609 31197 19338 I ReactNativeJS: <Loading /> render
12-23 02:27:35.603 31197 19338 I ReactNativeJS: Disconnected from DDP server.
12-23 02:27:35.613 31197 19338 I ReactNativeJS: <App /> render
12-23 02:27:35.613 31197 19338 I ReactNativeJS: Key home is already defined!
12-23 02:27:35.613 31197 19338 I ReactNativeJS: Key welcome is already defined!
12-23 02:27:35.613 31197 19338 I ReactNativeJS: Key loading is already defined!
12-23 02:27:35.613 31197 19338 I ReactNativeJS: Key root is already defined!
12-23 02:27:45.599 31197 19338 I ReactNativeJS: Disconnected from DDP server.
12-23 02:27:45.616 31197 19338 I ReactNativeJS: <App /> render
12-23 02:27:45.616 31197 19338 I ReactNativeJS: Key home is already defined!
12-23 02:27:45.616 31197 19338 I ReactNativeJS: Key welcome is already defined!
12-23 02:27:45.616 31197 19338 I ReactNativeJS: Key loading is already defined!
12-23 02:27:45.616 31197 19338 I ReactNativeJS: Key root is already defined!

index.android.js

import { AppRegistry } from 'react-native'

import App from './app/App.js'

AppRegistry.registerComponent('RNapp', () => App)

App Component

export class App extends Component {

    constructor(props) {
        super(props);
    }

    componentWillReceiveProps(nextProps) {
        if (nextProps.status.connected == false) {
            Actions.loading();
        } else {
            if (nextProps.user !== null) {
                Actions.home();
            } else {
                Actions.welcome();
            }
        }
    }


    render() {
       console.log('<App /> render')
        return (
            <Router>
                <Scene key="root">
                    <Scene key="home" component={Home} title="Home" hideNavBar={true} />
                    <Scene key="welcome" component={Welcome} title="Welcome" hideNavBar={true} />
                    <Scene key="loading" component={Loading} title="Loading" hideNavBar={true} />
                    <Scene key="profile" component={Profile} title="Home" hideNavBar={true} />
                    <Scene key="history" component={History} title="Home" hideNavBar={true} />
                    <Scene key="search" component={Search} title="Home" hideNavBar={true} />
                </Scene>
            </Router>
        )       
    }

}


export default createContainer(() => {
  return {
    status: Meteor.status(),
    user: Meteor.user(),
    loggingIn: Meteor.loggingIn(),
  };
}, App);

Answer 1

The warnings don't cause any harm, but you can get rid of them by defining your scenes in a variable and then including them as a prop to your router like so:

const scenes = Actions.create(
  <Scene key="root">
    <Scene key="home" component={Home} title="Home" hideNavBar={true} />
    <Scene key="welcome" component={Welcome} title="Welcome" hideNavBar={true} />
    <Scene key="loading" component={Loading} title="Loading" hideNavBar={true} />
    <Scene key="profile" component={Profile} title="Home" hideNavBar={true} />
    <Scene key="history" component={History} title="Home" hideNavBar={true} />
    <Scene key="search" component={Search} title="Home" hideNavBar={true} />
  </Scene>
)

export class App extends Component {

  constructor(props) {
    super(props);
  }

  componentWillReceiveProps(nextProps) {
    if (nextProps.status.connected == false) {
      Actions.loading();
    } else {
      if (nextProps.user !== null) {
        Actions.home();
      } else {
        Actions.welcome();
      }
    }
  }


  render() {
    console.log('<App /> render')
    return (
      <Router scenes={scenes} />
    )
  }

}

See the docs for more info.

Answer 2

In my experience the warnings do not cause any problems, but just in case, I usually define a shouldComponentUpdate method that returns false if the props/state have not changed in a way that would require the route to change (note that I use redux and redux persist to decide what is the initial scene, and so I need to connect the component to redux in order to get the required information via props, but later if the data in the store changes, I can safely block additional renders).

That is just my approach based on my experience and needs. The router-flux guys have a slightly 'stricter' approach to this, that you can see in their docs