Handle loading/errors

As we saw in the previous step, the useQuery returned an object with data, error, loading fields. loading and error are states that you typically want to handle in your app.

Now let's go back to the Todos file that you wrote in the previous step.

Todos.js

Copy
- if (!data) return null;

if (error) {
  console.error(error);
  return <Text>Error</Text>;
}

if (loading) {
  return <CenterSpinner />;
}

return (
  <View style={styles.container}>
    <LoadNewer show={this.state.newTodosExist && isPublic} toggleShow={this.dismissNewTodoBanner} styles={styles} isPublic={this.props.isPublic}/>
    <ScrollView style={styles.scrollView} contentContainerStyle={styles.scrollViewContainer}>
      <FlatList
        data={data.todos}
        renderItem={({item}) => <TodoItem item={item} isPublic={this.props.isPublic}/>}
        keyExtractor={(item) => item.id.toString()}
      />
      <LoadOlder
        isPublic={this.props.isPublic}
        styles={styles}
      />
    </ScrollView>
  </View>
)

When this component mounts, the GraphQL query sent in the background may not have been completed. But we need to handle that temporary state of no data and hence we return some useful text during loading state. In this loading state, typically you can do fancy things like displaying a loading spinner.

Now, the query could also end up in an error state due to various reasons. Sometimes the graphql query could be wrong, or the server isn't responding. Whatever may be the reason, the user facing UI should show something to convey that an error has occurred. In this error state, typically you can send these error messages to third-party services to track what went wrong.

All said and done, these are two important states that need to be handled inside your component. What you have written above is basic, but sufficient for this tutorial.