useLazyLoadQuery

useLazyLoadQuery

Hook used to fetch a GraphQL query during render. This hook can trigger multiple nested or waterfalling round trips if used without caution, and waits until render to start a data fetch (when it can usually start a lot sooner than render), thereby degrading performance. Instead, prefer usePreloadedQuery.

const React = require('React');

const {graphql, useLazyLoadQuery} = require('react-relay');

function App() {
  const data = useLazyLoadQuery(
    graphql`
      query AppQuery($id: ID!) {
        user(id: $id) {
          name
        }
      }
    `,
    {id: 4},
    {fetchPolicy: 'store-or-network'},
  );

 return <h1>{data.user?.name}</h1>;
}

Arguments

• query: GraphQL query specified using a graphql template literal.
• variables: Object containing the variable values to fetch the query. These variables need to match GraphQL variables declared inside the query.
• options: [Optional] options object
  • fetchPolicy: Determines if cached data should be used, and when to send a network request based on the cached data that is currently available in the Relay store (for more details, see our Fetch Policies and Garbage Collection guides):
    • "store-or-network": (default) will reuse locally cached data and will only send a network request if any data for the query is missing. If the query is fully cached, a network request will not be made.
    • "store-and-network": will reuse locally cached data and will always send a network request, regardless of whether any data was missing from the local cache or not.
    • "network-only": will not reuse locally cached data, and will always send a network request to fetch the query, ignoring any data that might be locally cached in Relay.
    • "store-only": will only reuse locally cached data, and will never send a network request to fetch the query. In this case, the responsibility of fetching the query falls to the caller, but this policy could also be used to read and operate on data that is entirely local.
  • fetchKey: A fetchKey can be passed to force a re-evaluation of the current query and variables when the component re-renders, even if the variables didn't change, or even if the component isn't remounted (similarly to how passing a different key to a React component will cause it to remount). If the fetchKey is different from the one used in the previous render, the current query will be re-evaluated against the store, and it might be refetched depending on the current fetchPolicy and the state of the cache.
  • networkCacheConfig: [Optional] Default value: {force: true}. Object containing cache config options for the network layer. Note that the network layer may contain an additional query response cache which will reuse network responses for identical queries. If you want to bypass this cache completely (which is the default behavior), pass {force: true} as the value for this option.

Return Value

• data: Object that contains data which has been read out from the Relay store; the object matches the shape of specified query.
  • The Flow type for data will also match this shape, and contain types derived from the GraphQL Schema. For example, the type of data above is: {| user: ?{| name: ?string |} |}.

Behavior

• It is expected for useLazyLoadQuery to have been rendered under a RelayEnvironmentProvider, in order to access the correct Relay environment, otherwise an error will be thrown.
• Calling useLazyLoadQuery will fetch and render the data for this query, and it may suspend while the network request is in flight, depending on the specified fetchPolicy, and whether cached data is available, or if it needs to send and wait for a network request. If useLazyLoadQuery causes the component to suspend, you'll need to make sure that there's a Suspense ancestor wrapping this component in order to show the appropriate loading state.
  • For more details on Suspense, see our Loading States with Suspense guide.
• The component is automatically subscribed to updates to the query data: if the data for this query is updated anywhere in the app, the component will automatically re-render with the latest updated data.
• After a component using useLazyLoadQuery has committed, re-rendering/updating the component will not cause the query to be fetched again.
  • If the component is re-rendered with different query variables, that will cause the query to be fetched again with the new variables, and potentially re-render with different data.
  • If the component unmounts and remounts, that will cause the current query and variables to be refetched (depending on the fetchPolicy and the state of the cache).

Differences with QueryRenderer

• useLazyLoadQuery no longer takes a Relay environment as a parameter, and thus no longer sets the environment in React Context, like QueryRenderer did. Instead, useLazyLoadQuery should be used as a descendant of a RelayEnvironmentProvider, which now sets the Relay environment in Context. Usually, you should render a single RelayEnvironmentProvider at the very root of the application, to set a single Relay environment for the whole application.
• useLazyLoadQuery will use Suspense to allow developers to render loading states using Suspense boundaries, and will throw errors if network errors occur, which can be caught and rendered with Error Boundaries. This as opposed to providing error objects or null props to the QueryRenderer render function to indicate errors or loading states.
• useLazyLoadQuery fully supports fetch policies in order to reuse data that is cached in the Relay store instead of solely relying on the network response cache.
• useLazyLoadQuery has better type safety guarantees for the data it returns, which was not possible with QueryRenderer since we couldn't parametrize the type of the data with a renderer api.

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