Infinite Queries

Rendering lists that can additively "load more" data onto an existing set of data or "infinite scroll" is also a very common UI pattern. Svelte Query supports a useful version of useQuery called useInfiniteQuery for querying these types of lists.

When using useInfiniteQuery, you'll notice a few things are different:

• data is now an object containing infinite query data:
• data.pages array containing the fetched pages
• data.pageParams array containing the page params used to fetch the pages
• The fetchNextPage and fetchPreviousPage functions are now available
• The getNextPageParam and getPreviousPageParam options are available for both determining if there is more data to load and the information to fetch it. This information is supplied as an additional parameter in the query function (which can optionally be overridden when calling the fetchNextPage or fetchPreviousPage functions)
• A hasNextPage boolean is now available and is true if getNextPageParam returns a value other than undefined.
• A hasPreviousPage boolean is now available and is true if getPreviousPageParam returns a value other than undefined.
• The isFetchingNextPage and isFetchingPreviousPage booleans are now available to distinguish between a background refresh state and a loading more state

Example

Let's assume we have an API that returns pages of projects 3 at a time based on a cursor index along with a cursor that can be used to fetch the next group of projects:

1 fetch('/api/projects?cursor=0')
2 // { data: [...], nextCursor: 3}
3 fetch('/api/projects?cursor=3')
4 // { data: [...], nextCursor: 6}
5 fetch('/api/projects?cursor=6')
6 // { data: [...], nextCursor: 9}
7 fetch('/api/projects?cursor=9')
8 // { data: [...] }

With this information, we can create a "Load More" UI by:

• Waiting for useInfiniteQuery to request the first group of data by default
• Returning the information for the next query in getNextPageParam
• Calling fetchNextPage function

Note: It's very important you do not call fetchNextPage with arguments unless you want them to override the pageParam data returned from the getNextPageParam function. eg. Do not do this: <button onClick={fetchNextPage} /> as this would send the onClick event to the fetchNextPage function.

1 <script>
2   import { useInfiniteQuery } from '@sveltestack/svelte-query'
3 
4   const fetchProjects = async ({ pageParam = 0 }) => {
5     const { data } = await axios.get(`/projects?cursor=${pageParam}`)
6     return data
7   }
8 
9   const queryResult = useInfiniteQuery('projects', fetchProjects, {
10     getNextPageParam: lastGroup => lastGroup.nextId || undefined,
11   })
12 </script>
13 
14 {#if $queryResult.status === 'loading'}
15   Loading...
16 {:else if $queryResult.status === 'error'}
17   <span>Error: {$queryResult.error.message}</span>
18 {:else}
19   <div>
20     {#each $queryResult.data.pages as page}
21       {#each page.data as project}
22         <p>{project.name}</p>
23       {/each}
24     {/each}
25   </div>
26   <div>
27     <button
28       on:click={() => $queryResult.fetchNextPage()}
29       disabled={!$queryResult.hasNextPage || $queryResult.isFetchingNextPage}>
30       {#if $queryResult.isFetching}
31         Loading more...
32       {:else if $queryResult.hasNextPage}
33         Load More
34       {:else}Nothing more to load{/if}
35     </button>
36   </div>
37 {/if}

What happens when an infinite query needs to be refetched?

When an infinite query becomes stale and needs to be refetched, each group is fetched sequentially, starting from the first one. This ensures that even if the underlying data is mutated we're not using stale cursors and potentially getting duplicates or skipping records. If an infinite query's results are ever removed from the queryCache, the pagination restarts at the initial state with only the initial group being requested.

What if I need to pass custom information to my query function?

By default, the variable returned from getNextPageParam will be supplied to the query function, but in some cases, you may want to override this. You can pass custom variables to the fetchNextPage function which will override the default variable like so:

1 <script>
2   import { useInfiniteQuery } from '@sveltestack/svelte-query'
3 
4   const fetchProjects = async ({ pageParam = 0 }) => {
5     const { data } = await axios.get(`/projects?cursor=${pageParam}`)
6     return data
7   }
8 
9   const queryResult = useInfiniteQuery('projects', fetchProjects, {
10     getNextPageParam: lastGroup => lastGroup.nextId || undefined,
11   })
12 
13   // Pass your own page param
14   const skipToCursor50 = () => $queryResult.fetchNextPage({ pageParam: 50 })
15 </script>

What if I want to implement a bi-directional infinite list?

Bi-directional lists can be implemented by using the getPreviousPageParam, fetchPreviousPage, hasPreviousPage and isFetchingPreviousPage properties and functions.

1 useInfiniteQuery('projects', fetchProjects, {
2   getNextPageParam: (lastPage, pages) => lastPage.nextCursor,
3   getPreviousPageParam: (firstPage, pages) => firstPage.prevCursor,
4 })

What if I want to show the pages in reversed order?

Sometimes you may want to show the pages in reversed order. If this is case, you can use the select option:

1 useInfiniteQuery('projects', fetchProjects, {
2   select: data => ({
3     pages: [...data.pages].reverse(),
4     pageParams: [...data.pageParams].reverse(),
5   }),
6 })

What if I want to manually update the infinite query?

Manually removing first page:

1 queryClient.setQueryData('projects', data => ({
2   pages: data.pages.slice(1),
3   pageParams: data.pageParams.slice(1),
4 }))

What if I want to enable/disable infinite queries reactively?

Sometimes you may want to enable or disable a Query by using a svelte reactive statement. In this case, you can use the setEnabled function:

1 <script>
2   import { useInfiniteQuery } from '@sveltestack/svelte-query'
3 
4   export let isEnabled = false;
5 
6   const queryResult = useInfiniteQuery('projects', fetchProjects, {
7     getNextPageParam: lastGroup => lastGroup.nextId || undefined,
8   })
9 
10   $: queryResult.setEnabled(isEnabled)
11 </script>
12 
13 {#if $queryResult.status === 'idle'}
14   <button on:click={() => isEnabled = true}>Enable</button>
15 {#if $queryResult.status === 'loading'}
16   Loading...
17 {:else if $queryResult.status === 'error'}
18   <span>Error: {$queryResult.error.message}</span>
19 {:else}
20   <div>
21     {#each $queryResult.data.pages as page}
22       {#each page.data as project}
23         <p>{project.name}</p>
24       {/each}
25     {/each}
26   </div>
27   <div>
28     <button
29       on:click={() => $queryResult.fetchNextPage()}
30       disabled={!$queryResult.hasNextPage || $queryResult.isFetchingNextPage}>
31       {#if $queryResult.isFetching}
32         Loading more...
33       {:else if $queryResult.hasNextPage}
34         Load More
35       {:else}Nothing more to load{/if}
36     </button>
37   </div>
38 {/if}