swr/esm/use-swr-pages.js

import React, { useCallback, useMemo, useState, useRef } from 'react';
import { cacheGet, cacheSet } from './config';
/*
The idea

A "Page" component renders the content of 1 API request, it accepts an offset (in this example it's from),
uses a SWR hook (useSWR(API + '?limit=' + limit + '&from=' + from)) and returns items (Projects).

The UI:
      +------------------------------------------+
      |   Projects                               |
+------------------------------------------------------+
|     |   +----------------+                     |     |
|     |                                          |     |
|     |   +------------+                         |     |
|     |                                          |     +--> 1 Page
|     |   +-----------------+                    |     |
|     |                                          |     |  /projects/list?limit=4
|     |   +---------+                            |     |
+------------------------------------------------------+
      |                                          |
      |   +------------+                         |     +  /projects/list?limit=4&from=123
      |                                          |     |
      |   +----------------+                     |     |
      |                                          |     |
      |   +---------+                            |     |
      |                                          |     |
      |   +--------------+                       |     +
      |                                          |
      |   +-------------------+                  |     +  /projects/list?limit=4&from=456
      |                                          |     |
      |   +------------+                         |     |
      |                                          |     |
      |   +----------------+                     |     |
      |                                          |     |
      |                                          |     +

The API
// (inside `render`)

function App () {
  const {
    pages,    // an array of each page component
    pageSWRs, // an array of SWRs of each page
    isLoadingMore,
    isReachingEnd,
    isEmpty,
    loadMore
  } = useSWRPages(
    'project-page', // key of this page

    // ======== the actual Page component!
    ({ offset, withSWR }) => {
      // required: use `withSWR` to wrap your main SWR (source of your pagination API)
      const { data } = withSWR(
        useSWR(API + '?limit=10&from=' + offset) // request projects with offset
      )
      if (!data) return <Placeholder>
      return data.projects.map(project => <Card project={project} team={team}>)
    },
    // ========

    // a function accepts a SWR's `data`, and returns the offset of the next page (or null)
    data => data && data.length >= 10 ? data[data.length - 1].createdAt : null,

    // (optional) outside deps of your Page component. in this case it's empty
    []
  )

  // ...

  if (isEmpty) return <EmptyProjectsPage/>

  return <div>
    {pages}
    {isReachingEnd
      ? null
      : <button loading={isLoadingMore} onClick={loadMore}>Load More</button>}
  </div>
}
*/
export function useSWRPages(pageKey, pageFn, SWRToOffset, deps = []) {
    const pageCountKey = `_swr_page_count_` + pageKey;
    const pageOffsetKey = `_swr_page_offset_` + pageKey;
    const [pageCount, setPageCount] = useState(cacheGet(pageCountKey) || 1);
    const [pageOffsets, setPageOffsets] = useState(cacheGet(pageOffsetKey) || [null]);
    const [pageSWRs, setPageSWRs] = useState([]);
    const pageCacheRef = useRef([]);
    const pageFnRef = useRef(pageFn);
    const emptyPageRef = useRef(false);
    // Page component (wraps `pageFn`)
    // for performance reason we need to memorize it
    const Page = useCallback(props => {
        // render the page component
        const dataList = pageFnRef.current(props);
        // if dataList is [], we can assume this page is empty
        // TODO: this API is not stable
        if (dataList && !dataList.length) {
            emptyPageRef.current = true;
        }
        else {
            emptyPageRef.current = false;
        }
        return dataList;
    }, []);
    // Doesn't have a next page
    const isReachingEnd = pageOffsets[pageCount] === null;
    const isLoadingMore = pageCount === pageOffsets.length;
    const isEmpty = isReachingEnd && pageCount === 1 && emptyPageRef.current;
    const loadMore = useCallback(() => {
        if (isLoadingMore || isReachingEnd)
            return;
        setPageCount(c => {
            cacheSet(pageCountKey, c + 1);
            return c + 1;
        });
    }, [isLoadingMore || isReachingEnd]);
    const _pageFn = useCallback(pageFn, deps);
    pageFnRef.current = _pageFn;
    const pages = useMemo(() => {
        const getWithSWR = id => swr => {
            if (!pageSWRs[id] ||
                pageSWRs[id].data !== swr.data ||
                pageSWRs[id].error !== swr.error ||
                pageSWRs[id].revalidate !== swr.revalidate) {
                setPageSWRs(swrs => {
                    const _swrs = [...swrs];
                    _swrs[id] = {
                        data: swr.data,
                        error: swr.error,
                        revalidate: swr.revalidate,
                        isValidating: swr.isValidating
                    };
                    return _swrs;
                });
                if (typeof swr.data !== 'undefined') {
                    // set next page's offset
                    const newPageOffset = SWRToOffset(swr, id);
                    if (pageOffsets[id + 1] !== newPageOffset) {
                        setPageOffsets(arr => {
                            const _arr = [...arr];
                            _arr[id + 1] = newPageOffset;
                            cacheSet(pageOffsetKey, _arr);
                            return _arr;
                        });
                    }
                }
            }
            return swr;
        };
        // render each page
        const p = [];
        const pageCache = pageCacheRef.current;
        for (let i = 0; i < pageCount; ++i) {
            if (!pageCache[i] ||
                pageCache[i].offset !== pageOffsets[i] ||
                pageCache[i].pageFn !== _pageFn) {
                // when props change or at init
                // render the page and cache it
                pageCache[i] = {
                    component: (React.createElement(Page, { key: `page-${pageOffsets[i]}-${i}`, offset: pageOffsets[i], withSWR: getWithSWR(i) })),
                    pageFn: _pageFn,
                    offset: pageOffsets[i]
                };
            }
            p.push(pageCache[i].component);
        }
        return p;
    }, [_pageFn, pageCount, pageSWRs, pageOffsets, pageKey]);
    return {
        pages,
        pageCount,
        pageSWRs,
        isLoadingMore,
        isReachingEnd,
        isEmpty,
        loadMore
    };
}