next/dist/esm/client/components/bfcache.js

The React Framework

import { useState } from 'react';
// When the flag is disabled, only track the currently active tree
const MAX_BF_CACHE_ENTRIES = process.env.__NEXT_ROUTER_BF_CACHE ? 3 : 1;
/**
 * Keeps track of the most recent N trees (FlightRouterStates) that were active
 * at a certain segment level. E.g. for a segment "/a/b/[param]", this hook
 * tracks the last N param values that the router rendered for N.
 *
 * The result of this hook precisely determines the number and order of
 * trees that are rendered in parallel at their segment level.
 *
 * The purpose of this cache is to we can preserve the React and DOM state of
 * some number of inactive trees, by rendering them in an <Activity> boundary.
 * That means it would not make sense for the the lifetime of the cache to be
 * any longer than the lifetime of the React tree; e.g. if the hook were
 * unmounted, then the React tree would be, too. So, we use React state to
 * manage it.
 *
 * Note that we don't store the RSC data for the cache entries in this hook —
 * the data for inactive segments is stored in the parent CacheNode, which
 * *does* have a longer lifetime than the React tree. This hook only determines
 * which of those trees should have their *state* preserved, by <Activity>.
 */ export function useRouterBFCache(activeTree, activeStateKey) {
    // The currently active entry. The entries form a linked list, sorted in
    // order of most recently active. This allows us to reuse parts of the list
    // without cloning, unless there's a reordering or removal.
    // TODO: Once we start tracking back/forward history at each route level,
    // we should use the history order instead. In other words, when traversing
    // to an existing entry as a result of a popstate event, we should maintain
    // the existing order instead of moving it to the front of the list. I think
    // an initial implementation of this could be to pass an incrementing id
    // to history.pushState/replaceState, then use that here for ordering.
    const [prevActiveEntry, setPrevActiveEntry] = useState(()=>{
        const initialEntry = {
            tree: activeTree,
            stateKey: activeStateKey,
            next: null
        };
        return initialEntry;
    });
    if (prevActiveEntry.tree === activeTree) {
        // Fast path. The active tree hasn't changed, so we can reuse the
        // existing state.
        return prevActiveEntry;
    }
    // The route tree changed. Note that this doesn't mean that the tree changed
    // *at this level* — the change may be due to a child route. Either way, we
    // need to either add or update the router tree in the bfcache.
    //
    // The rest of the code looks more complicated than it actually is because we
    // can't mutate the state in place; we have to copy-on-write.
    // Create a new entry for the active cache key. This is the head of the new
    // linked list.
    const newActiveEntry = {
        tree: activeTree,
        stateKey: activeStateKey,
        next: null
    };
    // We need to append the old list onto the new list. If the head of the new
    // list was already present in the cache, then we'll need to clone everything
    // that came before it. Then we can reuse the rest.
    let n = 1;
    let oldEntry = prevActiveEntry;
    let clonedEntry = newActiveEntry;
    while(oldEntry !== null && n < MAX_BF_CACHE_ENTRIES){
        if (oldEntry.stateKey === activeStateKey) {
            // Fast path. This entry in the old list that corresponds to the key that
            // is now active. We've already placed a clone of this entry at the front
            // of the new list. We can reuse the rest of the old list without cloning.
            // NOTE: We don't need to worry about eviction in this case because we
            // haven't increased the size of the cache, and we assume the max size
            // is constant across renders. If we were to change it to a dynamic limit,
            // then the implementation would need to account for that.
            clonedEntry.next = oldEntry.next;
            break;
        } else {
            // Clone the entry and append it to the list.
            n++;
            const entry = {
                tree: oldEntry.tree,
                stateKey: oldEntry.stateKey,
                next: null
            };
            clonedEntry.next = entry;
            clonedEntry = entry;
        }
        oldEntry = oldEntry.next;
    }
    setPrevActiveEntry(newActiveEntry);
    return newActiveEntry;
}

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