Get an immutable snapshot of a value and its children

[SamyPesse]

When synchronizing a Yjs doc with an external data structure, it'd be great if there was a way to transform a Yjs type into an immutable data structure so that we can implement memoization on it.

For example liveblocks has toImmutable on all its low level types: https://github.com/liveblocks/liveblocks/blob/f4096d55f16c237a4718a5dc7ec5018e4cbdf438/packages/liveblocks-core/src/crdts/AbstractCrdt.ts#L363
It's really useful as a WeakMap can be used to cache transformation from these values to a computed value.

With Yjs, we initially implemented a solution using:

const mutationKeys: WeakMap<YJSInput, number> = new WeakMap();

/**
 * Observe mutations on a Yjs input and return a number that increments on each mutation.
 */
function getYMutationClock(input): number {
    observeMutations(input);
    return mutationKeys.get(input) || 0;
}

function observeMutations(input: YJSInput) {
    if (mutationKeys.has(input)) {
        return;
    }

    mutationKeys.set(input, 0);

    input.observeDeep(() => {
        const current = mutationKeys.get(input) || 0;
        mutationKeys.set(input, current + 1);
    });
}

But it seems to be inconsistent and corrupts the cache.

Any suggestion on alternatives ?