On Tue, Nov 10, 2020 at 4:54 AM jres...@gmail.com <jresn...@gmail.com> wrote:
>
> I would like to cache Persistent handles in such a way that I can retrieve
> the same Persistent instance given the identical JS object, i.e. imagine a
> function like so:
>
> Persistent<Object> GetPersistentForObject(Local<Object> obj)
> {
> // If we have already created a Persistent for obj, find and return it.
> // Otherwise, create and return a Persistent for obj and cache it for
> // future calls.
> }
>
> The obvious thought is to store the Persistent handles in a map of some kind,
> but what to use as the key? I assume it would not be possible to actually use
> Local handles as keys in a map that outlives the current HandleScope.
>
> One thought I had was to assign a unique integer to the object using
> obj.SetPrivate(...), and use that as the key, but SetPrivate is documented as
> experimental, which is discouraging.
>
> Is there an easier way to achieve what I'm trying to do? I feel like I may be
> missing something obvious...
>
> Jonathan
A hashmap should work if you observe the following;
1. Use `v8::Object::GetIdentityHash()` to compute the hash key.
2. Use `Persistent::operator==()` to detect duplicates (also has an
overload that compares against a Local)
I.e., step 1 is to find the right bucket and step 2 is to find out if
that bucket already contains the object.
Using just the identity hash isn't sufficient, see [1] and [2] for discussion.
Caveat: use `v8::Value::SameValue()` - not `Persistent::operator==()`
- if you also store primitives (numbers, strings, etc.)
[1] https://github.com/denoland/deno/pull/7946#discussion_r504192165
[2] https://github.com/denoland/deno/issues/7969
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