Totally agree. WeakRef is what we want and WeakValueMap is what I want
to build with it. Sugar would be if the standard provides "an
out-of-the-box WeakValueMap *as well*".
Automatic removal of entries after they became obsolete would be
necessary - be it integrated or via a callback mechanism (i guess the
executor is sth. like that.)
On 30.03.2017 10:45, T.J. Crowder wrote:
Right -- but it's really weak references you're after, right? Because
with weak references, a Map can be a WeakValueMap, but other
structures are also possible. With that proposal's `executor` feature,
the map can even be proactive removing entries for objects that have
become only weakly reachable.
I'd like to see weak references (perhaps not quite that exact
proposal, but along those lines), not just a WeakValueMap. I've only
needed them rarely (your example is a good one), but when I've needed
them, they were pretty much the only option... I don't mind an
out-of-the-box WeakValueMap *as well*, I just don't want it *instead*.
-- T.J. Crowder
On Thu, Mar 30, 2017 at 9:21 AM, Michael Kriegel
<[email protected]
<mailto:[email protected]>> wrote:
I am also still very interested in Weak references / a weak map,
where the values are held weakly. Here is my example: I want to
easily build caches, which lose their values, when there are no
other references to them. Example pseudo code (assuming server
request answers synchronously for simplicity):
var Cache = new WeakValueMap();
function request(Key) {
var CachedResult = Cache.get(Key);
if (CachedResult) return CachedResult;
var UnprocessedResult = doServerRequest(Key);
var BigProcessedObject =
doVeryHeavyComputingTask(UnprocessedResult);
Cache.set(Key,BigProcessedObject);
return BigProcessedObject;
}
Thoughts:
- doHeavyComputingTask takes a long time, so it should be done
only once per key (until it is uncached)
- BigProcessedObject is really big, it shall be in memory at most
once at any time
- When the last reference to a BigProcessedObject is lost, it
shall be removed from cache to save memory
- removal could be delayed by holding a reference to a
BigProcessedObject and releasing it using a setTimeout-Timer.
Without WeakValueMap I have to build my own reference counting, so
everyone who requests the object for a Key has to "unrequest" it
again, else it would provoke a memory leak. This implies, that the
life cycle of an object is tracked, so each object, which has a
reference to a BigProcessedObject needs something like a
destructor. As there is no destructor in JS you have to write a
method and manually call it. Thus each object needs to have a
single known owner or reference counting/tracking has to be done
for that object, too. This propagates through your whole program.
Finally as a developer you end up building your own garbage
collector, which:
- is error prone
- consumes extra memory, as there are already all structures
present for garbage collecting in the JS engine, just it is not usable
- takes up lots of time for development and debugging
So it is a/the classical example case for
https://github.com/tc39/proposal-weakrefs
<https://github.com/tc39/proposal-weakrefs>
On 21.03.2017 08:34, T.J. Crowder wrote:
What are your use cases for it?
Rather than tying it to being a `Map`, I'd prefer to see
something like Java's [`WeakReference`][1]: You could store the
`WeakReference`s in a `Map` if you wanted strongly-referenced
keys with weakly-referenced values.
-- T.J. Crowder
[1]:
http://docs.oracle.com/javase/8/docs/api/java/lang/ref/WeakReference.html
<http://docs.oracle.com/javase/8/docs/api/java/lang/ref/WeakReference.html>
On Tue, Mar 21, 2017 at 6:15 AM, /#!/JoePea <[email protected]
<mailto:[email protected]>> wrote:
Hey all,
I'd like to have something like a WeakMap where the keys can
be primitives and the values are weakly-held Objects, so if
there are no more references to any of the Object values that
the entry gets removed from the map.
For example, it might look like this:
```
{
["foo"] => SomeObject,
["
bar
"] =>
OtherObject
,
}
```
where if there are no more references to `OtherObject`, then
`['bar'] => OtherObject` is removed from the map.
Usage would be very similar to WeakMap, like
```
let m = new ReversedWeakMap
m.add('foo', SomeObject)
m.add('
bar
',
OtherObject
)
console.log(m.get('bar')) // OtherObject
... time passes, no more references to OtherObject,
OtherObject is collected ...
console.log(m.get('bar')) // undefined
```
I thought of using WeakMap values as keys, and vice versa,
but it seems to be impossible.
I guess maybe it is difficult to add this natively because it
would mean that GC needs to be completely deterministic as
far as JS programs go. For example:
```js
let m = new ReversedWeakMap
function main() {
m.set('foo', {})
console.log(m.get('foo')) // {}
}()
main()
// GC would have to be guaranteed to have happened at this point.
console.log(m.get('foo')) // undefined
```
*/#/!//*JoePea
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• www.actifsource.com • +41 56 250 40 02
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