I see the points in that thread, but I am not sure, whether I completely
understand them. I can imagine, that there is an issue, because gc is
somewhat unpredictable. But shouldn't it be predictable, whether there
are still "non-weak" references to an object? So my intention slightly
differs from what WeakSet/WeakMap (and WeakReference would) do: whether
an object is in the SuperWeakSet (or a key in the SuperWeakMap, or a
SuperWeakReference still points to the object) does NOT depend on,
whether the object has actually been gc'ed, but whether the object does
have "non-weak" references to it.
I am not sure, which kinds of garbage collectors the implementations of
ECMAScript typically use. If they used a reference counter, which counts
"non-weak" references only, a simple check, whether the reference count
is 0 or not on access to a weak reference would be sufficient.
Of course, when I iterate through a WeakSet (respectively the keys of a
WeakMap), in the beginning of the iteration, a keys-array needs to be
retrieved, which is an array of all objects in the weak set, which have
reference count > 0 at that point in time. As this array then holds hard
references to all key objects, the key array cannot be changed by gc
during iteration.
On 13.07.2016 15:14, Bradley Meck wrote:
This thread from 2011 might help understand why they should not be
enumerable (it links to another thread as well inside).
https://mail.mozilla.org/pipermail/es-discuss/2011-April/013604.html
On Wed, Jul 13, 2016 at 8:08 AM, Isiah Meadows <[email protected]
<mailto:[email protected]>> wrote:
Could you explain a real world use case for this? I recall some
that it would've simplified some, but the best implementation
often didn't even require iteration, but just changing the data
layout altogether.
On Wed, Jul 13, 2016, 08:13 Michael Kriegel
<[email protected]
<mailto:[email protected]>> wrote:
Hi everyone,
not sure, whether this is the right place for discussing a feature
request - and how to find out, whether this was already
proposed/discussed before and with which result... My google
search did
not bring up anything about it.
On my opinion it should be made possible to iterate over all
elements in
a WeakSet & WeakMap. I already found many cases, in which I'd
like to
have used the neat feature of "garbage collect the object, if
it is not
referred to by others anymore", but with the necessity to
know, which
objects are in the set (not only, if a given object is). An
example:
Registering callbacks to objects:
class Foo {
onBla(Handler) {
this.BlaSuperWeakMap.push(Handler);
}
someOtherMethod() {
for (let X in this.BlaSuperWeakMap) {
X();
}
}
constructor() {
this.BlaSuperWeakMap = new SuperWeakMap();
}
}
const MyFoo = new Foo();
{
let MyHandler1 = function(){
console.log('MyHandler1 called');
};
MyFoo.onBla(MyHandler1);
console.log('Invokation1:');
MyFoo.someOtherMethod();
}
let MyHandler2 = function(){
console.log('MyHandler2 called');
};
MyFoo.onBla(MyHandler2);
MyFoo.onBla(function(){
console.log('MyHandler3 called');
});
console.log('Invokation2:');
MyFoo.someOtherMethod();
MyHandler2 = undefined;
console.log('Invokation3:');
MyFoo.someOtherMethod();
Expected result:
Invokation1:
MyHandler1 called
Invokation2:
MyHandler2 called
Invokation3:
Basically the example is: Invoke the callback of all objects
which still
"exist" (in the meaning of having "non-weak" references
towards them).
I know this may have some caveats. It would be necessary to
check the
reference count of the object in the SuperWeakSet, because the
object
may have no "non-weak" references left but still was not yet
garbage
collected. I do not know, whether current
ECMAScript-Implementations and
their garbage collecting solutions could handle this easily -
basically
it is the same decision, which the garbage collector would
make: If an
object can be disposed, because it has no non-Weak references
pointing
to it, the object is by definition not in the WeakSet anymore. And
implementations could in that case even give a hint to the garbage
collector, which may improve performance of applications,
which use the
SuperWeakSet alot.
Greetings, Michael
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