Weakly holding and circular references

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Weakly holding and circular references

Tm Jhnsn
Hi all,

I've tried to learn how to properly use Weak collections for purposes of
circular references, but I'm not sure I fully understand -- not enough
to actually use it in my implementation.

I made a fairly large project a couple of years ago where I ended up
with a lot of stale information in the image because I had circular
references keeping objects from being cleared by GC.  I learned my
lesson, and addressed the situation by building a 'release' mechanism
and explicitly tearing down objects when I was done with them.

 From what I understand, the Weak collection hierarchy will keep
references to objects but not in such a way that the GC won't clear them.

If I have, say, an ArchiveItem object, and the ArchiveItem has
ArchivedFiles, but I want ArchivedFiles to know which ArchiveItem they
belong to, then I've set myself up with a circular reference situation.

I understand I could have a separate object which allows ArchivedFiles
to *ask* which ArchiveItem they belong to, instead of storing it within
themselves.

But I also understand that I could potentially use, say, a WeakSet in
ArchiveItem, which will hold weakly onto ArchivedFiles since the
ArchivedFiles themselves refer back to the ArchiveItem. I get the
impression that this approach would free me from having to build out a
complete 'release' mechanism or from doing other explicit clearing and
removal of references.  But I don't understand if I could find myself in
a situation where an ArchiveItem thinks it doesn't have any
ArchivedFiles because they got GCed before I was truly through with them.

Can anyone recommend any approach or any materials on this subject?

Thanks,
Tim


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Re: Weakly holding and circular references

Ron Teitelbaum
Hi Tim,

Thanks for your email.  I believe you may be thinking of this backward.  The piece you want to pay careful attention is where the strong references are.  Objects will be cleared when they have no more strong references.  What is worse than having an object stick around is having it GC'd because there is nothing left to hold onto it.  Also, it doesn't matter how many connections the objects have to each other if they are disconnected from the running object tree they go poof (or should).

There are a lot of ways to hold onto objects but it's usually part of a model that is referenced when you start your program running.  Things attached to that model stay around.  You can also hold onto objects by using class variables or something else more permanent, but this usually is not necessary and is better left for static objects like something that configures your application.  

If you have an object with a circular reference and it's not held on to by anything else it all goes poof.  

The question you have to answer when you have this situation is what's holding onto my object?  You can use the pointer finder.  On an instance of your class (MyClass allInstances) right click and select, Explore pointers.

You will probably find an object in your model that is still holding onto it.  It's that pointer you need to remove.  Having a method like #delete that tears down all the references to your model is the right way to go.  When you find more pointers you forgot about when you designed your model you add a method to break the pointers there.  

There are places where a WeakArray is useful.  Usually when you need to reference something that might later go away.  Something like a background process is a good example.  Say you fork a method and want to know if that process is still running.  You can hold onto it with a WeakArray and then check to see if that process is still in the collection.  When the process stops it will get GC'd and removed from the WeakArray or it may still be there but marked as Terminated (until it's GC'd later).  

You are much better off designing your model with strong references in mind in a way that makes it easy to understand what objects are still active and which are not.  Removing connections to objects that are no longer needed is easier when you design the model that way.  Use WeakArrays only when you are holding onto objects that you know are disappearing, or being modified by something else that later removes pointers, like a Cache.  If you need to hold onto the array for some reason you don't want that array to accidentally hold all of your objects.  This is not the usual case and in most cases what you want is a regular Collection.

Does that help?

All the best,

Ron Teitelbaum
Chief Executive Officer
3D Immersive Collaboration Consulting
[hidden email]
Follow Me On Twitter: @RonTeitelbaum
www.3Dicc.com
https://www.google.com/+3Dicc


On Sun, Apr 29, 2018 at 9:35 AM, Tm Jhnsn <[hidden email]> wrote:
Hi all,

I've tried to learn how to properly use Weak collections for purposes of circular references, but I'm not sure I fully understand -- not enough to actually use it in my implementation.

I made a fairly large project a couple of years ago where I ended up with a lot of stale information in the image because I had circular references keeping objects from being cleared by GC.  I learned my lesson, and addressed the situation by building a 'release' mechanism and explicitly tearing down objects when I was done with them.

From what I understand, the Weak collection hierarchy will keep references to objects but not in such a way that the GC won't clear them.

If I have, say, an ArchiveItem object, and the ArchiveItem has ArchivedFiles, but I want ArchivedFiles to know which ArchiveItem they belong to, then I've set myself up with a circular reference situation.

I understand I could have a separate object which allows ArchivedFiles to *ask* which ArchiveItem they belong to, instead of storing it within themselves.

But I also understand that I could potentially use, say, a WeakSet in ArchiveItem, which will hold weakly onto ArchivedFiles since the ArchivedFiles themselves refer back to the ArchiveItem. I get the impression that this approach would free me from having to build out a complete 'release' mechanism or from doing other explicit clearing and removal of references.  But I don't understand if I could find myself in a situation where an ArchiveItem thinks it doesn't have any ArchivedFiles because they got GCed before I was truly through with them.

Can anyone recommend any approach or any materials on this subject?

Thanks,
Tim


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Re: Weakly holding and circular references

Ben Coman
In reply to this post by Tm Jhnsn


On 29 April 2018 at 21:35, Tm Jhnsn <[hidden email]> wrote:
Hi all,

I've tried to learn how to properly use Weak collections for purposes of circular references, but I'm not sure I fully understand -- not enough to actually use it in my implementation.

I made a fairly large project a couple of years ago where I ended up with a lot of stale information in the image because I had circular references keeping objects from being cleared by GC.  I learned my lesson, and addressed the situation by building a 'release' mechanism and explicitly tearing down objects when I was done with them.

Note that since the VM uses a tracing garbage collector rather than reference counting, circular references *only* between objects 'a' and 'b' will not in themselves prevent them both being garbage collected.  They will only stick around if one of them has a chain of references through from a root object.

cheers -ben 

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