Aggregation and UserCounter

[DanielSchick]

Hej Matthias,

It's me again with a question concerning the User Counter of G# objects.

I have the following situation:

A controller objects holds an array of stage objects as one of its properties.

So I can access each stage object over the controller object.

When the controller object is first created also the stages are created the first time in the contructor of the controller object.

Now I am working with the controller and stages in multiple applications. When I create the controller object its user counter updates accordingly to the number of applications which uses the controller object. The controller is implemented as Singleton and the stages as Named Singleton (Fewton).

The Problem is now:

The stages are available over the property of the controller object when the controller objects is created again in a new appliacation. But the user counters of these stage objects are not updated! So each stage's user counter stays at 1, independent of the number of applications the stage is used! When it comes to destruction in one of the applications all stage objects are destroyed although they should be still accessable as the property of the controller objects in the remaining applications.

Solutions:

I just wanna hear your opinion about that problem before I would start to hack my code.

But probably the user counter is not updated because the stage objects are not newly created when the controller object is created.

So I could write a workaround to implement the creation of the stage objects.

But wouldn't it be preferrable to have the user counter updated automatically by G# or is there a good reason againt that???

Looking forward to your enlighting answer

Regards,

Daniel

[Mattias_Ericsson]

Hi Daniel,

I'm not absolutely sure I understand your problem (as usual...). First, let me explain the user counter. The user counter only works if you are using named object, if you let the object name unwired or an empty string, you will always get a new object and user counter information will always be 1. This part of G# is different from Java or C#, since named objects are not native in these languages, you have to use fewton (multiton) design pattern to do this, but in Java or C#, the garbage collector isn't working very well with this design pattern. However, in LabVIEW and G# it is different and the named objects are inspired from how queues work and I think this is a really nice and useful feature of G#.

In your example, you have a controller that aggregates stage objects stored in an array. The Controller is set as a fewton (aka multiton), where you wire an object name so that you could look up an existing controller object. However, if this object aggregates other objects (like your stage objects) and you create these stage objects within the controller constructor (this aggregation is called composite aggregation). If these stage objects are unnamed you will create a new stage object each time you make a lookup of the controller object, actually overwriting the already stored stage references. In order to solve this, you have to put a case structure around the creating of the stage object, using the "created new?" flag that is returned from the "New" method. As an alternative you could also use named stage objects. I'm not sure if your stage objects are named or not.

I attach an example. I have created a stage class and two controllers with different solutions of the aggregation of stage objects.

Controller - this class only creates the stage objects when the controller object is first created. Stage objects are unnamed.

Controller2 - this class used named stage objects, make the stage user counter increase.

I usually use the "Controller" implementation, checking the "created new?" flag if the composite aggregates should be created or not. Please check the Destructor implementation as well. Very often I also name aggregated objects, even if I don't use the user counter, since named objects are presented really nice in the G#Debugger making it much easier to trace and identify object there. Therefore you could use a combination of Controller and Controller2 alternatives.

- Mattias

http://www.addq.se/document/Community/UserCounterExample.zip