Invalid Interface reference
I use an interface to provide some functionality to another class/module. During shutdown of my
application, the original object/form is destroyed beofre the module with the interface reference.
How should I solve this problem? There may exist several TBSShapeUI objects using the TFViewMap. And
since these objects are created dynamically I have not full control about the destroy order.
cu Christian
Here are the fragments of my code:
type
IBSShapeViewMap = interface
// some methodes
end;
TBSShapeUI = class(TDataModule)
strict private
FViewMap : IBSShapeViewMap
public
constructor Create(const AViewMap: IBSShapeViewMap); reintroduce;
end;
TFViewMap = class(TForm, IBSShapeViewMap)
end;
constructor TBSShapeUI.Create(const AViewMap: IBSShapeViewMap);
begin
inherited Create(nil);
FViewMap := AViewMap;
end;
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Christian
8/5/2015 2:27:13 PM
embarcadero.delphi.general
4258 articles. 
0 followers.
2 Replies
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Christian wrote: > I use an interface to provide some functionality to another > class/module. During shutdown of my application, the original > object/form is destroyed beofre the module with the interface > reference. TDataModule and TForm both derive from TComponent. TComponent disables reference counting on itself when its VCLComObject property is nil. So, if you create a TForm that implements an interface, the interface's reference count will have no effect and thus cannot be used to extend the TForm lifetime beyond the TDataModule's lifetime. > How should I solve this problem? You can either: 1. fix your destroy order so the TBSShapeUI objects are destroyed before the TFViewMap object. Or at least reset their IBSShapeViewMap interface references to nil before the TFViewMap object is destroyed. 2. make use of the TFViewMap.FreeNotification() method so you can detect when it is being destroyed and update the TBSShapeUI objects accordingly so they don't try to access it anymore. 3. Assign an IVCLComObject interface to the TFViewMap.VCLComObject property to re-enable its reference counting. You can either: a. create your own class that implements IVCLComObject, have it free the TFViewMap object in its destructor, and then you can create and assign an object of that class to the TFViewMap.VCLComObject property. b. read the TFViewMap.ComObject property, letting its getter method create its own IVCLComObject object internally for itself. You can add the System.Win.VCLCom unit to your uses clause to use the VCL's default creation factory, or you can assign your own function to the System.Classes.CreateVCLComObjectProc callback. It takes a TComponent as input and returns an IVCLComObject interface as output. In the case of #3, the TFViewMap might still get freed first, if it is TApplication that is freeing it while TBSShapeUI objects are still alive. Which goes back to #1. When you have objects that depend on a particular destruction order, you need to be more active about how the objects are getting destroyed, you cannot always rely on implicit destructions doing the right thing, at least when TComponent is involved anyway. -- Remy Lebeau (TeamB)
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Remy
8/5/2015 6:19:37 PM
>TDataModule and TForm both derive from TComponent. TComponent disables reference >counting on itself when its VCLComObject property is nil. So, if you create >a TForm that implements an interface, the interface's reference count will >have no effect and thus cannot be used to extend the TForm lifetime beyond >the TDataModule's lifetime. Yes I'm aware of that. We use that for some of our own classes as well. >You can either: Thanks for the suggestions. I thought, if I would need only one method, I could pass a "reference to procedure" but I need about 3-4 methods from TFViewMap. An abstract adapter class with a subclass of it in TFViewMap and then pass an instance of this class would be a solution as well. More coding, but then I have no ref counting in TBSShapeUI and can just free the adapter object in the Destroy method. cu Christian
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Christian
8/5/2015 8:30:12 PM
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