[OOP] Use extended interface instead of base interface as a method return object

2020-02-27T13:49:42.350000Z

Originally created by: scott_cunningham

QueryInterface can be used for two purposes: verify an object implements an interface, and cast the object. For it to work, the object must extend __System.IQueryInterface. Instead of remembering that all FBs must extend, I create a basic "ALL" interface that all FBs implement.

For example, I had a project where I wanted collections (arrays) of objects (because QueryInterface took too much time on my embedded controller for live-time determination)...

Created a base interface all object implement:

INTERFACE IAll EXTENDS __System.IQueryInterface
PROPERTY Instance : STRING

Created a interface for all OD objects:

INTERFACE IOdAll

Example object with automatic instance name!

{attribute 'reflection'}
FUNCTION_BLOCK ClassOdWord IMPLEMENTS IOdAll, IAll
VAR_OUTPUT
   {attribute 'instance-path'}
   {attribute 'noinit'}
   InstanceName   : STRING;   //object's full instance name - automatically
END_VAR

And the collection object:

FUNCTION_BLOCK ClassOdCollection
VAR
   Collection   : ARRAY[1..GVL_COLLECT.MAX_OBJECTS] OF IOdAll;
   ObjNames   : ARRAY[1..GVL_COLLECT.MAX_OBJECTS] OF STRING;
   J      : INT;
   _ArrayOver   : BOOL;
   _BadObj   : INT;
   _DupeObj   : INT;
   _OverObj   : INT;
   _TotalObj   : INT;
END_VAR
METHOD AddObject
VAR_INPUT
   Obj   : IAll; //general object
END_VAR
VAR
   Cast: IOdAll; //object for the collection
END_VAR
//try to cast the object
IF (__QUERYINTERFACE(Obj, Cast)) THEN
   //it is a member!
   // make sure haven't already added this object
   FOR J:= 1 TO _TotalObj DO
      IF Collection[J] = Cast THEN
         // object pointer matches - dupe! do not add it
         _DupeObj:= _DupeObj + 1;
         RETURN;
      END_IF
   END_FOR
   
   IF _TotalObj < GVL_COLLECT.MAX_OBJECTS THEN
      _TotalObj:= _TotalObj + 1;
      Collection[_TotalObj]:= Cast;
      ObjNames[_TotalObj]:= Obj.Instance;
   ELSE
      _ArrayOver:= TRUE;
      _OverObj:= _OverObj + 1;
   END_IF
   
ELSE
   //object was not a member!
   _BadObj:= _BadObj + 1;
END_IF

Some additional discussion can be found here: l viewtopic.php?f=11&t=5908 l .

[OOP] Use extended interface instead of base interface as a method return object

2020-02-27T13:49:42.216000Z

I am new to codesys interfaces and have only done a bit of reading. A couple points come to mind that you may find helpful.

IProduct is a subset of ProductTypeOne. If I understand correctly the _QUERRYINTERFACE macro can be used to cast ProductTypeOne to IProduct. Check help for usage.
An object (FB) may implement multiple interfaces

FUNCTION_BLOCK FactoryOne IMPLEMENTS IFactory, IProduct
VAR_INPUT
END_VAR
VAR_OUTPUT
END_VAR
VAR
   
END_VAR
METHOD getProduct : IProduct
   IF _QUERRYINTERFACE (...) THEN
      getProduct := ...;
   END_IF

[OOP] Use extended interface instead of base interface as a method return object

2020-02-27T13:49:41.998000Z

Originally created by: scott_cunningham

This code will not work:

product : ProductTypeOne := FactoryOne.getProduct()

I think for what you are trying to do, you need to look at the REFERENCE keyword. You want to define product to be of type ProductTypeOne but have it actually be whatever the product is out of FactoryOne.

Try this:

product : REFERENCE TO ProductTypeOne := FactoryOne.getProduct()

But I guess I am not understanding your use of interfaces... for me I understand it this way:

ProductTypeOne implements IProduct, but that doesn't mean ProductTypeOne is an IProduct. FBs can implement multiple interfaces, so whenever you write in code that you want to return an object IProduct, this does not mean the same as a ProductTypeOne.

For example:

INTERFACE IProduct
METHOD ProduceNoise
METHOD FlashLights
INTERFACE IWiFi
METHOD ConnectToWiFi
PROPERTY NeedsUpdate

And now you make the product FB:

FUNCTION BLOCK ProductTypeOne IMPLEMENTS IProduct, IWiFi
METHOD ProduceNoise
METHOD FlashLights
METHOD ConnectToWiFi
PROPERTY NeedsUpdate

So, the FB is required to have methods ProduceNoise and FlashLights, but also has other required features like a property NeedsUpdate and method ConnectToWiFi.

Now you do this:

PROGRAM PLC_PRG
VAR
   Toy1 : ProductTypeOne;
   Obj  : IProduct := Toy1;
END_VAR
Toy1.ProduceNoise(); //valid
Toy1.ConnectToWiFi(); //valid
Obj.ProduceNoise(); //valid
Obj.ConnectToWiFi(); //not valid - does not exist to interface

With REFERENCE TO, you would get:

PROGRAM PLC_PRG
VAR
   Toy1 : ProductTypeOne;
   Obj  : REFERENCE TO ProductTypeOne;
END_VAR
Toy1.ProduceNoise(); //valid
Toy1.ConnectToWiFi(); //valid
Obj.ProduceNoise(); //valid
Obj.ConnectToWiFi(); //valid

[OOP] Use extended interface instead of base interface as a method return object

2020-02-27T13:49:41.868000Z

Yes, I want to return instance of object type ProductTypeOne.

As long as ProductTypeOne implements IProduct, everywhere IProduct is correct also ProductTypeOne is also correct because it can be cast to IProduct.
So if FactoryOne implements IFactory then method getProduct should return anything what implements IProduct.
But in Java I can also specify that method getProduct in FactoryOne can only return object which extends ProductTypeOne (they also implements IProduct). I hope that this is also possible in Codesys.

The benefit of "narrowing return type" is that later I can write without any casting

product : ProductTypeOne := FactoryOne.getProduct()
//instead of
product : IProduct := FactoryOne.getProduct()

and call on that object methods specific to ProductTypeOne

My english is not so very well so maybe I haven't express correctly what I am trying to achieve.

[OOP] Use extended interface instead of base interface as a method return object

2020-02-27T13:49:41.735000Z

Originally created by: scott_cunningham

IProduct is not the same as ProductTypeOne. Therefor the error is correct. You can return an instance of ProductTypeOne with your method (because it implements the interface and it will be cast automatically), but you cannot return the type ProductTypeOne as this is a different object. Maybe you meant to actually return the type ProductTypeOne (FBs are objects!) - but then you don't need the interface IProduct.

You can do this:

FB Factory...
VAR
   MyProduct : ProductTypeOne;
END_VAR
METHOD getProduct : IProduct
getProduct := MyProduct;

[OOP] Use extended interface instead of base interface as a method return object

2020-02-27T13:49:41.577000Z

Hi guys,
I come from "Java world" so OOP is good know to me. But in codesys 3 I have some problems. I think codesys OOP is not so rich as Java and other high-level object oriented languages.

I have created two base interfaces - IProduct and IFactory (with method returning instance of IProduct)

INTERFACE IProduct

INTERFACE IFactory
METHOD getProduct : IProduct
VAR_INPUT
END_VAR

And two Functions Blocks - ProductTypeOne, and FactoryOne which should return instances to ProductTypeOne

FUNCTION_BLOCK ProductTypeOne IMPLEMENTS IProduct
VAR_INPUT
END_VAR
VAR_OUTPUT
END_VAR
VAR
END_VAR

FUNCTION_BLOCK FactoryOne IMPLEMENTS IFactory
VAR_INPUT
END_VAR
VAR_OUTPUT
END_VAR
VAR
END_VAR
METHOD getProduct : ProductTypeOne

Unfortunately, when I try to compile this I get an error: "Interface of overridden method 'GETPRODUCT' of interface 'IFACTORY' doesn't match declaration getProduct"
When I change the return type to the basic one - IProduct it works. I have checked that this work in Java (method declaration with return ProductTypeOne), so maybe it is also possible to do this in Codesys.

I want to mark that FactoryOne return only ProductTypeOne because I plan to use FactoryOne as a base FunctionBlock for FBs which should extends it and they should only return ProductTypeOne - eg. FB FactoryOneA, FactoryOneB,......
Also FactoryTwoA, FactoryTwoB,...... should return only ProcutTypeTwo and so on.

I hope you can point me out how to achieve this in codesys.

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