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Implementation of an Interface

Implementation of an Interface

Implementing interfaces is based on the concept of object-oriented programming. With common interfaces, you can use different but similar function blocks the same way.

A function block that implements an interface has to include all methods and properties that are defined in that interface (interface methods and interface properties). This means that the name and the inputs and outputs of the methods or attributes must be exactly the same.

When you create a new function block which implements an interface, CODESYS automatically adds all methods and attributes of the interface to the tree below the new function block.

Important

If you add more interface methods afterwards, then CODESYS does not automatically add these methods to the affected function block. To perform this update, you must execute the Implement Interfaces command explicitly.

For inherited function blocks, you have to make sure that any methods or attributes that were derived through the "inheritance" of an interface also receive the appropriate implementation. Otherwise they should be deleted in case the implementation that was provided in the basis should be used. Respective compile error messages or warnings are displayed, prompted automatically by added pragma attributes. For more information, see the help page for the Implementing Interfaces command.

Important

  • You must assign the interface of a function block to a variable of the interface type before a method can be called via the variable.

  • A variable of the interface type is always a reference of the assigned function block instance.

A variable of the interface type is a reference to instances of function blocks. This kind of variable can refer to every function block that implements the interface. If there is no assignment to a variable, then the variable in online mode contains the value 0.

Example 30. Examples

The I1 interface contains the GetName method.

METHOD GetName : STRING

The functions blocks A and B implements the interface I1:

FUNCTION_BLOCK A IMPLEMENTS I1
FUNCTION_BLOCK B IMPLEMENTS I1

For this reason, both function blocks must include a method named GetName and the return type STRING. Otherwise the compiler reports an error.

A function includes the declaration of a variable of interface I1 type.

FUNCTION DeliverName : STRING
VAR_INPUT
    l_i : I1;
END_VAR

Function blocks that implement the I1 interface can be assigned to these input variables.

Examples of function calls:

DeliverName(l_i := A_instance); // call with instance of type A
DeliverName(l_i := B_instance); // call with instance of type B

Calling of interface methods:

In this case, it depends on the actual type of l_i whether the application calls A.GetName or B.GetName.

DeliverName := l_i.GetName();


Implementing an interface in a new function block

Requirement: The project which is currently open has at least one interface object.

  1. In the device tree, select Application and click Project → Add Object → POU.

    The Add POU dialog opens.

  2. Enter a name for the new function block in the input field Name, e.g. POU_Im.

  3. Select Function block.

  4. Click Implements and then the _cds_icon_input_assistent.png button.

  5. In the Input Assistant, select the interface from the Interfaces category (example: ITF1) and click OK.

  6. To insert more interfaces, click _cds_icon_input_assistent.png again and select another interface.

  7. Optionally, you could also select an Access Modifier from the list box for the new function block.

  8. Select from the Implementation language combo box (example: Structured text (ST).

  9. Click Add.

    CODESYS adds the POU_Ex function block to the device tree and opens the editor. The first line contains the text:

    FUNCTION_BLOCK POU_Im IMPLEMENTS ITF1

    The interface and its methods and properties are now inserted below the function block in the device tree. Now you can type program code into the implementation part of the interface and its methods.

Implementing an interface in an existing function block

Requirement: The currently open project has a function block (example: POU_Im) and at least one interface object (example: ITF1).

  1. Double-click the POU POU_Ex(FB) in the device tree.

    The editor of the POU opens.

  2. Extend the existing entry in the top line FUNCTION_BLOCK POU_Im with IMPLEMENTS ITF1.

    The POU_Im function block implements the ITF1 interface.

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