Methods and Properties

A function block can do more than run one body. A method adds an operation, a property adds a value that looks like a variable but runs code.

Methods

A method is declared inside the function block, it has its own parameters and its own result, and it works on the data of the instance:

FUNCTION_BLOCK Tank
    VAR
        level: DINT;
    END_VAR

    METHOD Fill: DINT
        VAR_INPUT
            amount: DINT;
        END_VAR

        level := level + amount;
        Fill := level;
    END_METHOD
END_FUNCTION_BLOCK

Call it through an instance:

VAR
    inlet: Tank;
    value: DINT;
END_VAR

value := inlet.Fill(amount := 3);

value is now 3, the new level. A method is called like a function: the arguments come by position, in the order of the declaration, or by name. Give every parameter a value, because a parameter that the call leaves out takes the default value of its declaration, and a parameter without a default holds no defined value. The body of the block and its methods share the data of the instance, but a local variable of a method does not survive the call.

Inside a method, THIS^ names the instance itself. You need it when a parameter and a member have the same name, because the parameter hides the member:

METHOD SetLevel
    VAR_INPUT
        level: DINT;
    END_VAR

    THIS^.level := level;
END_METHOD

THIS works in a function block, in its methods, and in its actions. A CLASS does not have it, and a use of THIS there is rejected.

Properties

Where a method is called, a property is read and written. It is a value with code behind it, and it has a getter, a setter, or both. Inside an accessor, the name of the property is the value, exactly as the name of a function is its result:

FUNCTION_BLOCK Tank
    VAR
        level: DINT;
    END_VAR

    PROPERTY_GET Percent: DINT
        Percent := level * 10;
    END_PROPERTY

    PROPERTY_SET Percent: DINT
        level := Percent / 10;
    END_PROPERTY
END_FUNCTION_BLOCK

From outside, the property is used like a member, and the accessor runs:

VAR
    inlet: Tank;
    reading: DINT;
END_VAR

inlet.Percent := 50;      (* runs the setter, level becomes 5 *)
reading := inlet.Percent; (* runs the getter, 50 *)

When a property has both accessors, both must name the same type. A property with a getter only is read-only, and a write to it is reported:

error[E048]: PROPERTY_SET for property `Percent` is not defined

A property with a setter only is write-only, and a read of it is reported the same way.

A CLASS declares a property like a function block does. An INTERFACE declares one without bodies, and every block that implements the interface must supply the accessors that the interface names.

Method or property

Use a property when the caller thinks of the thing as a value: a level, a limit, a state. Use a method when the caller thinks of it as an action. An accessor takes no parameters, so an operation that needs arguments is always a method. Keep the work behind a property small, because the caller reads it as a plain variable.

What’s next

The next chapter lets one function block build on another, with inheritance and interfaces.