How do you control the order of executing different blocks of ST code?

I have a process recipe with 8 steps, each of which I have defined in ST. The user should be able to however, select the order in which these 8 steps are executed. I'm trying to come up with flags or variables which could be used for this but drawing a blank so far. Does anyone have any thoughts on how this could be implemented?

Solution

My proposal is to let the user fill an array with the step numbers in the order how it should be executed. Then put your different blocks of ST code in a CASE statement. For the CASE-variable use the array. This way the order of execution is fully flexible.

Here is an Sandwich Decorator as an example. The important part starts at E_SandwichDecoratorStep.RecipeExecution

Enumerations:

TYPE E_SandwichDecoratorStep :
(
    UserSelecting := 1,
    RecipeExecution,
    Finished
);
END_TYPE

TYPE E_SandwichDecoratorUserRecipe :
(
    Pepperoni := 1,
    Ham,
    Cheese,
    Tomato,
    Salad,
    Sauce,
    Salt,
    Pepper
);
END_TYPE

Program:

PROGRAM SANDWICHDECORATOR
VAR
    arrnStepOrder       : ARRAY[1..8] OF E_SandwichDecoratorStep;   (*User Recipe Configuration*)

    bRestart            : BOOL;
    bUserRecipeStart    : BOOL; (*Start the execution of the Recipe*)
    eCurStep            : E_SandwichDecoratorStep;
    nCurUserRecipeIndex : INT := 1;
END_VAR

CASE eCurStep OF
    E_SandwichDecoratorStep.UserSelecting:
        (*Example Order*)
        arrnStepOrder[1] := E_SandwichDecoratorUserRecipe.Ham;
        arrnStepOrder[2] := E_SandwichDecoratorUserRecipe.Tomato;
        arrnStepOrder[3] := E_SandwichDecoratorUserRecipe.Pepperoni;
        arrnStepOrder[4] := E_SandwichDecoratorUserRecipe.Cheese;
        arrnStepOrder[5] := E_SandwichDecoratorUserRecipe.Salt;
        arrnStepOrder[6] := E_SandwichDecoratorUserRecipe.Sauce;
        arrnStepOrder[7] := E_SandwichDecoratorUserRecipe.Pepper;
        arrnStepOrder[8] := E_SandwichDecoratorUserRecipe.Salad;

        IF bUserRecipeStart THEN
            bUserRecipeStart := FALSE;
            eCurStep := E_SandwichDecoratorStep.RecipeExecution;
        END_IF

    E_SandwichDecoratorStep.RecipeExecution:
        IF nCurUserRecipeIndex <= E_SandwichDecoratorUserRecipe.Pepper THEN
            CASE arrnStepOrder[nCurUserRecipeIndex] OF
                E_SandwichDecoratorUserRecipe.Pepperoni:
                    nCurUserRecipeIndex := nCurUserRecipeIndex + 1;

                E_SandwichDecoratorUserRecipe.Ham:
                    nCurUserRecipeIndex := nCurUserRecipeIndex + 1;

                E_SandwichDecoratorUserRecipe.Cheese:
                    nCurUserRecipeIndex := nCurUserRecipeIndex + 1;

                E_SandwichDecoratorUserRecipe.Tomato:
                    nCurUserRecipeIndex := nCurUserRecipeIndex + 1;

                E_SandwichDecoratorUserRecipe.Salad:
                    nCurUserRecipeIndex := nCurUserRecipeIndex + 1;

                E_SandwichDecoratorUserRecipe.Sauce:
                    nCurUserRecipeIndex := nCurUserRecipeIndex + 1;

                E_SandwichDecoratorUserRecipe.Salt:
                    nCurUserRecipeIndex := nCurUserRecipeIndex + 1;

                E_SandwichDecoratorUserRecipe.Pepper:
                    nCurUserRecipeIndex := nCurUserRecipeIndex + 1;
            END_CASE
        ELSE
            nCurUserRecipeIndex := 1;
            eCurStep := E_SandwichDecoratorStep.Finished;
        END_IF

    E_SandwichDecoratorStep.Finished:
        IF bRestart THEN
            bRestart := FALSE;
            eCurStep := E_SandwichDecoratorStep.UserSelecting;
        END_IF

END_CASE