Reaching post-conditions in tasks

Implementing use cases in Adf.Net is covered by the task pattern. Each smart use case in the model is implemented as a descendant of the Task class in Adf.Net.

The task pattern consists of three major parts:

• Starting the task, either using a parameterized Init() method, or the default Start() method.
• After a task calls another task, and this second task is finished, control goes back to the calling task, using any of the specific ContinueFrom() methods, or the default Continue() method.
• When a task reaches any of the post-conditions of a task, the task needs to be ended, usually through the Ok() or Cancel() method.

In this specific post I’ll look at reaching the post-conditions. In the pattern, the Task class itself acts as the layer super type and implements a number of method to end itself. A use case can have multiple post-conditions. Some of them are positive, some can be negative. And sometimes a specific case needs to be addressed.

When a reaches it positive post-condition, it is a best practices to end it using the Ok() method. This method takes a params object[] p, which allows you to pass back results from the task to the calling task, using the following signature.

public virtual void OK(params object[] p);

A similar method Cancel() exists for task that end in a negative post-condition, usually because the user cancels the interaction:

public virtual void Cancel(params object[] p);

But, Adf.Net also supplies a similar method Error(), to allow tasks to finish with an error, possibly technical.

Under the covers, these three methods call on another method, which is called Finish(). This method takes care of passing back an instance of TaskResult. This result also gets posted back to the calling task, so it knows how the called task has ended. As in the following example, you could also use Finish() yourself.

if (persoon.IsNullOrEmpty())
{
    ValidationManager.AddError(GEENPERSOONGESELECTEERD);
    Finish(TaskResult.Error);
}

Please note that the code above is equal to the following code.

if (persoon.IsNullOrEmpty())
{
    ValidationManager.AddError(GEENPERSOONGESELECTEERD);
    Error();
}

By default, TaskResult has the following values in Adf.Net: Ok, Cancel, Error.

However, as TaskResult is implemented using the descriptor pattern, additional project specific value can be added easily, by inheriting from TaskResult, and added the specific values. Thus you would be able to end your task in more project specific ways, such as in the code example below.

if (persoon.IsNullOrEmpty())
{
    ValidationManager.AddError(GEENPERSOONGESELECTEERD);
    Finish(Dashboard8TaskResult.Aborted);
}