Task

(TcoTask : ITcoTask)

API

TcoTask is a block for managing chunks of logic in asynchronous execution. Task controls the run of a component's function (servo movement, piston movement, barcode reader trigger, etc.).

There are two key methods for managing the task:

  • Invoke call to fire the execution of the task (can be called fire&forget or cyclically)

  • Execute method must be called cyclically (typically in the body of a FB). The method returns TRUE when required the execution from a call of Invoke method until the task enters Done state.

TcoTask diagram

FUNCTION_BLOCK BlocWithATask EXTENDS TcoCore.TcoObject
VAR
    _counter : INT;
    _myTask  : TcoCore.TcoTask(THIS^);
END_IF

// Body of a FB
IF(_myTask.Exectute()) THEN
    _counter := _counter + 1;
    _myTask.DoneWhen(_counter = 100);
END_IF;

The task executes upon the Invoke method call. Invoke fires the execution of Execute logic upon the first call, and it does not need cyclical calling.

_myTask.Invoke();

Invoke method returns ITcoTaskStatus with the following members:

  • Busy indicates the execution started and is running.
  • Done indicates the execution completed with success.
  • Error indicates the execution terminated with a failure.
// Wait for the task to complete
IF(_myTask.Invoke().Done) THEN
    ; // Do something
END_IF;

// ALTERNATIVELY

_myTask.Invoke();

IF(_myTask.Done) THEN
    ; // Do something
END_IF;


// Make sure that the task is executing
IF(_myTask.Invoke().Busy) THEN
    ; // Do something
END_IF;    

// Check for task's error.
IF(_myTask.Invoke().Error) THEN
    ; // Do something
END_IF;

Restore is a function of IRestoreable (implemented by TcoTask, TcoComponent...) It provides an initialization routine for the object; it recovers the object from any state into Ready.

After task completion, the state of the task will remain in Done, unless:

  1. Task's Restore method is called (task moves to Ready state).

  2. Invoke method is not called for two or more cycles of its context (that usually means the same as PLC cycle); successive call of Invoke will switch the task into Ready and immediately to Requested state.

  3. If the task is part of complex coordination primitive, the transition between states will bring the task into a Ready state if that primitive (StateController and derivatives) has this option enabled.

The task may finish in an Error state. In that case, two recovery scenarios are possible:

  1. Task's Restore method is called (task goes to Readystate).
  2. Restore from on transition methods of a coordination block.