3.4 Task Controller Needs an Owner

Task Controller Needs an Owner
The IOmniTaskController interface returned from the CreateTask must always be stored in a variable/field with a scope that exceeds the lifetime of the background task. In other words, don’t store a long-term background task interface in a local variable.

The simplest example of the wrong approach can be written in one line:

CreateTask(MyWorker).Run;

This code looks fine, but it doesn’t work. In this case, the IOmniTaskController interface is stored in a hidden temporary variable which is destroyed at the end of the current method. This then causes the task controller to be destroyed which in turn causes the background task to be destroyed. Running this code would therefore just create and then destroy the task.

A common solution is to just store the interface in some field.

FTaskControl := CreateTask(MyWorker).Run;

When you don’t need background worker anymore, you should terminate the task and free the task controller.

FTaskControl.Terminate;
FTaskControl := nil;

Another solution is to provide the task with an implicit owner. You can, for example, use the event monitor to monitor tasks lifetime or messages sent from the task and that will make the task owned by the monitor. The following code is therefore valid:

CreateTask(MyWorker).MonitorWith(eventMonitor).Run;

Yet another possibility is to call the Unobserved) before the Run. This method makes the task being observed by an internal monitor.

CreateTask(MyWorker).Unobserved.Run;

When you use a thread pool to run a task, the thread pool acts as a task owner so there’s no need for an additional explicit owner.

procedure Beep(const task: IOmniTask);
begin
   MessageBeep(MB_ICONEXCLAMATION);
 end;
 
 CreateTask(Beep, 'Beep').Schedule;