Get a function result asynchronously in Delphi using Omni Thread Library

Question

I am trying to call a function from another unit/class which would take some time in performing the task and would return a string value. I couldn\'t find a good reference somet

Answer 1

You would normally use a future in a case where you want something executed in the background but still need the result in the same execution path. It basically lets you do something in the background while doing another thing in the main thread and you can then use the result of the background thread.

What you need to use is the Async abstraction that TLama linked to:

In your case it would be:

procedure TForm1.DoSomething;
var
  theFunctionResult: string;
begin
  memo1.Lines.Add('calling a asynchronous function..');
  Parallel.Async(
    procedure
    begin
      // executed in background thread
      theFunctionResult := GetMagicString;
    end,

    procedure
    begin
      // executed in main thread after the async has finished
      memo1.Lines.Add(theFunctionResult);

      // notify that the task has been completed
      memo1.Lines.Add('Results fetched successfully.');
    end
  );
end;

This is a bit messy but you should get the idea. You need to make sure that your async code is completed before you destroy the form that owns this code (TForm1).

If you want to try and setup a system that will call an event when the code completes then you can do something like this:

type
  TMyEvent = procedure(const aValue: string) of object;

procedure GetMagicStringInBackground(AEvent: TMyEvent);
var
  theFunctionResult: string;
begin
  Parallel.Async(
    procedure
    begin
      // executed in background thread
      theFunctionResult := GetMagicString;
    end,

    Parallel.TaskConfig.OnTerminated(
      procedure (const task: IOmniTaskControl)
      begin
        // executed in main thread after the async has finished
        if Assigned(AEvent) then
          AEvent(theFunctionResult );
      end
    )
  );
end;

You can then put the threaded code in the GetMagicString unit and just call the method above from your form passing in an event that will get called when it completes.