Get a function result asynchronously in Delphi using Omni Thread Library - multithreading

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 something similar to C# async-await like simple approach in Delphi. Using Omni Thread library seems a good idea for me.
A simple example will be a great start for me.
Sample approach:
procedure TForm1.button1Click(Sender: TObject);
begin
// notify before starting the task
memo1.Lines.Add('calling a asynchronous function..');
// call to the function that takes some time and returns a string value
memo1.Lines.Add(GetMagicString);
// notify that the task has been completed
memo1.Lines.Add('Results fetched successfully.');
end;
Here, the function GetMagicString should process the result asynchronously. Once the result is obtained, only then the program should notify that the task has been completed. By the way, I'm using Delphi-XE.
Edit1:
Here is what I tried. But I am still unable to figure out the proper way to make the job done. The problem is how to return the value.
.....
private
ResultValue: IOmniFuture<string>;
.........
.....
function TForm1.FutureGet: string;
begin
Sleep(3000);
Result := 'my sample magic string response ' + IntToStr(Random(9999));
end;
procedure TForm1.FutureGetTerminated;
begin
// This code fired when the task is completed
memo1.Lines.Add(ResultValue.Value);
end;
function TForm1.GetMagicString: string;
begin
ResultValue := Parallel.Future<string>(FutureGet,
Parallel.TaskConfig.OnTerminated(FutureGetTerminated));
end;
Here, using Result := ResultValue.Value feezes the UI.
Edit2
I made changes as per the answer provided.
MainForm Code:
unit Unit1;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls, Unit2;
type
TForm1 = class(TForm)
memo1: TMemo;
button1: TButton;
procedure button1Click(Sender: TObject);
private
FOnStringReceived: TMyEvent;
procedure StringReceived(const AValue: string);
property OnStringReceived: TMyEvent read FOnStringReceived write FOnStringReceived;
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.button1Click(Sender: TObject);
var
MyObject: TMyClass;
begin
// notify before starting the task
memo1.Lines.Add('calling a asynchronous function..');
// call to the function that takes some time and returns a string value
MyObject := TMyClass.Create;
OnStringReceived := StringReceived;
try
MyObject.GetMagicStringInBackground(OnStringReceived);
finally
MyObject.Free;
end;
end;
procedure TForm1.StringReceived(const AValue: string);
begin
memo1.Lines.Add(AValue);
// notify that the task has been completed
memo1.Lines.Add('Results fetched successfully.');
end;
end.
Other Unit Code:
unit Unit2;
interface
uses SysUtils, OtlTask, OtlParallel, OtlTaskControl;
type
TMyEvent = procedure(const aValue: string) of object;
type
TMyClass = class
private
FOnStringReceived: TMyEvent;
function GetMagicString: string;
public
procedure GetMagicStringInBackground(AEvent: TMyEvent);
end;
implementation
{ TMyClass }
function TMyClass.GetMagicString: string;
begin
Sleep(3000);
Result := 'my sample magic string response ' + IntToStr(Random(9999));
end;
procedure TMyClass.GetMagicStringInBackground(AEvent: TMyEvent);
var
theFunctionResult: string;
begin
Parallel.Async(
procedure
begin
theFunctionResult := GetMagicString;
end,
Parallel.TaskConfig.OnTerminated(
procedure (const task: IOmniTaskControl)
begin
if Assigned(AEvent) then
AEvent(theFunctionResult);
end)
);
end;
end.
Yes, the code works as expected. I just want to know if this is the best way of doing what I really want to perform.

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.