How to terminate a parallel.foreach from OTL in Delphi xe2 - delphi

Question

I am learning how to use OmniThreadLibrary in Delphi XE2, i wonder if someone can show me how to cancel a parallel.foreach.
I read that I should use a cancellation token, but I cannot find an example of some sort on how to use it.
This is the original for loop inside the function.
function SomeFunction() : string;
begin
for value := 0 to length(listOfThings)-1 do
begin
Chain := Function1( listOfThings[value] );
if Evaluate( Chain , Solution) then
Parameters[value] := Solution
else
begin
Result := 'ERROR';
exit;
end;
end;
end;
And this is how I am using the Parallel.ForEach
function SomeFunction() : string;
begin
Parallel.ForEach(0, length(listOfThings)-1 ).Execute(
procedure (const value: integer)
var Chain : string;
begin
Chain := Function1(listOfThings[value]);
if Evaluate(Chain , Solution) then
Parameters[value] := Solution
else
begin
Result := 'ERROR'; //Here is where it won't work
exit;
end;
end
);
end;
Inside the Parallel.ForEach I can't do Result := 'ERROR' because it is not captured inside the procedure, so I think if I can cancel the Parallel.ForEach and report that cancellation, then I can just assign Result := 'ERROR' outside.
But I am new to OmniThreadLibrary and I don't know how to do such a thing, please help me :)

Answer 1

You need to use a cancellation token:
var
cancelToken: IOmniCancellationToken;
You obtain the cancellation token by calling CreateOmniCancellationToken from the OtlSync unit.
cancelToken := CreateOmniCancellationToken;
You then supply the token to the parallel loop:
Parallel.ForEach(...)
.CancelWith(cancelToken)
.Execute(...);
And you signal the cancellation token by calling its Signal method.
cancelToken.Signal;
From outside the parallel loop you can use
cancelToken.IsSignaled
to detect that you cancelled. Or you can capture a boolean variable from the surrounding scope and pass the information through that variable.
The example here gives an illustration.

Answer 2

The cancellation token is only half of it. If you need it to return a value, you'll need to use Aggregate, because you can have an arbitrary number of elements in the sequence, but only one return value, so you need to collapse (aggregate) an arbitrary number of return values into one final value. So you want something like this:
function SomeFunction() : string;
var
cancelToken: IOmniCancellationToken;
error: TOmniValue;
begin
cancelToken := CreateOmniCancellationToken;
error := Parallel.ForEach(0, length(listOfThings)-1 ).
CancelWith(cancelToken).
Aggregate('',
procedure(var aggregate: TOmniValue; const value: TOmniValue)
var Chain : string;
begin
Chain := Function1(listOfThings[value]);
if Evaluate(Chain , Solution) then
Parameters[value] := Solution
else
begin
aggregate := 'ERROR';
cancelToken.signal;
end;
end).
Execute(
procedure(const value: TOmniValue; var result: TOmniValue)
begin
if value <> '' then
result := value;
end);
if error <> '' then
//something went wrong
end;
This may not be exactly perfect, but it should get you on the right track.