Delphi cannot execute procedure in Task with object references - delphi

Question

I have a simple class that has the following interface implementation.
Note: TPolyBase is an abstract class and TPolyResult is an array of double; it's not important to see their code, it's not relevant here.
//INTERFACE
type
TPolynomialList = class
strict private
FPolynomialList: TObjectList<TPolyBase>;
FResult: TList<TPolyResult>;
FCanGet: boolean;
function GetResult: TList<TPolyResult>;
procedure DoSolve;
public
constructor Create(PolynomialList: TObjectList<TPolyBase>);
destructor Destroy; override;
procedure SolvePolynomials(CompletionHandler: TProc);
property Solutions: TList<TPolyResult> read GetResult;
end;
//IMPLEMENTATION
constructor TPolynomialList.Create(PolynomialList: TObjectList<TPolyBase>);
begin
FPolynomialList := PolynomialList;
FResult := TList<TPolyResult>.Create;
FCanGet := false;
end;
destructor TPolynomialList.Destroy;
begin
FResult.Free;
inherited;
end;
procedure TPolynomialList.DoSolve;
var
i: integer;
begin
for i := 0 to FPolynomialList.Count - 1 do
FResult.Add(FPolynomialList[i].GetSolutions);
FCanGet := true;
end;
function TPolynomialList.GetResult: TList<TPolyResult>;
begin
if FCanGet = false then
raise TEquationError.Create('You must solve the equation first!');
Result := FResult;
end;
procedure TPolynomialList.SolvePolynomials(CompletionHandler: TProc);
begin
TTask.Run(procedure
var
ex: TObject;
begin
try
DoSolve;
TThread.Synchronize(nil, procedure
begin
CompletionHandler;
end);
except
on E: Exception do
begin
ex := AcquireExceptionObject;
TThread.Synchronize(nil, procedure
begin
Writeln( (ex as Exception).Message );
end);
end;
end;
end);
end;
This class takes a list of objects as input and it has an internal important field called FResult that gives the results to the user. It can be accessed from the getter only if the method SolvePolynomials has finished his work.
The problem is in the SolvePolynomials. The code I have shown uses a task because the size of the object list may be very big and I don't want to freeze the UI. Why do I always get an access violation in the task code?
Note that the following code works fine but this is not what I want because if I input 15000 the program freezes for a few seconds.
procedure TPolynomialList.SolvePolynomials(CompletionHandler: TProc);
begin
DoSolve;
CompletionHandler;
end;
Could the FPolynomialList variable be a the problem? If you look at my class the only thing "taken from outside" is the TObjectList<TPolyBase> because in the constructor I simply assing the reference (I'd like to avoid the copy ok 15k items). All the other variables are not shared with anything.
I have seen in many books I have read like "Delphi High Performance" that is good to have a task that calls an inner "slow" method but in this case there could be those reference that are messing up something. Any idea?
This is the code that I am using as test:
var
a: TObjectList<TPolyBase>;
i, j: integer;
f: TPolynomialList;
s: string;
function GetRandom: integer;
begin
Result := (Random(10) + 1);
end;
begin
a := TObjectList<TPolyBase>.Create(true);
try
for i := 0 to 15000 do
begin
a.Add({*Descendant of TPolyBase*})
end;
f := TPolynomialList.Create(a);
try
f.SolvePolynomials(procedure
var
i, j: integer;
begin
for i := 0 to f.Solutions.Count - 1 do
begin
for j := Low(f.Solutions[i]) to High(f.Solutions[i]) do
Writeln({output the results...})
end;
end);
finally
f.Free;
end;
finally
a.Free;
end;
end.

Answer 1

Your SolvePolynomials method delegates solving to another thread and returns before that thread is finished with its task. While that task thread is running it is necessary that all data it operates on is still alive. But, in your code you are releasing necessary object instances right after SolvePolynomials exits - while your task is still running, hence the error.
You have to move releasing of those objects into completion handler.
Basically, your code simplified looks like:
type
TPolynomialList = class
public
destructor Destroy; override;
procedure DoSolve;
procedure SolvePolynomials(CompletionHandler: TProc);
end;
destructor TPolynomialList.Destroy;
begin
Writeln('Destroyed');
inherited;
end;
procedure TPolynomialList.DoSolve;
begin
Writeln('Solving');
end;
procedure TPolynomialList.SolvePolynomials(CompletionHandler: TProc);
begin
TTask.Run(
procedure
begin
try
DoSolve;
TThread.Synchronize(nil,
procedure
begin
CompletionHandler;
end);
except
on E: Exception do
Writeln(E.ClassName, ': ', E.Message);
end;
end);
end;
procedure Test;
var
f: TPolynomialList;
begin
f := TPolynomialList.Create;
try
f.SolvePolynomials(
procedure
begin
Writeln('Solved');
end);
finally
f.Free;
end;
end;
If you run it output will be:
Destroyed
Solving
Solved
However, if you move releasing of your variables into completion handler order of execution will be correct.
procedure Test;
var
f: TPolynomialList;
begin
f := TPolynomialList.Create;
f.SolvePolynomials(
procedure
begin
Writeln('Solved');
f.Free;
end);
end;
Solving
Solved
Destroyed
For your code, that means moving both a.Free and f.Free into completion handler.