Threading & Multi-Threading

Threading & Multi-Threading — Skill

Use this skill when working with threads, asynchronous tasks and parallelism in Delphi projects.

When to Use

• When performing time-consuming operations without blocking the UI (VCL/FMX)
• When implementing parallel data processing
• When creating servers/workers that process concurrent requests
• When synchronizing access to shared resources
• When managing thread pools and work queues
• By implementing graceful thread cancellation

Golden Rule of Threading in Delphi

NEVER access visual components (VCL/FMX) directly from a secondary thread. Use TThread.Synchronize or TThread.Queue to update the UI.

Available Approaches

Approach	When to Use	Complexity
TThread	Full control, long-running threads	Average
TThread.CreateAnonymousThread	Simple, one-shot tasks	Low
TTask (PPL)	Modern parallelism, lightweight tasks	Low
TParallel.For (PPL)	Parallel loops in collections	Low
TFuture<T> (PPL)	Asynchronous result with return value	Low
TThreadPool	Reusable Thread Pool	Average
Dedicated thread (inheritance)	Permanent workers, servers, queues	High

TThread — Classical Approach

Thread with Inheritance (Recommended for Workers)

type
  ///<summary>
  ///Worker thread for background processing.
  ///Demonstrates TThread inheritance with cancellation via Terminated.
  ///</summary>
  TDataProcessorThread = class(TThread)
  private
    FItems: TThreadList<string>;
    FOnProgress: TProc<Integer, Integer>;
    FOnComplete: TProc<Boolean>;
  protected
    procedure Execute; override;
  public
    constructor Create(AItems: TThreadList<string>);
    property OnProgress: TProc<Integer, Integer> write FOnProgress;
    property OnComplete: TProc<Boolean> write FOnComplete;
  end;

constructor TDataProcessorThread.Create(AItems: TThreadList<string>);
begin
  inherited Create(True);   //Create dropdown
  FreeOnTerminate := True;  //Auto-releases when finished
  FItems := AItems;
end;

procedure TDataProcessorThread.Execute;
var
  LList: TList<string>;
  LTotal, I: Integer;
begin
  try
    LList := FItems.LockList;
    try
      LTotal := LList.Count;
    finally
      FItems.UnlockList;
    end;

    for I := 0 to LTotal - 1 do
    begin
      { Verificar cancelamento em cada iteração }
      if Terminated then
        Exit;

      { Processar item }
      ProcessItem(I);

      { Atualizar UI via Queue (não-bloqueante) }
      if Assigned(FOnProgress) then
        TThread.Queue(nil,
          procedure
          begin
            FOnProgress(I + 1, LTotal);
          end);
    end;

    { Notificar conclusão na main thread }
    if Assigned(FOnComplete) then
      TThread.Queue(nil,
        procedure
        begin
          FOnComplete(not Terminated);
        end);
  except
    on E: Exception do
    begin
      TThread.Queue(nil,
        procedure
        begin
          raise EThreadException.Create('Erro no processamento: ' + E.Message);
        end);
    end;
  end;
end;