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

TThread — Classical Approach

Thread with Inheritance (Recommended for Workers)

type
  /// <summary>
  ///   Worker thread para processamento em background.
  ///   Demonstra herança de TThread com cancelamento 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);   // Criar suspensa
  FreeOnTerminate := True;  // Auto-libera ao terminar
  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;

Use of Dedicated Thread

procedure TfrmMain.btnProcessClick(Sender: TObject);
var
  LThread: TDataProcessorThread;
begin
  LThread := TDataProcessorThread.Create(FSharedItems);
  LThread.OnProgress :=
    procedure(ACurrent, ATotal: Integer)
    begin
      pbrProgress.Max := ATotal;
      pbrProgress.Position := ACurrent;
      lblStatus.Caption := Format('Processando %d de %d...', [ACurrent, ATotal]);
    end;
  LThread.OnComplete :=
    procedure(ASuccess: Boolean)
    begin
      if ASuccess then
        ShowMessage('Concluído com sucesso!')
      else
        ShowMessage('Processamento cancelado.');
    end;
  LThread.Start;  // Iniciar a thread
end;

procedure TfrmMain.btnCancelClick(Sender: TObject);
begin
  { Solicitar cancelamento gracioso }
  if Assigned(FCurrentThread) then
    FCurrentThread.Terminate;
end;

CreateAnonymousThread (Simple Tasks)

/// <summary>
///   Forma mais simples de executar código em background.
///   Ideal para one-shot tasks sem necessidade de controle avançado.
/// </summary>
procedure TfrmMain.LoadDataAsync;
begin
  btnLoad.Enabled := False;

  TThread.CreateAnonymousThread(
    procedure
    var
      LData: TStringList;
    begin
      LData := TStringList.Create;
      try
        { Trabalho pesado (thread secundária — OK!) }
        LData.LoadFromFile('C:\dados\arquivo_grande.csv');
        Sleep(2000); // Simular processamento

        { Atualizar UI (DEVE usar Synchronize ou Queue) }
        TThread.Synchronize(nil,
          procedure
          begin
            mmoOutput.Lines.Assign(LData);
            btnLoad.Enabled := True;
            lblStatus.Caption := Format('Carregados %d registros', [LData.Count]);
          end);
      finally
        LData.Free;
      end;
    end
  ).Start;
end;

Synchronize vs Queue

{ Synchronize: BLOQUEIA a thread até a main thread processar }
TThread.Synchronize(nil,
  procedure
  begin
    lblStatus.Caption := 'Processando...';
  end);
// A thread só continua AQUI after que a main thread executou o código acima

{ Queue: NÃO BLOQUEIA — enfileira e continua imediatamente }
TThread.Queue(nil,
  procedure
  begin
    lblStatus.Caption := 'Processando...';
  end);
// A thread continua IMEDIATAMENTE, sem esperar a main thread

Recommendation: Prefer Queue whenever possible. Use Synchronize only when you need a result from the UI back in the thread.

PPL — Parallel Programming Library (System.Threading)

TTask — Light Tasks

uses
  System.Threading;

/// <summary>
///   TTask é a forma moderna de executar tarefas em background.
///   Gerenciado automaticamente pelo pool de threads do sistema.
/// </summary>
procedure TfrmMain.ExecuteMultipleTasks;
var
  LTask1, LTask2, LTask3: ITask;
begin
  LTask1 := TTask.Create(
    procedure
    begin
      { Tarefa 1: Carregar dados do banco }
      LoadCustomers;
    end);

  LTask2 := TTask.Create(
    procedure
    begin
      { Tarefa 2: Processar relatório }
      GenerateReport;
    end);

  LTask3 := TTask.Create(
    procedure
    begin
      { Tarefa 3: Enviar emails }
      SendPendingEmails;
    end);

  { Iniciar todas as tarefas em paralelo }
  LTask1.Start;
  LTask2.Start;
  LTask3.Start;

  { Aguardar todas completarem (com timeout) }
  TTask.WaitForAll([LTask1, LTask2, LTask3], 30000); // 30s timeout

  TThread.Queue(nil,
    procedure
    begin
      ShowMessage('Todas as tarefas concluídas!');
    end);
end;

TTask.Run — Direct Shortcut

{ Forma mais simples de executar em background via PPL }
TTask.Run(
  procedure
  begin
    { Código executado no ThreadPool }
    PerformHeavyCalculation;

    TThread.Queue(nil,
      procedure
      begin
        lblResult.Caption := 'Cálculo concluído';
      end);
  end);

TParallel.For — Parallel Loops

uses
  System.Threading,
  System.SyncObjs;

/// <summary>
///   TParallel.For distribui iterations do loop entre múltiplas threads.
///   Ideal para processamento de coletions independentes.
/// </summary>
procedure TfrmMain.ProcessImagesParallel;
var
  LFiles: TArray<string>;
  LProcessed: Integer;
  LLock: TCriticalSection;
begin
  LFiles := TDirectory.GetFiles('C:\Images', '*.jpg');
  LProcessed := 0;
  LLock := TCriticalSection.Create;
  try
    TParallel.For(0, High(LFiles),
      procedure(AIndex: Integer)
      begin
        { Cada imagem processada em thread separada }
        ResizeImage(LFiles[AIndex]);

        { Atualizar contador de forma thread-safe }
        LLock.Enter;
        try
          Inc(LProcessed);
        finall