Synchronizing Threads and GUI in a Delphi Application

Multi-threading in Delphi lets you create applications that include several simultaneous paths of execution.

A "normal" Delphi application is single-threaded, which means all (VCL) objects access their properties and execute their methods within this single thread. To speed up data processing in your application, you can decide to include one or more "secondary" threads.

Threads & GUI

When several threads are running in the application, the question arises of how you can update your graphical user interface (GUI) as a result of a thread execution.

The answer lies in the TThread class Synchronize method.

To update your application's user interface, or main thread, from a secondary thread, you need to call the Synchronize method. This is a thread-safe method that avoids multi-threading conflicts that can arise from accessing object properties or methods that are not thread-safe, or using resources not in the main thread of execution.

Below is an example demo that uses several buttons with progress bars, each progress bar displaying the current "state" of the thread execution.

 unit MainU;
interface
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ComCtrls, StdCtrls, ExtCtrls;
type
  //interceptor class
  TButton = class(StdCtrls.TButton)
    OwnedThread: TThread;
    ProgressBar: TProgressBar;
  end;
  TMyThread = class(TThread)
  private
    FCounter: Integer;
    FCountTo: Integer;
    FProgressBar: TProgressBar;
    FOwnerButton: TButton;
    procedure DoProgress;
    procedure SetCountTo(const Value: Integer) ;
    procedure SetProgressBar(const Value: TProgressBar) ;
    procedure SetOwnerButton(const Value: TButton) ;
  protected
    procedure Execute; override;
  public
    constructor Create(CreateSuspended: Boolean) ;
    property CountTo: Integer read FCountTo write SetCountTo;
    property ProgressBar: TProgressBar read FProgressBar write SetProgressBar;
    property OwnerButton: TButton read FOwnerButton write SetOwnerButton;
  end;
  TMainForm = class(TForm)
    Button1: TButton;
    ProgressBar1: TProgressBar;
    Button2: TButton;
    ProgressBar2: TProgressBar;
    Button3: TButton;
    ProgressBar3: TProgressBar;
    Button4: TButton;
    ProgressBar4: TProgressBar;
    Button5: TButton;
    ProgressBar5: TProgressBar;
    procedure Button1Click(Sender: TObject) ;
  end;
var
  MainForm: TMainForm;
implementation
{$R *.dfm}
{ TMyThread }
constructor TMyThread.Create(CreateSuspended: Boolean) ;
begin
  inherited;
  FCounter := 0;
  FCountTo := MAXINT;
end;
procedure TMyThread.DoProgress;
var
  PctDone: Extended;
begin
  PctDone := (FCounter / FCountTo) ;
  FProgressBar.Position := Round(FProgressBar.Step * PctDone) ;
  FOwnerButton.Caption := FormatFloat('0.00 %', PctDone * 100) ;
end;
procedure TMyThread.Execute;
const
  Interval = 1000000;
begin
  FreeOnTerminate := True;
  FProgressBar.Max := FCountTo div Interval;
  FProgressBar.Step := FProgressBar.Max;
  while FCounter < FCountTo do
  begin
    if FCounter mod Interval = 0 then Synchronize(DoProgress) ;
    Inc(FCounter) ;
  end;
  FOwnerButton.Caption := 'Start';
  FOwnerButton.OwnedThread := nil;
  FProgressBar.Position := FProgressBar.Max;
end;
procedure TMyThread.SetCountTo(const Value: Integer) ;
begin
  FCountTo := Value;
end;
procedure TMyThread.SetOwnerButton(const Value: TButton) ;
begin
  FOwnerButton := Value;
end;
procedure TMyThread.SetProgressBar(const Value: TProgressBar) ;
begin
  FProgressBar := Value;
end;
procedure TMainForm.Button1Click(Sender: TObject) ;
var
  aButton: TButton;
  aThread: TMyThread;
  aProgressBar: TProgressBar;
begin
  aButton := TButton(Sender) ;
  if not Assigned(aButton.OwnedThread) then
  begin
    aThread := TMyThread.Create(True) ;
    aButton.OwnedThread := aThread;
    aProgressBar := TProgressBar(FindComponent(StringReplace(aButton.Name, 'Button', 'ProgressBar', []))) ;
    aThread.ProgressBar := aProgressBar;
    aThread.OwnerButton := aButton;
    aThread.Resume;
    aButton.Caption := 'Pause';
  end
  else
  begin
    if aButton.OwnedThread.Suspended then
      aButton.OwnedThread.Resume
    else
      aButton.OwnedThread.Suspend;
    aButton.Caption := 'Run';
  end;
end;
end.

Note: The code used here was submitted by Jens Borrisholt.