Synchronizing Threads and GUI in Delphi application

^{Code submitted by Jens Borrisholt}

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

A "normal" Delphi application is single-threaded: all (VCL) objects access their properties and execute their methods within this single thread.

Threads and GUI

To speed up data processing in your application you can decide to include one or more "secondary" threads.

When several threads are running in the application a question arises: how to update your GUI (user interface) as a result of a thread execution. The question lies in the TThread class Synchronize method.

To update your application's user interface - main thread - from a secondary thread you need to call the Synchronize method - 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.

An example demo uses several buttons with progress bar, each progress bar displaying the current "state" of the thread execution...

Download source code

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.