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.