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, 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 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 technique 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.

Thanks to Jens Borrisholt for submitting this code sample.