Events handling, threads and gui interface [duplicate] - delphi

Question

I have to modify and change some visual components in a thread and as you know it's not safe to doing this.
My question is how to write a completely thread-safe code? It is possible? if it is then can you please give me a simple example?
my code that is not threadsafe:
type
tMyWorkerThread = class(TThread)
public
procedure Execute; override;
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
procedure tMyWorkerThread.Execute;
begin
//codes
//working with visual components
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
TMyWorkerThread.Create(false);
end;
Thank you.

Answer 1

Writing a thread safe code in Delphi involves the basic care you would have in any other language, which means to deal with race conditions. A race condition happens when different threads access the same data. A good way to deal with that is to declare an instance of TCriticalSection and wrap the dangerous code in it.
The code below shows a getter and a setter of a property that, by hypotesis, has a race condition.
constructor TMyThread.Create;
begin
CriticalX := TCriticalSection.Create;
end;
destructor TMyThread.Destroy; override;
begin
FreeAndNil(CriticalX);
end;
function TMyThread.GetX: string;
begin
CriticalX.Enter;
try
Result := FX;
finally
CriticalX.Leave;
end;
end;
procedure TMyThread.SetX(const value: string);
begin
CriticalX.Enter;
try
FX := Value;
finally
CriticalX.Leave;
end;
end;
Notice the use of a single instance of TCriticalSection (CriticalX) to serialize the access to the data member FX.
However, with Delphi you have an aditional consideration! VCL is not thread safe, so in order to avoid VCL race conditions, any operation that results in screen changing must run in the main thread. You get that by calling such a code inside a Synchronize method. Considering the class above, you should do something like this:
procedure TMyThread.ShowX;
begin
Synchronize(SyncShowX);
end;
procedure TMyThread.SyncShowX;
begin
ShowMessage(IntToStr(FX));
end;
If you have Delphi 2010 or later, there is an easier way that makes use of anonymous methods:
procedure TMyThread.ShowX;
begin
Synchronize(procedure begin
ShowMessage(IntToStr(FX));
end);
end;
I hope this helps!

Answer 2

You should only access VCL objects from main VCL thread.
Some reading methods (property getters) do work from other threads in practice - but you have to prove it in advance reading VCL sources for the specific Delphi build. Or not use it.
PS: Synchronize method runs given procedure in main VCL thread, pausing the caller thread, which may lead to a deadlock, if main thread was also blocked.
Read more: (actually making this answer to list some links)
http://www.michael-puff.de/Programmierung/Delphi/Code-Snippets/VCLThreadDemo.shtml
http://www.drbob42.com/uk-bug/hood-04.htm
http://delphi.about.com/od/kbthread/a/thread-gui.htm
Is it better to use TThread's "Synchronize" or use Window Messages for IPC between main and child thread?
Delphi 6 : breakpoint triggered on non-VCL thread stops main thread repaints
http://docs.embarcadero.com/products/rad_studio/delphiAndcpp2009/HelpUpdate2/EN/html/devwin32/win32_mthreadusemainthread_xml.html
Simplifying VCL thread wrapper code
Update a VCL component from CreateAnonymousThread
http://thaddy.co.uk/threads/ - the mirror of "Multithreading - The Delphi Way" by Martin Harvey
http://otl.17slon.com/ - new Delphi approach to threading

Answer 3

My problem solved with Synchronize!
type
tMyWorkerThread = class(TThread)
public
procedure Execute; override;
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
procedure tMyWorkerThread.Execute;
begin
//codes that takes long time
Synchronize(procedure begin
//working with visual components
end
);
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
TMyWorkerThread.Create(false);
end;
Thank you all for helping me.