How to make GIF animate on "please, wait form"? - delphi

Question

I would like make a quick non-closable modal dialog, that pops up while do some tasks and goes away when tasks finish.
There are some inherent difficulties:
Don't block the main UI thread;
Don't leave system ghosts windows;
Move tasks to running into a separate thread;
Allow update the waiting message to the user;
Handling exceptions from thread to the application;
Show animated GIF in the dialog;
How to get around these pitfalls?
Below, a practical example of how I would use it:
TWaiting.Start('Waiting, loading something...');
try
Sleep(2000);
TWaiting.Update('Making something slow...');
Sleep(2000);
TWaiting.Update('Making something different...');
Sleep(2000);
finally
TWaiting.Finish;
end;

Answer 1

type
TWaiting = class(TForm)
WaitAnimation: TImage;
WaitMessage: TLabel;
WaitTitle: TLabel;
procedure FormClose(Sender: TObject; var Action: TCloseAction);
procedure FormCreate(Sender: TObject);
strict private
class var FException: Exception;
private
class var WaitForm : TWaiting;
class procedure OnTerminateTask(Sender: TObject);
class procedure HandleException;
class procedure DoHandleException;
public
class procedure Start(const ATitle: String; const ATask: TProc);
class procedure Status(AMessage : String);
end;
implementation
{$R *.dfm}
procedure TWaiting.FormCreate(Sender: TObject);
begin
TGIFImage(WaitAnimation.Picture.Graphic).Animate := True;
end;
procedure TWaiting.FormClose(Sender: TObject; var Action: TCloseAction);
begin
Action := caFree;
end;
class procedure TWaiting.Start(const ATitle: String; const ATask: TProc);
var
T : TThread;
begin
if (not Assigned(WaitForm))then
WaitForm := TWaiting.Create(nil);
T := TThread.CreateAnonymousThread(
procedure
begin
try
ATask;
except
HandleException;
end;
end);
T.OnTerminate := OnTerminateTask;
T.Start;
WaitForm.WaitTitle.Caption := ATitle;
WaitForm.ShowModal;
DoHandleException;
end;
class procedure TWaiting.Status(AMessage: String);
begin
TThread.Synchronize(TThread.CurrentThread,
procedure
begin
if (Assigned(WaitForm)) then
begin
WaitForm.WaitMessage.Caption := AMessage;
WaitForm.Update;
end;
end);
end;
class procedure TWaiting.OnTerminateTask(Sender: TObject);
begin
if (Assigned(WaitForm)) then
begin
WaitForm.Close;
WaitForm := nil;
end;
end;
class procedure TWaiting.HandleException;
begin
FException := Exception(AcquireExceptionObject);
end;
class procedure TWaiting.DoHandleException;
begin
if (Assigned(FException)) then
begin
try
if (FException is Exception) then
raise FException at ReturnAddress;
finally
FException := nil;
ReleaseExceptionObject;
end;
end;
end;
end.
Usage:
procedure TFSales.FinalizeSale;
begin
TWaiting.Start('Processing Sale...',
procedure
begin
TWaiting.Status('Sending data to database');
Sleep(2000);
TWaiting.Status('Updating Inventory');
Sleep(2000);
end);
end;