Wait for a global variable to change its value - delphi

Question

I have created following abstract code where the user has 2 buttons:
One button is starting some kind of process. The global variable PleaseStop will tell the running process that it should stop its work.
The other button sets the global variable PleaseStop which will tell the procedure to stop.
-
var
PleaseStop: boolean;
IsRunning: boolean;
procedure TForm1.RunActionClick(Sender: TObject);
var
rnd: integer;
tic: Cardinal;
begin
try
IsRunning := true;
rnd := Random(100);
while not PleaseStop do
begin
tic := GetTickCount;
while (GetTickCount-tic < 1000) and not PleaseStop do
begin
Application.ProcessMessages;
Sleep(10);
end;
Memo1.Lines.Add(IntToStr(rnd));
end;
finally
IsRunning := false;
PleaseStop := false;
end;
end;
procedure TForm1.StopBtnClick(Sender: TObject);
begin
PleaseStop := true;
end;
Everything works as expected.
Now there will be problems if the user doesn't click the Stop button, but instead clicks the Run button again (which should be allowed).
I have now modified my code like this:
var
PleaseStop: boolean;
IsRunning: boolean;
procedure TForm1.Button1Click(Sender: TObject);
var
rnd: integer;
tic: Cardinal;
begin
// ---- BEGIN NEW ----
if IsRunning then
begin
PleaseStop := true; // End the previous actions
while PleaseStop do // Wait until the previous actions are done
begin
// TODO: this loop goes forever. PleaseStop will never become false
Application.ProcessMessages;
Sleep(10);
end;
// Now we can continue
end;
// ---- END NEW ----
try
IsRunning := true;
rnd := Random(100);
while not PleaseStop do
begin
tic := GetTickCount;
while (GetTickCount-tic < 1000) and not PleaseStop do
begin
Application.ProcessMessages;
Sleep(10);
end;
Memo1.Lines.Add(IntToStr(rnd));
end;
finally
IsRunning := false;
PleaseStop := false;
end;
end;
procedure TForm1.Button2Click(Sender: TObject);
begin
PleaseStop := true;
end;
Clicking the Start button again will cause a deadlock.
I assume the compiler thinks that while PleaseStop do is equal to while true do since I just set PleaseStop to true earlier. But in fact, this variable should be monitored...
I also tried putting [volatile] in front of the variables, and make them member of TForm1, but that doesn't work either.
Why didn't I use threads?
The code is heavily VCL dependent.
The run button will start a dia show. Every time, the run button is clicked, a random picture folder will be chosen.
So, when the user doesn't like the pictures, he will click "Run" again to switch to a new folder and start the new dia show automatically. The previous run should be stoppped therefore.

Answer 1

Your diagnosis is not exactly accurate, ProcessMessages, simply, cannot cause a previously retrieved message's processing to continue. You have to stop processing and let the execution continue from where re-entrancy occurred. Re-entrancy is the primary avoidance reason of Application.ProcessMessages, and you're doing it on purpose. Hard to work it out...
If you don't want to use synchronization and threading, you can use a timer instead. The code will be much simpler too.
procedure TForm1.FormCreate(Sender: TObject);
begin
Timer1.Enabled := False;
Randomize;
end;
var
rnd: Integer;
procedure TForm1.StartClick(Sender: TObject);
begin
rnd := Random(100);
Timer1.Enabled := True;
end;
procedure TForm1.StopClick(Sender: TObject);
begin
Timer1.Enabled := False;
end;
procedure TForm1.Timer1Timer(Sender: TObject);
begin
Memo1.Lines.Add(IntToStr(rnd));
end;

Answer 2

Try to rewrite your event handler like this:
var Needtorestart:Boolean = false;
procedure TForm1.Button1Click(Sender: TObject);
var
rnd: integer;
tic: Cardinal;
begin
// ---- BEGIN NEW ----
if IsRunning then
begin
PleaseStop := true; // End the previous action
Needtorestart := true;
Exit;
// Now we can continue
end;
// ---- END NEW ----
try
repeat
IsRunning := true;
rnd := Random(100);
while not PleaseStop do
begin
Tic := GetTickCount;
while (GetTickCount-tic < 1000) and not PleaseStop do
begin
Application.ProcessMessages;
Sleep(10);
end;
Memo1.Lines.Add(IntToStr(rnd));
end;
PleaseStop := false;
until not Needtorestart;
finally
IsRunning := false;
PleaseStop := false;
Needtorestart := false;
end;
end;