Get WM_LBUTTONDOWN message while ShellExecAndWait? - delphi

Question

In the Delphi IDE, create a VCL Forms Application. Then add a TApplicationEvents component and a TButton on the form. Then add these two event-handlers:
uses
JclShell;
procedure TForm3.ApplicationEvents1Message(var Msg: tagMSG; var Handled: Boolean);
begin
if Msg.Message = WM_LBUTTONDOWN then
begin
Self.Caption := 'WM_LBUTTONDOWN';
end
else if Msg.Message = WM_LBUTTONUP then
begin
Self.Caption := 'WM_LBUTTONUP';
end
end;
procedure TForm3.Button1Click(Sender: TObject);
begin
JclShell.ShellExecAndWait('C:\Windows\system32\notepad.exe');
Self.Caption := 'Notepad closed';
end;
Now click on the button. The following happens:
"WM_LBUTTONDOWN" appears on the form's title-bar.
"WM_LBUTTONUP" appears on the form's title-bar.
Notepad is executed.
Then click again on the button which starts another instance of Notepad BUT this time without writing anything on the form's title-bar.
Obviously, the program is stuck in JclShell.ShellExecAndWait which returns only when Notepad is closed. So when Notepad is closed any mouse-click again writes to the form's title-bar.
So we can see that while Notepad is running in JclShell.ShellExecAndWait everything in the program works normally: You could even make mathematical calculations while Notepad is running in JclShell.ShellExecAndWait. Only the ApplicationEvents1Message is not triggered while Notepad is running.
So how can I get a WM_LBUTTONDOWN message while Notepad is running in JclShell.ShellExecAndWait?

Answer 1

While JclShell.ShellExecAndWait is waiting for the spawned process to exit it uses following basic message pump:
while WaitForSingleObject(Sei.hProcess, 10) = WAIT_TIMEOUT do
repeat
Msg.hwnd := 0;
Res := PeekMessage(Msg, Sei.Wnd, 0, 0, PM_REMOVE);
if Res then
begin
TranslateMessage(Msg);
DispatchMessage(Msg);
end;
until not Res;
CloseHandle(Sei.hProcess);
I say basic, because the looped part of the VCLs message pump looks like this:
if PeekMessage(Msg, 0, 0, 0, PM_NOREMOVE) then
begin
Unicode := (Msg.hwnd = 0) or IsWindowUnicode(Msg.hwnd);
if Unicode then
MsgExists := PeekMessageW(Msg, 0, 0, 0, PM_REMOVE)
else
MsgExists := PeekMessageA(Msg, 0, 0, 0, PM_REMOVE);
if MsgExists then
begin
Result := True;
if Msg.Message <> WM_QUIT then
begin
Handled := False;
if Assigned(FOnMessage) then FOnMessage(Msg, Handled);
if not IsPreProcessMessage(Msg) and not IsHintMsg(Msg) and
not Handled and not IsMDIMsg(Msg) and
not IsKeyMsg(Msg) and not IsDlgMsg(Msg) then
begin
TranslateMessage(Msg);
if Unicode then
DispatchMessageW(Msg)
else
DispatchMessageA(Msg);
end;
end
else
begin
{$IF DEFINED(CLR)}
if Assigned(FOnShutDown) then FOnShutDown(self);
DoneApplication;
{$IFEND}
FTerminate := True;
end;
end;
The code line
if Assigned(FOnMessage) then FOnMessage(Msg, Handled);
is what will hand the message to your TApplicationEvents.OnMessage via the TMultiCaster that TApplicationEvents has assigned to Application.OnMessage. This is not done in the JCL source.
Additional issues might be that JCL is not UniCode aware regarding messages at this point and there is no handling for WM_QUIT.
What to do about this is depending on what you want to achieve, too. Why do you want to receive this messages in the first place?
I mean it is possible to alter the JCL source - if you are willing to do so - and to add VCL.Forms to the uses, and then call the event handler if assigned like the VCL does:
while WaitForSingleObject(Sei.hProcess, 10) = WAIT_TIMEOUT do
repeat
Msg.hwnd := 0;
Res := PeekMessage(Msg, Sei.Wnd, 0, 0, PM_REMOVE);
if Res then
begin
Handled := False;
if Assigned(Application.OnMessage) then
Application.OnMessage(Msg, Handled);
if not Handled then
begin
TranslateMessage(Msg);
DispatchMessage(Msg);
end;
end;
until not Res;
CloseHandle(Sei.hProcess);
Or even call Application.ProcessMessages, to have the same message processing in place the VCL does:
while WaitForSingleObject(Sei.hProcess, 10) = WAIT_TIMEOUT do
Application.ProcessMessages;
CloseHandle(Sei.hProcess);
This is working, I could not see any side effects like commenters suggested. But before changing the JCL source in this way I would probably implement my own ShellExecAndWait. Depending on what you would like to achieve, the normal dispatching of messages should still work. So if your TFrom has e.g.
procedure WMLButtonDown(var Message: TWMLButtonDown); message WM_LBUTTONDOWN;
implemented, it should be called. This is, however, only the case if the message was directed at your form. If the button is clicked, the resulting messages will be directed to the button itself. You would need to implement your own descendant class then.
Maybe a completely different suggestion?
If your goal is to let certain controls not to be clicked while JclShell.ShellExecAndWait is waiting, and using message handling is your approach to make this possible, there might be other things you can try.
Why not disable them? This will also give the user a visible indication that clicking is a no-go.
Button1.Enabled := False;
try
JclShell.ShellExecAndWait('C:\Windows\system32\notepad.exe');
Self.Caption := 'Notepad closed';
finally
Button1.Enabled := True;
end;
If you are concerned about multiple controls create a TAction, assign it to every control you want to disable and disable all of them together with the Enabled property of the TAction.
Or, just hide your form while notepad.exe is opened?
Hide;
try
JclShell.ShellExecAndWait('C:\Windows\system32\notepad.exe');
Self.Caption := 'Notepad closed';
finally
Show;
end;
Or you could remove the event handlers:
var
ATmpOnClick: TNotifyEvent;
begin
ATmpOnClick := Button1.OnClick;
Button1.OnClick := nil;
try
JclShell.ShellExecAndWait('C:\Windows\system32\notepad.exe');
Self.Caption := 'Notepad closed';
finally
Button1.OnClick := ATmpOnClick;
end;
end;