Design Multi-Threaded SDI Application

SDI (Single Document Interface) has been pushed by Microsoft Office years ago. Previously, it was MDI (Multiple Document Interface) in few older Office Suite. Delphi Win32 VCL Form Application support MDI natively. It requires some works to get SDI done in Delphi.

A new document interface model: Tabbed Browsing has been pushed by Firefox that grab lot of attention too. A more recent Google Chrome can even undock a tab to make it as a separate desktop window.

It is no doubt that SDI has some advantages over MDI. Research shows that using more than one desktop do improve productivity of user. Dual-View environment will become popular in end user's day to day work. MDI application can't utilize multi-desktop environment. It may only show on one desktop only. However, SDI doesn't has the restriction and may fully utilize multi-desktop environment. Perhaps, the only drawback is the over created SDI window flow on desktop may irritate user's eye.

Show form as a separate desktop button

By default, all newly form create form may just show as is flowing on desktop. There is only one desktop button for the VCL application you launch regardless of how many forms you created. Running the following code for each form instance will create a desktop button:

procedure TChildForm.CreateParams(var Params: TCreateParams);
begin
inherited CreateParams(Params);
with Params do begin
ExStyle := ExStyle or WS_EX_APPWINDOW;
end;
end;

Now, all TChildForm instances will have separate desktop button. That makes your application start looks like SDI.

Make it more like SDI

After play around with the above application for while, you start notice few things:

1. If my child form overlap the main form, my main form always stay behind the child form
2. Minimize main form will minimize all child forms and desktop button shrink to only one.
3. Press Alt-Tab to bring the window selector doesn't show all correct number of main form and child form windows.

This is because all child form instance's WndParent handle is set to Application.Handle or Application.MainformHandle. To overcome above problems, we can code like this:

procedure TChildForm.CreateParams(var Params: TCreateParams);
begin
inherited CreateParams(Params);
with Params do begin
ExStyle := ExStyle or WS_EX_APPWINDOW;
WndParent := GetDesktopwindow;
end;
end;

Now all main form and child form behave similar and independent on your desktop. Only different is closing the main form will close all child forms and end the process.

Child Form invoke modal form freeze whole application

So far, you should happy with the new SDI looks. It works great. But if your child form run the following codes:

begin
ShowMessage('Modal Form Launched');
end;

begin
raise Exception.Create('Exception raised');
end;

You will notice that you can't activate other child form or main form anymore. This reason behind this is simple. TCustomForm.ShowModal is running in a repeat until loop waiting for the form to return a ModalResult:

function TCustomForm.ShowModal: Integer;
begin
...
Application.ModalStarted;
try
...
try
Show;
try
ModalResult := 0;
repeat
Application.HandleMessage;
if Application.Terminated then ModalResult := mrCancel else
if ModalResult <> 0 then CloseModal;
until ModalResult <> 0;
...
finally
Hide;
end;
finally
...
end;
finally
Application.ModalFinished;
end;
end;

For simple message dialog like ShowMessage or exception dialog, we may use Windows.MessageBox to prevent the freezing by introduce a parent form handle to the API function.

Other modal dialog may need extra coding to prevent the freezing is mimic TForm.ShowModal by using TForm.Show method instead. The following code reveals a possible solution:

procedure ShowOwnModal(const aParentForm: TCustomForm; const aForm: TForm;
const Proc: TProc<TModalResult>);
begin
aForm.PopupParent := aParentForm;
aForm.Show;

aParentForm.Enabled := False;
try
while aForm.Visible do
Application.HandleMessage;
finally
aParentForm.Enabled := True;
end;

Proc(aForm.ModalResult);
end;

procedure TChildForm.btnModalFormClick(Sender: TObject);
var F: TForm;
begin
F := TModalForm.Create(Self);
try
ShowOwnModal(Self, F,
procedure(M: TModalResult)
begin
if M = mrOK then
ShowMessage('OK pressed')
else
ShowMessage('Cancel pressed');
end
);
finally
F.Free;
end;
end;

We may reuse procedure ShowOwnModal else where in code that need to mimic ShowModal. Furthermore, the ShowOwnModal strategy allow us to retain the classic coding style of using ShowModal in try...finally block:

F := TModalForm.Create(Self);
try
if F.ShowModal = mrOK then
...
finally
F.Free;
end;

Child form running lengthy task freeze whole application

If the child form is running a task that takes a while to complete. Whole application will freeze. This doesn't seems to be a acceptable behavior as user may have a perception that each child window should work independently from others.

A straight solution to the problem is using thread to run the task.

Hybrid Mode: Working with SDI and MDI together

There is an obvious disadvantage with SDI mode. If too many SDI form has been open on desktop, user desktop getting messy and become harder for user to find and switch form. It is also harder for user to identify a window was instantiated from which application. In this scenarios, user may miss the classic MDI mode that collect all related forms under one roof.

Perhaps a solution would be mixing both mode together. If we make MDI as a primary mode, all new instantiated window will become MDI child form by default. There is a gadget to switch the MDI child form to SDI form floating on desktop window and vice versa allowing user working on multi-desktop environment. This design may be a favor solution for user who need SDI in ad-hoc manner.

The following code reveal a possible design:

type
TfmMDIChild = class(TForm)
Button1: TButton;
Button2: TButton;
procedure Button1Click(Sender: TObject);
procedure Button2Click(Sender: TObject);
protected
procedure CreateParams(var Params: TCreateParams); override;
end;

procedure TfmMDIChild.Button1Click(Sender: TObject);
begin
Self.FormStyle := fsNormal;
end;

procedure TfmMDIChild.Button2Click(Sender: TObject);
begin
Self.Hide; // Hide the form first else CreateParams will invoke twice
Self.FormStyle := fsMDIChild;
end;

procedure TfmMDIChild.CreateParams(var Params: TCreateParams);
begin
inherited;
if FormStyle = fsNormal then begin
with Params do begin
ExStyle := Params.ExStyle or WS_EX_APPWINDOW;
WndParent := GetDesktopWindow;
end;
end else if FormStyle = fsMDIChild then begin
Params.Width := Self.Width;
Params.Height := Self.Height;
end;
end;

The form provide 2 buttons allowing user to switch from MDI to SDI and SDI to MDI. CreateParams will be invoked if FormStyle has changed.