How to simulate drop-down form in Delphi?

How can i create a "drop-down" window using Delphi?

Research Effort

Making a proper drop-down requires a lot of pieces to carefully work together. I assume people don't like the difficult question, and would rather i asked seven separate questions; each one addressing one tiny piece of the problem. Everything that follows is my research effort into solving the deceptively simple problem.

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Note the defining characteristics of a drop-down window:

• 1. The drop-down extends outside it's "owner" window


• 2. The "owner" window keeps focus; the drop-down never steals focus


• 3. The drop-down window has a drop-shadow

This is the Delphi variation of the same question i asked about in WinForms:

• How to simulate a drop-down window in WinForms?

The answer in WinForms was to use the ToolStripDropDown class . It is a helper class that turns any form into a drop-down.

Lets do it in Delphi

I will start by creating a gaudy dropdown form, that serves as the example:

Next i will drop a button, that will be the thing i click to make the drop-down appear:

And finally i will wire-up some initial code to show the form where it needs to be in the OnClick:

procedure TForm3.Button1MouseDown(Sender: TObject; 
      Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
var
    frmPopup: TfrmPopup;
    pt: TPoint;
begin
    frmPopup := TfrmPopup.Create(Self);

    //Show the form just under, and right aligned, to this button
    pt := Self.ClientToScreen(Button1.BoundsRect.BottomRight);
    Dec(pt.X, frmPopup.ClientWidth);

    frmPopup.Show(Self, Self.Handle, pt);
end;

Edit: Changed it to MouseDown rather than Click. Click is incorrect, as the drop-down is shown without the need to click. One of the unresolved issues is how to hide a drop-down if the user mouse-downs the button again. But we'll leave that for the person who answers the question to solve. Everything in this question is research effort - not a solution.

And we're off:

Now how to do it the right way?

First thing we notice right away is the lack of a drop-shadow. That's because we need to apply the CS_DROPSHADOW window style:

procedure TfrmPopup.CreateParams(var Params: TCreateParams);
const
    CS_DROPSHADOW = $00020000;
begin
    inherited CreateParams({var}Params);

    Params.WindowClass.Style := Params.WindowClass.Style or CS_DROPSHADOW;
end;

That fixes that:

Focus Stealing

The next issue is that calling .Show on the popup causes it to steal focus (the title bar of the application indicates that it has lost focus). Sertac comes up with the solution to this.

• when the popup receives it's WM_Activate message indicating that it is receiving focus (i.e. Lo(wParam) <> WA_INACTIVE):


• send the parent form a WM_NCActivate (True, -1) to indicate that it should draw itself like it still has focus

We handle the WM_Activate:

protected
   procedure WMActivate(var Msg: TWMActivate); message WM_ACTIVATE;

and the implementation:

procedure TfrmPopup.WMActivate(var Msg: TWMActivate);
begin
    //if we are being activated, then give pretend activation state back to our owner
    if (Msg.Active <> WA_INACTIVE) then
        SendMessage(Self.PopupParent.Handle, WM_NCACTIVATE, WPARAM(True), -1);

    inherited;
end;

So the owner window looks like it still has focus (who knows if that is the correct way to do it - it only looks like it still has focus):

Rolling up

Fortunately, Sertac already solves the problem of how to dismiss the window whenever the user clicks away:

• when the popup receives it's WM_Activate message indicating that it is losing focus (i.e. Lo(wParam) = WA_INACTIVE):


• send the owner control a notification that we are rolling up


• Free the popup form

We add that to our existing WM_Activate handler:

procedure TfrmPopup.WMActivate(var Msg: TWMActivate);
begin
    //if we are being activated, then give pretend activation state back to our owner
    if (Msg.Active <> WA_INACTIVE) then
        SendMessage(Self.PopupParent.Handle, WM_NCACTIVATE, WPARAM(True), -1);

    inherited;

    //If we're being deactivated, then we need to rollup
    if Msg.Active = WA_INACTIVE then
    begin
        //TODO: Tell our owner that we've rolled up

        //Note: The parent should not be using rollup as the time to read the state of all controls in the popup.
        //      Every time something in the popup changes, the drop-down should give that inforamtion to the owner
        Self.Release; //use Release to let WMActivate complete
    end;
end;

Sliding the dropdown

Dropdown controls use AnimateWindow to slide the drop-down down. From Microsoft's own combo.c:

if (!(TEST_EffectPUSIF(PUSIF_COMBOBOXANIMATION))
      || (GetAppCompatFlags2(VER40) & GACF2_ANIMATIONOFF)) {
   NtUserShowWindow(hwndList, SW_SHOWNA);
} 
else 
{
   AnimateWindow(hwndList, CMS_QANIMATION, (fAnimPos ? AW_VER_POSITIVE :
            AW_VER_NEGATIVE) | AW_SLIDE);
}

After checking if animations should be used, they use AnimateWindow to show the window. We can use SystemParametersInfo with SPI_GetComboBoxAnimation:

Determines whether the slide-open effect for combo boxes is enabled. The pvParam parameter must point to a BOOL variable that receives TRUE for enabled, or FALSE for disabled.

Inside our newly consecrated TfrmPopup.Show method, we can check if client area animations are enabled, and call either AnimateWindow or Show depending on the user's preference:

procedure TfrmPopup.Show(Owner: TForm; NotificationParentWindow: HWND;
      PopupPosition: TPoint);
var
    pt: TPoint;
    comboBoxAnimation: BOOL;
begin
    FNotificationParentWnd := NotificationParentWindow;

    //We want the dropdown form "owned" by (i.e. not "parented" to) the OwnerWindow
    Self.Parent := nil; //the default anyway; but just to reinforce the idea
    Self.PopupParent := Owner; //Owner means the Win32 concept of owner (i.e. always on top of, cf Parent, which means clipped child of)
    Self.PopupMode := pmExplicit; //explicitely owned by the owner

    //Show the form just under, and right aligned, to this button
    Self.BorderStyle := bsNone;
    Self.Position := poDesigned;
    Self.Left := PopupPosition.X;
    Self.Top := PopupPosition.Y;

    if not Winapi.Windows.SystemParametersInfo(SPI_GETCOMBOBOXANIMATION, 0, @comboBoxAnimation, 0) then
        comboBoxAnimation := False;

    if comboBoxAnimation then
    begin
        //200ms is the shell animation duration
        AnimateWindow(Self.Handle, 200, AW_VER_POSITIVE or AW_SLIDE or AW_ACTIVATE);
    end
    else
        inherited Show;
end;

Edit: Turns out there is SPI_GETCOMBOBOXANIMATION which should probably use over SPI_GETCLIENTAREAANIMATION. Which points to the depths of difficulty hidden behind the subtle "How to simulate a drop-down". Simulating a drop-down requires a lot of stuff.

The problem is that Delphi forms pretty much fall over dead if you try to use ShowWindow or AnimateWindow behind their back:

How to solve that?

It's also odd that Microsoft itself uses either:

• ShowWindow(..., SW_SHOWNOACTIVATE), or


• AnimateWindow(...) *(without AW_ACTIVATE)

to show the drop-down listbox without activation. And yet spying on a ComboBox with Spy++ i can see WM_NCACTIVATE flying around.

In the past people have simulated a slide window using repeated calls to change the Height of the drop-down form from a timer. Not only is this bad; but it also changes the size of the form. Rather than sliding down, the form grows down; you can see all the controls change their layout as the drop-down appears. No, having the drop-down form remain it's real size, but slide down is what is wanted here.

I know AnimateWindow and Delphi have never gotten along. And the question has been asked, a lot, long before Stackoverflow arrived. I even asked about it in 2005 on the newsgroups. But that can't stop me from asking again.

I tried to force my form to redraw after it animates:

AnimateWindow(Self.Handle, 200, AW_VER_POSITIVE or AW_SLIDE or AW_ACTIVATE);
Self.Repaint;
Self.Update;
Self.Invalidate;

But it doesn't work; it just sits there mocking me:

Now showing again when i want to close-up

If a combobox is dropped down, and the user tries to MouseDown on the button, the real Windows ComboBox control does not simply show the control again, but instead hides it:

The drop-down also knows that it is currently "dropped-down", which is useful so that it can draw itself as if it is in "dropped down" mode. What we need is a way to know that the drop-down is dropped down, and a way to know that the drop-down is no longer dropped down. Some kind of boolean variable:

private
   FDroppedDown: Boolean;

And it seems to me that we need to tell the host that we're closing up (i.e. losing activation). The host then needs to be responsible for destroying the popup. (the host cannot be responsible for destroying the popup; it leads to an unresolvable race condition). So i create a message used to notify the owner that we're closing up:

const
   WM_PopupFormCloseUp = WM_APP+89;

Note: I don't know how people avoid message constant conflicts (especially since CM_BASE starts at $B000 and CN_BASE starts at $BC00).

Building on Sertac's activation/deactivation routine:

procedure TfrmPopup.WMActivate(var Msg: TWMActivate);
begin
    //if we are being activated, then give pretend activation state back to our owner
    if (Msg.Active <> WA_INACTIVE) then
        SendMessage(Self.PopupParent.Handle, WM_NCACTIVATE, WPARAM(True), -1);

    inherited;

    //If we're being deactivated, then we need to rollup
    if Msg.Active = WA_INACTIVE then
    begin
        //DONE: Tell our owner that we've rolled up
        //Note: We must post the message. If it is Sent, the owner
        //will get the CloseUp notification before the MouseDown that
        //started all this. When the MouseDown comes, they will think
        //they were not dropped down, and drop down a new one.
        PostMessage(FNotificationParentWnd, WM_PopupFormCloseUp, 0, 0);

        Self.Release; //use release to give WM_Activate a chance to return
    end;
end;

And then we have to change our MouseDown code to understand that the drop-down is still there:

procedure TForm3.Edit1MouseDown(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
var
    frmPopup: TfrmPopup;
    pt: TPoint;
begin
    //If we (were) dropped down, then don't drop-down again.
    //If they click us, pretend they are trying to close the drop-down rather than open a second copy
    if FDroppedDown then
    begin
        //And since we're receiving mouse input, we by defintion must have focus.
        //and since the drop-down self-destructs when it loses activation, 
        //it can no longer be dropped down (since it no longer exists)
        Exit;
    end;

    frmPopup := TfrmPopup.Create(Self);

    //Show the form just under, and right aligned, to this button
    pt := Self.ClientToScreen(Edit1.BoundsRect.BottomRight);
    Dec(pt.X, frmPopup.ClientWidth);

    frmPopup.Show(Self, Self.Handle, pt);
    FDroppedDown := True;
end;

And i think that's it

Aside from the AnimateWindow conundrum, i may have been able use my research effort to solve all the problems i can think of in order to:

Simulate a drop-down form in Delphi

Of course, this could all be for naught. It might turn out there's a VCL function:

TComboBoxHelper = class;
public
   class procedure ShowDropDownForm(...);
end;

In which case that would be the correct answer.