How do combo boxes implement their dropdown lists?

[Stevie-O]

I have a program, I'll call it 'SS'. SS stores its information in a database. I'm developing an administration program to create databases for SS to use. I need to let the user choose where the database should be stored. Because the database server has to store the database, and not the local machine, I can't use a typical browse-for-folder dialog, since they work off the users's machine. I have a TreeView control and am perfectly able to populate it with the server's directory sturcture. However, tree views can be fairly big - so I'd really like to be able to make it appear at the click of a button, much like the list box part of a CBS_DROPDOWN/CBS_DROPDOWNLIST combo box.

My question is, how does the combo box accomplish this feat? After some simple observations, you'll see that the combo box is able to draw its list area OUTSIDE of its owner's window area - quite a feat, given that Windows doesn't generally let applications do such things.

Another thing is that clicking outside the list area will close the list back up. I tried using the LostFocus event, but that only works if the user clicks another control. If the user clicks the form, or a windowless 'control' (i.e. Line, Shape, Label, Image), nothing happens.

[Sam Finch]

it's a seperate window

[Stevie-O]

Okay...so how do I go about doing this myself? What are this windows' styles? What is its parent window? etc. etc.

[Sam Finch]

hmm, you probably want to add a form to your usercontroll and make it appearwith height = 0 and make the height bigger gradually.

[Stevie-O]

usercontrol? I'm not using a usercontrol. Usercontrols are restricted to their parents' client area, so if I made a usercontrol and made it bigger, the bottom and right sides would just be cut off. I need it to be 'over' everything else.

[Sam Finch]

right, I've got a very simple example for you, in a usercontrol project, have a command button to the right of your control, change the background colour so you can see it and add this code.

Option Explicit

Private Type RECT
        Left As Long
        Top As Long
        Right As Long
        Bottom As Long
End Type
Private Declare Function GetWindowRect Lib "user32" (ByVal hwnd As Long, lpRect As RECT) As Long

Dim frmDropDown As New Form1

Private Sub Command1_Click()
Dim apiControlRect As RECT
Dim apiButtonRect As RECT


GetWindowRect UserControl.hwnd, apiControlRect
GetWindowRect Command1.hwnd, apiButtonRect

frmDropDown.DropDown apiControlRect.Left, _
                     apiControlRect.Bottom, _
                     apiButtonRect.Left - apiControlRect.Left, _
                     (apiControlRect.Bottom - apiControlRect.Top) * 5
                     

End Sub

now add a form to your usercontrol project, set the borderstyle to 0 and change the backcolour, add a commandbutton near the top left and add this code

Option Explicit
Const DROPDOWNTIME = 2
Private Declare Function MoveWindow Lib "user32" (ByVal hwnd As Long, ByVal X As Long, ByVal Y As Long, ByVal nWidth As Long, ByVal nHeight As Long, ByVal bRepaint As Long) As Long

Public Function DropDown(X As Long, Y As Long, Width As Long, Height As Long)

Dim sngStart As Single
Dim lngCurrentHeight As Long


MoveWindow Me.hwnd, X, Y, Width, 0, True

Me.Visible = True

sngStart = Timer


Do While Timer < sngStart + DROPDOWNTIME

    MoveWindow Me.hwnd, _
               X, Y, _
               Width, _
               Height * (Timer - sngStart) / DROPDOWNTIME, _
               True
               
    DoEvents
               
Loop

MoveWindow Me.hwnd, X, Y, Width, Height, True

End Function

Private Sub Command1_Click()
Me.Hide
End Sub

then go to the file menu...AddProject and add a standard exe, right click project2 in your project explorer and select set as start up

add the controll to the form of this new project so you can see the command button and run the code.

[Stevie-O]

Cool, thanks...but how do I make it disappear when it loses focus?