BBC BASIC Programmers' Reference

by Richard Russell, January 2007

Ordinary comboboxes can be created using the routines provided in the WINLIB2 (for use in a dialogue box) and WINLIB5 (for use on the main output window) libraries. However there is a different kind, the extended combobox, which allows for the inclusion of a bitmap image as well as a text string in each of the selections:



This article explains how to incorporate extended comboboxes in your BBC BASIC programs.

To start off with we need to include this initialisation code:

      DIM icex{dwSize%, dwICC%}
      ICC_USEREX_CLASSES = &200
      icex.dwSize% = DIM(icex{})
      icex.dwICC% = ICC_USEREX_CLASSES
      SYS "InitCommonControlsEx", icex{}

The set of images to be used in the extended combobox must be supplied as an image list. The first step is to create an empty image list as follows:

      dx% = 16
      dy% = 16
      maximages% = 8
      SYS "ImageList_Create", dx%, dy%, 0, maximages%, 0 TO himglist%
      IF himglist% = 0 ERROR 100, "Couldn't create image list"

Here dx% and dy% are the width and height, respectively, of each image in pixels and maximages% is the maximum number of images that the image list will ever need to hold.

The next step is to populate the image list with images. You can do that either from a set of individual bitmaps or from one or more image strips (or a mixture of the two). An image strip is a bitmap containing several identical-sized images side-by-side.

For the purposes of this example I will use the bitmap file WIDGETS.BMP supplied with BBC BASIC for Windows; this contains five 24×24 images in a strip 120 pixels wide by 24 pixels high. Since these images are a little on the large size for use in a combobox the bitmap can be resized when it is loaded, thus:

      nimages% = 5
      bmpfile$ = @lib$+"..\examples\windows\widgets.bmp"
      LR_LOADFROMFILE = 16
      SYS "LoadImage", 0, bmpfile$, 0, dx%*nimages%, dy%, LR_LOADFROMFILE TO hbitmap%
      IF hbitmap% = 0 ERROR 100, "Couldn't load bitmap file"

Now it is straightforward to populate the first five images in the image list from the bitmap:

      SYS "ImageList_Add", himglist%, hbitmap%, 0
      SYS "DeleteObject", hbitmap%

More images (or strips) can be added, up to the limit specified when the image list was created, using further calls to ImageList_Add.

Now the image list is ready we can concentrate on creating the extended combobox itself. The process depends on whether you want to display the combobox in a dialogue box or directly on your program's main window.

As usual, we must install the appropriate library for working with dialogue boxes:

      INSTALL @lib$+"WINLIB2"

(alternatively install WINLIB2A or WINLIB2B if you need the facilities they provide).

Now we will create a simple dialogue box containing just a cancel button and the extended combobox:

      dlg% = FN_newdialog("Extended combobox", 250, 20, 150, 140, 8, 1000)
      PROC_pushbutton(dlg%, "Cancel", 2, 20, 120, 56, 14, 0)

      WS_VISIBLE = &10000000
      WS_CHILD = &40000000
      WS_VSCROLL = &200000
      CBS_AUTOHSCROLL = &40
      CBS_DROPDOWN = 2
      cbid% = 101
      PROC_dlgctrl(dlg%, "", cbid%, 20, 20, 90, 150, WS_VISIBLE OR WS_CHILD OR \
      \            WS_VSCROLL OR CBS_AUTOHSCROLL OR CBS_DROPDOWN, \
      \            "ComboBoxEx32")

Note that the combobox has been allocated the ID number 101. Refer to the documentation for PROC_dlgitem for the meaning of the other parameters.

The dialogue box is displayed in the usual way:

      PROC_showdialog(dlg%)

We've now created the dialogue box and the extended combobox, but the combobox is still empty. The next step is to tell the combobox which image list to use:

      CBEM_SETIMAGELIST = &402
      SYS "SendDlgItemMessage", !dlg%, cbid%, CBEM_SETIMAGELIST, 0, himglist%

Note the use of the combobox's ID number cbid%.

To add items to the combobox we first have to declare some constants and a data structure:

      CBEIF_TEXT = 1
      CBEIF_IMAGE = 2
      CBEIF_SELECTEDIMAGE = 4
      CBEM_INSERTITEMA = &401
      DIM cbxi{mask%, iItem%, pszText%, cchTextMax%, iImage%, iSelectedImage%, \
      \        iOverlay%, iIndent%, lParam}

Now we can define each item as follows:

        text$ = "Combobox item"+CHR$0
        cbxi.mask% = CBEIF_TEXT OR CBEIF_IMAGE OR CBEIF_SELECTEDIMAGE
        cbxi.iItem% = itemno%
        cbxi.pszText% = !^text$
        cbxi.iImage% = imageno%
        cbxi.iSelectedImage% = imageno%
        SYS "SendDlgItemMessage", !dlg%, cbid%, CBEM_INSERTITEMA, 0, cbxi{}

Here itemno% is the item to define (starting at zero for the first item), imageno% is the image to display alongside the text (zero for the first image in the image list) and text$ is the text for this item. Note particularly the CHR$0 termination for the text string. You can set itemno% to the special value -1, which adds it at the end of the list.

Note that is possible to use a different image for the currently selected item from that shown against the item in the drop-down list. If it's not convenient to define the text and image(s) at the same time, you can set the value of cbxi.mask% according to which you want to set.

To test which item in the extended combobox is selected you can use exactly the same code as for a regular combobox:

      CB_GETCURSEL = &147
      SYS "SendDlgItemMessage", !dlg%, cbid%, CB_GETCURSEL, 0, 0 TO selected%

The process for creating an extended combobox on the main output window is very similar to that for creating one in a dialogue box. As usual, we must install the appropriate library for working with boxes and buttons:

      INSTALL @lib$+"WINLIB5"

(alternatively install WINLIB5A, and remember to pass the parent window handle @hwnd% as the first parameter of FN_createwindow).

The extended combobox is created as follows:

      WS_VSCROLL = &200000
      CBS_AUTOHSCROLL = &40
      CBS_DROPDOWN = 2
      hcbex% = FN_createwindow("ComboBoxEx32", "", 200, 50, 140, 200, 0, \
      \        WS_VSCROLL OR CBS_AUTOHSCROLL OR CBS_DROPDOWN, 0)

Refer to the documentation for FN_createwindow for the meaning of the parameters.

The next step is to tell the combobox which image list to use:

      CBEM_SETIMAGELIST = &402
      SYS "SendMessage", hcbex%, CBEM_SETIMAGELIST, 0, himglist%

Note the use of the combobox's window handle hcbex%.

We declare some constants and a data structure exactly as before:

      CBEIF_TEXT = 1
      CBEIF_IMAGE = 2
      CBEIF_SELECTEDIMAGE = 4
      CBEM_INSERTITEMA = &401
      DIM cbxi{mask%, iItem%, pszText%, cchTextMax%, iImage%, iSelectedImage%, \
      \        iOverlay%, iIndent%, lParam}

Now we can define each item as follows:

        text$ = "Combobox item"+CHR$0
        cbxi.mask% = CBEIF_TEXT OR CBEIF_IMAGE OR CBEIF_SELECTEDIMAGE
        cbxi.iItem% = itemno%
        cbxi.pszText% = !^text$
        cbxi.iImage% = imageno%
        cbxi.iSelectedImage% = imageno%
        SYS "SendMessage", hcbex%, CBEM_INSERTITEMA, 0, cbxi{} TO res%

Here itemno% is the item to define (starting at zero for the first item), imageno% is the image to display alongside the text (zero for the first image in the image list) and text$ is the text for this item. Note particularly the CHR$0 termination for the text string. You can set itemno% to the special value -1, which adds it at the end of the list.

Note that is possible to use a different image for the currently selected item from that shown against the item in the drop-down list. If it's not convenient to define the text and image(s) at the same time, you can set the value of cbxi.mask% according to which you want to set.

To test which item in the extended combobox is selected you can use exactly the same code as for a regular combobox:

      CB_GETCURSEL = &147
      SYS "SendMessage", hcbex%, CB_GETCURSEL, 0, 0 TO selected%