Storing Data in Hidden List Boxes
Using Child Windows as Temporary Data Storage Gets Added Benefits
We look at how using list boxes in Windows Win32 GUI programming can provide a simple solution to application data storage using automatic API features.
Introduction
In this article we look at a couple of interesting ways to use the features provided by the Win32 API relating to list boxes. The reader should understand the Windows programming paradigm on the one hand, and some basic programming skills on the other. Although the examples are provided in C code, they ought to be fairly easily portable to other languages.
Creating the Hidden List Box
The list box is created with a simple call to CreateWindow, making sure that the style does not include WS_VISIBLE. The location and size parameters can also all be set to zero. Both of these together will ensure that the list box is never revealed to the user. The handle to it should be retained, or a static child ID used.
Searching and Sorting
List boxes can be created with the style LBS_SORT, which will sort the items, after a fashion. It is not as strict as an alphabetical sort, but works well with most string based data. When items are added with the LB_ADDSTRING message, the list box will automatically insert at the right position and return the index from the SendDlgItemMessage function call.
Searching the list box is also a useful feature of the Windows API. The LB_FINDSTRING and LB_FINDSTRINGEXACT messages work in a similar way:
- wParam : start index, integer
- lParam : string to search for
The difference between the 2 messages is that LB_FINDSTRING will return the index of the first occurrence of a string that starts with the text provided, while LB_FINDSTRINGEXACT works on an exact match only. To get the next occurrence, one needs only to supply the index returned by the first call. If LB_ERR was returned, the string was not found.
Caution: The start index is the index of the item before the item to start searching at. Therefore, to start at the first item, a value of -1 must be used.
An example of this message in action might be:
do
{
nIndex = SendDlgItemMessage( hParent, nID, LB_FINDSTRING, nIndex, (LPARAM) szText );
if (nIndex != LB_ERR)
{
// Do something
}
} while (nIndex != LB_ERR);
The LB_FINDSTRING and LB_FINDSTRINGEXACT messages do not search in a wrap around way - when the last item has been searched, LB_ERR is returned.
Practical Applications
One practical application of this technique occurs when processing files where the user should be presented with a list of file names (as the result of a search, for example), but where the original full file name with path needs to be retained. We then create 2 list boxes:
- Full Path List Box (hidden)
- Filename only List Box (sorted)
We link the 2 together by stripping the path from the file name, inserting it into the Filename list box, and keeping the index to use as the index parameter into the call to insert the full path file name into the hidden list box. If the user selects an item from the Filename list box, we can then extract the correct item from the hidden list box.
When doing this, the programmer has to be sure to use:
- LB_ADDSTRING for the sorted listbox
- LB_INSERTSTRING for the unsorted list box
As a final note, hidden list boxes can also be used, with the LB_DIR message, to get a directory listing, rather than using the FindFirstFile and FindNextFile API functions. All of these LB_ messages are sent with the SendDlgItemMessage function.
