Win32 Dynamic User Interface Trick
Using the Windows API to Generate the UI Dynamically at Run Time
Jun 13, 2007
Introduction
Building an application that has a dynamic user interface is a very useful technique to be able to master. Anything from programmable button bars, wizard interfaces, polls, teaching and training software can benefit from a dynamic user interface. Generally speaking, a dynamic GUI consists of:
- Edit Areas
- Buttons
- Combo-Boxes
- Static Text
Each of these can be created dynamically, and can also be manipulated at run-time, using some simple Windows API calls. In fact, the manipulation of graphical user interface objects is key to the Win32 programming paradigm. There are 3 techniques used to manipulate the dynamic user interface:
- Creating (and destroying) the window
- Responding to client window messages
- Showing and hiding the child windows
We shall study each of these in turn and see how a dynamic user interface is built up by creating a series of child windows.
Using the CreateWindow Call
In Win32 API programming, a window is created with the CreateWindow call, and destroyed with the DestroyWindow. The handle returned by the CreateWindow call is a specific reference to that window, and is required by the DestroyWindow call to know which window is to be destroyed.
The CreateWindow function takes 11 parameters, and since we are creating child windows, some of these can be set to default or known values:
LPCSTR szClassName, // Either "edit", "button", "combobox", or "static"
LPCSTR szWindowName, // Can be set to ""
DWORD dwStyle, // Will be a combination of window styles including WS_CHILD|WS_VISIBLE
int nX, nY, nWidth, nHeight, // Positioning information
HWND hParent, // The parent window handle
HMENU nID, // The Identifier
HINSTANCE hInstance, // Global application instance
NULL); // Additional data, not required
The HMENU nID parameter needs some explanation. Usually it is set aside for a handle to the menu, but GUI elements do not have menus, and so it is re-used for the child window identifier, which is an integer. However, since the HMENU and integer types are not directly compatible, it is necessary to cast the integer to an HMENU type in the call. For example, the following code will create 10 child windows (buttons) in the parent window client area:
#define MAX_BUTTONS 10
// The handle pool
HWND hButtonBar[MAX_BUTTONS];
// Get the client area
RECT rcClient;
GetClientRect(hwnd, &rcClient);
// Decide on button dimensions
int nButtonWidth = rcClient.right / 10;
int nButtonHeight = rcClient.bottom / 10;
// Create the buttons
for (int i = 0; i MAX_BUTTONS; i++)
{
char szButtonName[25];
sprintf(szButtonName,"%02d", i);
hButtonBar[i] = CreateWindow( "button", szButtonName,
WS_CHILD|WS_VISIBLE|BS_PUSHBUTTON,
i * nButtonWidth, 0, nButtonWidth, nButtonHeight,
hwnd, (HMENU) BUTTON_START + i, hInstance, NULL);
}
We have retained the window handle, so we can destroy all the windows in a similar loop using a call such as
One can also destroy a window by sending it the WM_DESTROY message in some cases, but since this does not clean up menus, timers, and flush the message queue for some windows, it is not advisable.
Intercepting the WM_COMMAND Message
When the GUI element is interacted with, a message is sent to the parent window. This message comes as part of the WM_COMMAND message, as if a menu item was selected. There are some caveats when working with edit, list and combo-boxes, but these are well documented in the Win32 API reference, and on the Microsoft MSDN web site. Generally speaking, processing the notification is easy, requiring a loop-and-test approach:
{
for (i = BUTTON_START; i BUTTON_START + MAX_BUTTONS; i++)
{
if (wParam == i)
{
// Process the child window notification
}
}
switch (wParam) // Catch everything else
{
// Process menus, etc.
}
}
break;
Exactly what processing is done in the child window processing is application-dependent.
Showing and Hiding the GUI Elements
If we need to dynamically show and hide the elements, we can call the ShowWindow function. This is easy to use as long as we have positioned the elements correctly:
ShowWindow( hButtonBar[BUTTON_START], SW_SHOW ); // Show the child window
The SW_SHOW flag will restore the child window in its current position. We can use MoveWindow to move it around in between the SW_HIDE and SW_SHOW calls, to arrange the interface behind the scenes.
