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Part I

Let’s see in some more detail the basic interaction mechanism:

We’ve seen that the VBCE WindowProc does not provide functionality to react programmatically to WM_NOTIFY callbacks. So, we would have to "Subclass" the VBCE window which is nothing else than extending WindowProc with extra functionality designed by the programmer.

To implement Subclassing, we would have to inform the system that our WindowProc is somewhere else and return the control to the original WindowProc once we’re done with the callback:

And we came to another drawback on VBCE: the lack of the AddressOf operator. Without AddressOf we’re not able to inform the Month Calendar Control that we want its messages to be sent to our own handler so we’re not able to implement this mechanism.

In our case, it would be excellent to be informed by the Month-Calendar control every time the user clicks on a date. As we are unable to receive these messages, we have to implement another approach: we show the calendar, wait for the user to click an "OK" button, and then we send a message to the control requesting the selected date.

On the example, our VBCE window calls just two functions to use the Month-Calendar control.

First we load the calendar on the "Form_Load()" event with:

'-- Insert Calendar Common Control
MCalendar_Load Me.hwnd, App.hInstance, 8, 40

where the last two values are just the position (in pixels) of the upper-left corner of the calendar, and we call:

'-- Get Calendar Date selected
mDate = MCalendar_Date

to get the date selected. All the Month-Calendar related code is included on the "McalendarMod" module. We also use the GetLocalTime and SetLocalTime API functions to retrieve and set our system date but this is not directly related to the Month-Calendar control (check the UDT demonstration code).

Before we can create or use any common controls, we have to register them. There are actually two options available: we either register the specific control we want to use or we register all common controls at once. The MCalendar_Load sub uses the first option by calling the InitCommonControlsEx API function with the ICC_DATE_CLASSES parameter on its INITCOMMONCONTROLSEX udt.

Then we create the Month-Calendar window, CreateWindowEx, with the desired profile. "dwStyle", the 4th call parameter of CreateWindowEx, let us change the control style. Example: if we add MCS_NOTODAY (&H10), the Month-Calendar control will not display the "today" date at the bottom of the control:

Const MCS_NOTODAY = &H10
'-- Create the month calendar (resize it later)
MCalendar_hWnd = CreateWindowEx( _
0, MONTHCAL_CLASS, "", _
WS_BORDER + WS_CHILD + WS_VISIBLE + MCS_NOTODAY, _
0, 0, 0, 0, _
hwndOwner, vbNull, g_hinst, vbNull)

After creation we’ll interact with the control through messages. MCalendar_Load uses two messages: first it uses the MCM_GETMINREQRECT message to get the required width and height to show an entire month. These values will be used further on by the SetWindowPos API function. Then it uses the MCM_SETCOLOR with the MCSC_MONTHBK parameter to set the background color (RGB) to light yellow.

This function just sends a MCM_GETCURSEL message to get the currently selected date on a SYSTEMTIME udt and returns it. The time part is extracted because it’s irrelevant on a Month-Calendar control.

Written on: 27/Oct/99
By: Antonio Paneiro
ComMEDIA,Lda.
apaneiro@commedia.pt
http://www.commedia.pt/ce