TN061: ON_NOTIFY and WM_NOTIFY Messages
Note
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This technical note provides background information on the new WM_NOTIFY message and describes the recommended (and most common) way of handling WM_NOTIFY messages in your MFC application.
Notification Messages in Windows 3.x
In Windows 3.x, controls notify their parents of events such as mouse clicks, changes in content and selection, and control background painting by sending a message to the parent. Simple notifications are sent as special WM_COMMAND messages, with the notification code (such as BN_CLICKED) and control ID packed into wParam and the control's handle in lParam. Note that since wParam and lParam are full, there is no way to pass any additional data — these messages can be only simple notification. For instance, in the BN_CLICKED notification, there's no way to send information about the location of the mouse cursor when the button was clicked.
When controls in Windows 3.x need to send a notification message that includes additional data, they use a variety of special-purpose messages, including WM_CTLCOLOR, WM_VSCROLL, WM_HSCROLL, WM_DRAWITEM, WM_MEASUREITEM, WM_COMPAREITEM, WM_DELETEITEM, WM_CHARTOITEM, WM_VKEYTOITEM, and so on. These messages can be reflected back to the control that sent them. For more information, see TN062: Message Reflection for Windows Controls.
Notification Messages in Win32
For controls that existed in Windows 3.1, the Win32 API uses most of the notification messages that were used in Windows 3.x. However, Win32 also adds a number of sophisticated, complex controls to those supported in Windows 3.x. Frequently, these controls need to send additional data with their notification messages. Rather than adding a new WM_* message for each new notification that needs additional data, the designers of the Win32 API chose to add just one message, WM_NOTIFY, which can pass any amount of additional data in a standardized fashion.
WM_NOTIFY messages contain the ID of the control sending the message in wParam and a pointer to a structure in lParam. This structure is either an NMHDR structure or some larger structure that has an NMHDR structure as its first member. Note that since the NMHDR member is first, a pointer to this structure can be used as either a pointer to an NMHDR or as a pointer to the larger structure depending on how you cast it.
In most cases, the pointer will point to a larger structure and you'll need to cast it when you use it. In only a few notifications, such as the common notifications (whose names start with NM_) and the tool tip control's TTN_SHOW and TTN_POP notifications, is an NMHDR structure actually used.
The NMHDR structure or initial member contains the handle and ID of the control sending the message and the notification code (such as TTN_SHOW). The format of the NMHDR structure is shown below:
typedef struct tagNMHDR {
HWND hwndFrom;
UINT idFrom;
UINT code;
} NMHDR;
For a TTN_SHOW message, the code member would be set to TTN_SHOW.
Most notifications pass a pointer to a larger structure that contains an NMHDR structure as its first member. For instance, consider the structure used by the list view control's LVN_KEYDOWN notification message, which is sent when a key is pressed in a list view control. The pointer points to an LV_KEYDOWN structure, which is defined as shown below:
typedef struct tagLV_KEYDOWN {
NMHDR hdr;
WORD wVKey;
UINT flags;
} LV_KEYDOWN;
Note that since the NMHDR member is first in this structure, the pointer you're passed in the notification message can be cast to either a pointer to an NMHDR or a pointer to an LV_KEYDOWN.
Notifications Common to All New Windows Controls
Some notifications are common to all of the new Windows controls. These notifications pass a pointer to an NMHDR structure.
| Notification code | Sent because |
|---|---|
| NM_CLICK | User clicked left mouse button in the control |
| NM_DBLCLK | User double-clicked left mouse button in the control |
| NM_RCLICK | User clicked right mouse button in the control |
| NM_RDBLCLK | User double-clicked right mouse button in the control |
| NM_RETURN | User pressed the ENTER key while control has input focus |
| NM_SETFOCUS | Control has been given input focus |
| NM_KILLFOCUS | Control has lost input focus |
| NM_OUTOFMEMORY | Control could not complete an operation because there was not enough memory available |
ON_NOTIFY: Handling WM_NOTIFY Messages in MFC Applications
The function CWnd::OnNotify handles notification messages. Its default implementation checks the message map for notification handlers to call. In general, you do not override OnNotify. Instead, you provide a handler function and add a message-map entry for that handler to the message map of your owner window's class.
ClassWizard, via the ClassWizard property sheet, can create the ON_NOTIFY message-map entry and provide you with a skeleton handler function. For more information on using ClassWizard to make this easier, see Mapping Messages to Functions.
The ON_NOTIFY message-map macro has the following syntax:
ON_NOTIFY(wNotifyCode, id, memberFxn)
where the parameters are:
wNotifyCode
The code for the notification message to be handled, such as LVN_KEYDOWN.
id
The child identifier of the control for which the notification is sent.
memberFxn
The member function to be called when this notification is sent.
Your member function must be declared with the following prototype:
afx_msg void memberFxn(NMHDR* pNotifyStruct, LRESULT* result);
where the parameters are:
pNotifyStruct
A pointer to the notification structure, as described in the section above.
result
A pointer to the result code you'll set before you return.
Example
To specify that you want the member function OnKeydownList1 to handle LVN_KEYDOWN messages from the CListCtrl whose ID is IDC_LIST1, you would use ClassWizard to add the following to your message map:
ON_NOTIFY(LVN_KEYDOWN, IDC_LIST1, OnKeydownList1)
In the example above, the function provided by ClassWizard is:
void CMessageReflectionDlg::OnKeydownList1(NMHDR* pNMHDR, LRESULT* pResult)
{
LV_KEYDOWN* pLVKeyDow = (LV_KEYDOWN*)pNMHDR;
// TODO: Add your control notification handler
// code here
*pResult = 0;
}
Note that ClassWizard provides a pointer of the proper type automatically. You can access the notification structure through either pNMHDR or pLVKeyDow.
ON_NOTIFY_RANGE
If you need to process the same WM_NOTIFY message for a set of controls, you can use ON_NOTIFY_RANGE rather than ON_NOTIFY. For instance, you may have a set of buttons for which you want to perform the same action for a certain notification message.
When you use ON_NOTIFY_RANGE, you specify a contiguous range of child identifiers for which to handle the notification message by specifying the beginning and ending child identifiers of the range.
ClassWizard does not handle ON_NOTIFY_RANGE; to use it, you need to edit your message map yourself.
The message-map entry and function prototype for ON_NOTIFY_RANGE are as follows:
ON_NOTIFY_RANGE(wNotifyCode, id, idLast, memberFxn)
where the parameters are:
wNotifyCode
The code for the notification message to be handled, such as LVN_KEYDOWN.
id
The first identifier in the contiguous range of identifiers.
idLast
The last identifier in the contiguous range of identifiers.
memberFxn
The member function to be called when this notification is sent.
Your member function must be declared with the following prototype:
afx_msg void memberFxn(UINT id, NMHDR* pNotifyStruct, LRESULT* result);
where the parameters are:
id
The child identifier of the control that sent the notification.
pNotifyStruct
A pointer to the notification structure, as described above.
result
A pointer to the result code you'll set before you return.
ON_NOTIFY_EX, ON_NOTIFY_EX_RANGE
If you want more than one object in the notification routing to handle a message, you can use ON_NOTIFY_EX (or ON_NOTIFY_EX_RANGE) rather than ON_NOTIFY (or ON_NOTIFY_RANGE). The only difference between the EX version and the regular version is that the member function called for the EX version returns a BOOL that indicates whether or not message processing should continue. Returning FALSE from this function allows you to process the same message in more than one object.
ClassWizard does not handle ON_NOTIFY_EX or ON_NOTIFY_EX_RANGE; if you want to use either of them, you need to edit your message map yourself.
The message-map entry and function prototype for ON_NOTIFY_EX and ON_NOTIFY_EX_RANGE are as follows. The meanings of the parameters are the same as for the non-EX versions.
ON_NOTIFY_EX(nCode, id, memberFxn)
ON_NOTIFY_EX_RANGE(wNotifyCode, id, idLast, memberFxn)
The prototype for both of the above is the same:
afx_msg BOOL memberFxn(UINT id, NMHDR* pNotifyStruct, LRESULT* result);
In both cases, id holds the child identifier of the control that sent the notification.
Your function must return TRUE if the notification message has been completely handled or FALSE if other objects in the command routing should have a chance to handle the message.