Using text callbacks

If you store you application data and you wish to save some
time and memory, you can have the list ask your application
for the strings to display in the list on the fly, instead of
having the list store the strings explicitly. This will reduce
the amount of memory your final app needs and will make filling
and deleting the list contents quicker, at the expense of slightly
slower display time. (If you are displaying several thousand items
in the list, the time saved in filling and cleaning the list
more than makes up for fractionally slower display). Furthermore,
list sorting can become lightning fast.

Suppose your data is stored in your app in structures “ItemStruct”
declared:

typedef struct {
	int nItemNo;
	CString strName;
} ItemStruct;

and each structure is stored in an array, list or map. The easiest
way to let the list know about each item is to store a pointer
to each item in the lParam field of the LV_ITEM structure you
pass to the CListCtrl::InsertItem function.

Firstly, when you add a new item to the list you should set
the LVIF_PARAM bit in the mask field of your LV_ITEM structure
that you are using. This lets the list know that the lParam field
of your LV_ITEM contains data.

You may want to create a helper function to add a new item to the
list like:

BOOL CMyListCtrl::AddItem(ItemStruct* pItem, int nPos /* = -1 *
)
{
	int nNumItems = GetItemCount();
	int nInsertPos = (nPos >= 0 && nPos <= nNumItems)? nPos : nNumItems;

	LV_ITEM Item;
	Item.lParam	  = (LPARAM) pItem;		// Store the pointer to the data
	Item.pszText  = LPSTR_TEXTCALLBACK;		// using callbacks to get the text
	Item.mask	  = LVIF_TEXT | LVIF_PARAM;	// lParam and pszText fields active
	Item.iItem	  = nInsertPos;			// position to insert new item
	Item.iSubItem = 0;

	if (InsertItem(&Item) < 0)
		return FALSE;
	else
		return TRUE;
}

The LPSTR_TEXTCALLBACK value for the pszText field tells the list that
it should use callbacks to get the text to display, instead of storing
the text itself. Every time the list needs to display text, it sends
a LVN_GETDISPINFO. We don’t add text for the subitems, since they will
be dealt with in the LVN_GETDISPINFO handler as well.

You need to handle the windows notification LVN_GETDISPINFO using the
following:

BEGIN_MESSAGE_MAP(CMyListCtrl, CListCtrl)
	//{{AFX_MSG_MAP(CMyListCtrl)
	ON_NOTIFY_REFLECT(LVN_GETDISPINFO, OnGetDispInfo)
	//}}AFX_MSG_MAP
END_MESSAGE_MAP()

and have an accompanying function declared in you class definition:

class CMyListCtrl
{
// .. other declarations

// Generated message map functions
protected:
	//{{AFX_MSG(CMyListCtrl)
	afx_msg void OnGetDispInfo(NMHDR* pNMHDR, LRESULT* pResult);
	//}}AFX_MSG
};

The handler function should look something like:

void CMyListCtrl::OnGetDispInfo(NMHDR* pNMHDR, LRESULT* pResult)
{
	LV_DISPINFO* pDispInfo = (LV_DISPINFO*)pNMHDR;

	if (pDispInfo->item.mask & LVIF_TEXT)
	{
		ItemStruct* pAppItem = (ItemStruct*) pDispInfo->item.lParam;
		CString strField = QueryItemText(pDispInfo->item.iItem,
							   pDispInfo->item.iSubItem);
		LPTSTR pstrBuffer = AddPool(&strField);

		pDispInfo->item.pszText = pstrBuffer;
	}
	*pResult = 0;
}

The NMHDR variable contains the list view display info, which in turn holds
a LV_ITEM member specifying the item and subitem it’s after, and what sort
of info it’s after. In our case we are only specifying text, so we only
deal with notifications where the pDispInfo->item.mask equals LVIF_TEXT.
We get a pointer to our Apps data through the lParam field of LV_ITEM, and
from this we get a text representation of our data (I’m using another
helper function “QueryItemText” implemented as

// returns a CString representation for the data in row nRow, column nCol
CString CMyListCtrl::QueryItemText(int nRow, int nCol)
{
	CString strField;
	ItemStruct* pAppItem = (ItemStruct*) GetItemData(nRow);
	if (!pAppItem) return strField;

	switch (nCol) {
		case 0:	strField.Format("%d",pAppItem->nItemNo);	break;
		case 1: strField = pAppItem->strName;			break;
		default: ASSERT(FALSE);
	}

	return strField;
}

The main reason I use the “QuesryItemText” function is that since we
are now using text callbacks, functions like CListCtrl::GetItemText
no longer work. If you use GetItemText (for instance in TitleTips
or InPlaceEdit’s) then you must replace them with QueryItemText in
order for them to work. The two lines:

	LPTSTR pstrBuffer = AddPool(&strField);
	pDispInfo->item.pszText = pstrBuffer;

in the OnGetDispInfo function were taken from Mike Blaszczak’s program
“ApiBrow” (get it either online or from his book “Proffessional MFC
with Visual C++ 5”). It handles the bizarre situtation that the list
control requires the buffer you pass back from the OnGetDispInfo to
be valid for 2 more LVN_GETDISPINFO notifications. His solution was
a simple caching to store sufficient copies so the list control
doesn’t have a cow. It goes something like this:

LPTSTR CMyListCtrl::AddPool(CString* pstr)
{
	LPTSTR pstrRetVal;
	int nOldest = m_nNextFree;

	m_strCPool[m_nNextFree] = *pstr;
	pstrRetVal = m_strCPool[m_nNextFree].LockBuffer();
	m_pstrPool[m_nNextFree++] = pstrRetVal;
	m_strCPool[nOldest].ReleaseBuffer();

	if (m_nNextFree == 3)
		m_nNextFree = 0;
	return pstrRetVal;
}

You will need to declare the protected attributes

	CString m_strCPool[3];
	LPTSTR	m_pstrPool[3];
	int	m_nNextFree;

in your class definition.

And that’s all there is to it!

One advantage of storing the data in the lParam field is that sorting
become very quick. A column sort function as already been provided in
in the MFC programmers sourcebook. It is prototyped:

BOOL CMasterListCtrl::SortTextItems(int nCol, BOOL bAscending,
					int low /*= 0*
, int high /*= -1*
 );

I will ignore the high and low parameters (sorry!) and instead
implement a different version: (see the docs on CListCtrl::SortItems
for details)

typedef struct {
	int nColumn;
	BOOL bAscending;
} SORTINFO;

BOOL CMasterListCtrl::SortTextItems(int nCol, BOOL bAscending)
{
	CWaitCursor waiter;

	SORTINFO si;
	si.nColumn = m_nSortColumn;
	si.bAscending = m_bSortAscending;
	return SortItems(CompareFunc, (LPARAM) &si);
}

int CALLBACK CompareFunc(LPARAM lParam1, LPARAM lParam2, LPARAM lParamSort)
{
	ItemStruct *pItem1 = (ItemStruct*) lParam1;
	ItemStruct *pItem2 = (ItemStruct*) lParam2;
	SORTINFO* pSortInfo = (SORTINFO*) lParamSort;
	ASSERT(pItem1 && pItem2 && pSortInfo);

	int result;
	switch (pSortInfo->nColumn) {
		case 0: if (pItem1->nItemNo < pItem2->nItemNo)
					result = -1;
				else if (pItem1->nItemNo == pItem2->nItemNo)
					result = 0;
				else
					result = 1;
				break;
		case 1: result = pItem1->strName.Compare(pItem2->strName);
				break;
		default: ASSERT(FALSE);
	}

	if (!pSortInfo->bAscending) result = -result;
	return result;
}

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