A tree control allocates memory for storing each item; the text of the item labels takes up a significant portion of this memory. If your application maintains a copy of the strings in the tree control, you can decrease the memory requirements of the control by specifying the **LPSTR_TEXTCALLBACK** value in the *pszText* member of `TV_ITEM` or the *lpszItem* parameter instead of passing actual strings to the tree control. Using **LPSTR_TEXTCALLBACK** causes the tree control to retrieve the text of an item's label from the application whenever the item needs to be redrawn. To retrieve the text, the tree control sends a [TVN_GETDISPINFO](http://msdn.microsoft.com/library/windows/desktop/bb773518) notification message, which includes the address of a [NMTVDISPINFO](http://msdn.microsoft.com/library/windows/desktop/bb773418) structure. You must respond by setting the appropriate members of the included structure.

When the selection changes from one item to another, a tree control ([CTreeCtrl](../mfc/reference/ctreectrl-class.md)) sends [TVN_SELCHANGING](http://msdn.microsoft.com/library/windows/desktop/bb773547) and [TVN_SELCHANGED](http://msdn.microsoft.com/library/windows/desktop/bb773544) notification messages. Both notifications include a value that specifies whether the change is the result of a mouse click or a keystroke. The notifications also include information about the item that is gaining the selection and the item that is losing the selection. You can use this information to set item attributes that depend on the selection state of the item. Returning **TRUE** in response to `TVN_SELCHANGING` prevents the selection from changing; returning **FALSE** allows the change.

An item's current state is specified by the *nState* parameter. A tree control might change an item's state to reflect a user action, such as selecting the item or setting the focus to the item. In addition, an application might change an item's state to disable or hide the item or to specify an overlay image or state image.

When you specify or change an item's state, the *nStateMask* parameter specifies which state bits to set, and the *nState* parameter contains the new values for those bits. For example, the following example changes the current state of a parent item (specified by *hParentItem*) in a `CTreeCtrl` object (`m_treeCtrl`) to `TVIS_EXPANDPARTIAL`:

Another example of changing the state would be to set an item's overlay image. To accomplish this, *nStateMask* must include the `TVIS_OVERLAYMASK` value, and *nState* must include the one-based index of the overlay image shifted left eight bits by using the [INDEXTOOVERLAYMASK](http://msdn.microsoft.com/library/windows/desktop/bb761408) macro. The index can be 0 to specify no overlay image. The overlay image must have been added to the tree control's list of overlay images by a previous call to the [CImageList::SetOverlayImage](../mfc/reference/cimagelist-class.md#setoverlayimage) function. The function specifies the one-based index of the image to add; this is the index used with the INDEXTOOVERLAYMASK macro. A tree control can have up to four overlay images.

To set an item's state image, *nStateMask* must include the `TVIS_STATEIMAGEMASK` value, and *nState* must include the one-based index of the state image shifted left 12 bits by using the [INDEXTOSTATEIMAGEMASK](http://msdn.microsoft.com/library/windows/desktop/bb775597) macro. The index can be 0 to specify no state image. For more information about overlay and state images, see [Tree Control Image Lists](../mfc/tree-control-image-lists.md).

When label editing is canceled or completed, a tree control sends a [TVN_ENDLABELEDIT](http://msdn.microsoft.com/library/windows/desktop/bb773515) notification message. The *lParam* parameter is the address of a [NMTVDISPINFO](http://msdn.microsoft.com/library/windows/desktop/bb773418) structure. The **item** member is a [TVITEM](http://msdn.microsoft.com/library/windows/desktop/bb773456) structure that identifies the item and includes the edited text. You are responsible for updating the item's label, if appropriate, perhaps after validating the edited string. The *pszText* member of `TV_ITEM` is 0 if editing is canceled.

A tree control ([CTreeCtrl](../mfc/reference/ctreectrl-class.md)) sends the following notification messages as WM_NOTIFY messages:

|TVN_BEGINDRAG|Signals the start of a drag-and-drop operation|

|TVN_BEGINLABELEDIT|Signals the start of in-place label editing|

|TVN_BEGINRDRAG|Signals the start of a drag-and-drop operation, using the right mouse button|

|TVN_DELETEITEM|Signals the deletion of a specific item|

|TVN_ENDLABELEDIT|Signals the end of label editing|

|TVN_GETDISPINFO|Requests information that the tree control requires to display an item|

|TVN_ITEMEXPANDED|Signals that a parent item's list of child items was expanded or collapsed|

|TVN_ITEMEXPANDING|Signals that a parent item's list of child items is about to be expanded or collapsed|

|TVN_KEYDOWN|Signals a keyboard event|

|TVN_SELCHANGED|Signals that the selection has changed from one item to another|

|TVN_SELCHANGING|Signals that the selection is about to be changed from one item to another|

|TVN_SETDISPINFO|Notification to update the information maintained for an item|

You add an item to a tree control by calling the [InsertItem](../mfc/reference/ctreectrl-class.md#insertitem) member function. This function returns a handle of the **HTREEITEM** type, which uniquely identifies the item. When adding an item, you must specify the handle of the new item's parent item. If you specify **NULL** or the **TVI_ROOT** value instead of a parent item handle in the [TVINSERTSTRUCT](http://msdn.microsoft.com/library/windows/desktop/bb773452) structure or *hParent* parameter, the item is added as a root item.

Tree control ([CTreeCtrl](../mfc/reference/ctreectrl-class.md)) styles govern aspects of a tree control's appearance. You set the initial styles when you create the tree control. You can retrieve and change the styles after creating the tree control by using the [GetWindowLong](http://msdn.microsoft.com/library/windows/desktop/ms633584) and [SetWindowLong](http://msdn.microsoft.com/library/windows/desktop/ms633591) Windows functions, specifying **GWL_STYLE** for the *nIndex* parameter. For a complete list of styles, see [Tree View Control Window Styles](http://msdn.microsoft.com/library/windows/desktop/bb760013) in the Windows SDK.