Tree Control Parent and Child Items
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Any item in a tree control (CTreeCtrl) can have a list of subitems, which are called child items, associated with it. An item that has one or more child items is called a parent item. A child item is displayed below its parent item and is indented to indicate it is subordinate to the parent. An item that has no parent is at the top of the hierarchy and is called a root item.
At any given time, the state of a parent item's list of child items can be either expanded or collapsed. When the state is expanded, the child items are displayed below the parent item. When it is collapsed, the child items are not displayed. The list automatically toggles between the expanded and collapsed states when the user double-clicks the parent item or, if the parent has the TVS_HASBUTTONS style, when the user clicks the button associated with the parent item. An application can expand or collapse the child items by using the Expand member function.
You add an item to a tree control by calling the 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 structure or hParent parameter, the item is added as a root item.
A tree control sends a TVN_ITEMEXPANDING notification message when a parent item's list of child items is about to be expanded or collapsed. The notification gives you the opportunity to prevent the change or to set any attributes of the parent item that depend on the state of the list of child items. After changing the state of the list, the tree control sends a TVN_ITEMEXPANDED notification message.
When a list of child items is expanded, it is indented relative to the parent item. You can set the amount of indentation by using the SetIndent member function or retrieve the current amount by using the GetIndent member function.