Parent and Child Items

Any item in a tree-view control 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.

To add an item to a tree-view control, send the TVM_INSERTITEM message to the tree-view control. The message 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 TV_INSERTSTRUCT structure, the item is added as 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 TVM_EXPAND message.

A tree-view control sends the parent window a TVN_ITEMEXPANDING notification message when a parent item's list of child items is about to be expanded or collapsed. The notification gives an application 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-view control sends the parent window 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 TVM_SETINDENT message or retrieve the current amount by using the TVM_GETINDENT message.

A tree-view control uses memory allocated from the heap of the process that creates the tree-view control. The maximum number of items in a tree view is based on the amount of memory available in the heap. Each item takes 64 bytes.

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