## wxTreeCtrl overview

Classes: wxTreeCtrl, wxImageList

The tree control displays its items in a tree like structure. Each item has its own (optional) icon and a label. An item may be either collapsed (meaning that its children are not visible) or expanded (meaning that its children are shown). Each item in the tree is identified by its itemId which is of opaque data type wxTreeItemId. You can test whether an item is valid by calling wxTreeItemId::IsOk.

The items text and image may be retrieved and changed with GetItemText/SetItemText and GetItemImage/SetItemImage. In fact, an item may even have two images associated with it: the normal one and another one for selected state which is set/retrieved with SetItemSelectedImage/GetItemSelectedImage functions, but this functionality might be unavailable on some platforms.

Tree items have several attributes: an item may be selected or not, visible or not, bold or not. It may also be expanded or collapsed. All these attributes may be retrieved with the corresponding functions: IsSelected, IsVisible, IsBold and IsExpanded. Only one item at a time may be selected, selecting another one (with SelectItem) automatically unselects the previously selected one.

In addition to its icon and label, a user-specific data structure may be associated with all tree items. If you wish to do it, you should derive a class from wxTreeItemData which is a very simple class having only one function GetId() which returns the id of the item this data is associated with. This data will be freed by the control itself when the associated item is deleted (all items are deleted when the control is destroyed), so you shouldn't delete it yourself (if you do it, you should call SetItemData(NULL) to prevent the tree from deleting the pointer second time). The associated data may be retrieved with GetItemData() function.

Working with trees is relatively straightforward if all the items are added to the tree at the moment of its creation. However, for large trees it may be very inefficient. To improve the performance you may want to delay adding the items to the tree until the branch containing the items is expanded: so, in the beginning, only the root item is created (with AddRoot). Other items are added when EVT_TREE_ITEM_EXPANDING event is received: then all items lying immediately under the item being expanded should be added, but, of course, only when this event is received for the first time for this item - otherwise, the items would be added twice if the user expands/collapses/re-expands the branch.

The tree control provides functions for enumerating its items. There are 3 groups of enumeration functions: for the children of a given item, for the sibling of the given item and for the visible items (those which are currently shown to the user: an item may be invisible either because its branch is collapsed or because it is scrolled out of view). Child enumeration functions require the caller to give them a cookie parameter: it is a number which is opaque to the caller but is used by the tree control itself to allow multiple enumerations to run simultaneously (this is explicitly allowed). The only thing to remember is that the cookie passed to GetFirstChild and to GetNextChild should be the same variable (and that nothing should be done with it by the user code).

Among other features of the tree control are: item sorting with SortChildren which uses the user-defined comparison function OnCompareItems (by default the comparison is the alphabetic comparison of tree labels), hit testing (determining to which portion of the control the given point belongs, useful for implementing drag-and-drop in the tree) with HitTest and editing of the tree item labels in place (see EditLabel).

Finally, the tree control has a keyboard interface: the cursor navigation (arrow) keys may be used to change the current selection. <HOME> and <END> are used to go to the first/last sibling of the current item. '+', '-' and '*' expand, collapse and toggle the current branch. Note, however, that <DEL> and <INS> keys do nothing by default, but it is common to associate them with deleting an item from a tree and inserting a new one into it.