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12. Tree Widget

The purpose of tree widgets is to display hierarchically-organized data. The Tree widget itself is a vertical container for widgets of type TreeItem. Tree itself is not terribly different from CList - both are derived directly from Container, and the Container methods work in the same way on Tree widgets as on CList widgets. The difference is that Tree widgets can be nested within other Tree widgets. We'll see how to do this shortly.

The Tree widget has its own window, and defaults to a white background, as does CList. Also, most of the Tree methods work in the same way as the corresponding CList ones. However, Tree is not derived from CList, so you cannot use them interchangeably.

12.1 Creating a Tree

A Tree is created in the usual way, using:

GtkWidget *gtk_tree_new( void );

Like the CList widget, a Tree will simply keep growing as more items are added to it, as well as when subtrees are expanded. For this reason, they are almost always packed into a ScrolledWindow. You might want to use gtk_widget_set_usize() on the scrolled window to ensure that it is big enough to see the tree's items, as the default size for ScrolledWindow is quite small.

Now that you have a tree, you'll probably want to add some items to it. The Tree Item Widget below explains the gory details of TreeItem. For now, it'll suffice to create one, using:

GtkWidget *gtk_tree_item_new_with_label( gchar *label );

You can then add it to the tree using one of the following (see Functions and Macros below for more options):

void gtk_tree_append( GtkTree    *tree,
                       GtkWidget *tree_item );

void gtk_tree_prepend( GtkTree   *tree,
                       GtkWidget *tree_item );

Note that you must add items to a Tree one at a time - there is no equivalent to gtk_list_*_items().

12.2 Adding a Subtree

A subtree is created like any other Tree widget. A subtree is added to another tree beneath a tree item, using:

void gtk_tree_item_set_subtree( GtkTreeItem *tree_item,
                                GtkWidget   *subtree );

You do not need to call gtk_widget_show() on a subtree before or after adding it to a TreeItem. However, you must have added the TreeItem in question to a parent tree before calling gtk_tree_item_set_subtree(). This is because, technically, the parent of the subtree is not the GtkTreeItem which "owns" it, but rather the GtkTree which holds that GtkTreeItem.

When you add a subtree to a TreeItem, a plus or minus sign appears beside it, which the user can click on to "expand" or "collapse" it, meaning, to show or hide its subtree. TreeItems are collapsed by default. Note that when you collapse a TreeItem, any selected items in its subtree remain selected, which may not be what the user expects.

12.3 Handling the Selection List

As with CList, the Tree type has a selection field, and it is possible to control the behaviour of the tree (somewhat) by setting the selection type using:

void gtk_tree_set_selection_mode( GtkTree          *tree,
                                  GtkSelectionMode  mode );

The semantics associated with the various selection modes are described in the section on the CList widget. As with the CList widget, the "select_child", "unselect_child" (not really - see Signals below for an explanation), and "selection_changed" signals are emitted when list items are selected or unselected. However, in order to take advantage of these signals, you need to know which Tree widget they will be emitted by, and where to find the list of selected items.

This is a source of potential confusion. The best way to explain this is that though all Tree widgets are created equal, some are more equal than others. All Tree widgets have their own X window, and can therefore receive events such as mouse clicks (if their TreeItems or their children don't catch them first!). However, to make GTK_SELECTION_SINGLE and GTK_SELECTION_BROWSE selection types behave in a sane manner, the list of selected items is specific to the topmost Tree widget in a hierarchy, known as the "root tree".

Thus, accessing the selection field directly in an arbitrary Tree widget is not a good idea unless you know it's the root tree. Instead, use the GTK_TREE_SELECTION (Tree) macro, which gives the root tree's selection list as a GList pointer. Of course, this list can include items that are not in the subtree in question if the selection type is GTK_SELECTION_MULTIPLE.

Finally, the "select_child" (and "unselect_child", in theory) signals are emitted by all trees, but the "selection_changed" signal is only emitted by the root tree. Consequently, if you want to handle the "select_child" signal for a tree and all its subtrees, you will have to call gtk_signal_connect() for every subtree.

12.4 Tree Widget Internals

The Tree's struct definition looks like this:

struct _GtkTree
{
  GtkContainer container;

  GList *children;
  
  GtkTree* root_tree; /* owner of selection list */
  GtkWidget* tree_owner;
  GList *selection;
  guint level;
  guint indent_value;
  guint current_indent;
  guint selection_mode : 2;
  guint view_mode : 1;
  guint view_line : 1;
};

The perils associated with accessing the selection field directly have already been mentioned. The other important fields of the struct can also be accessed with handy macros or class functions. GTK_IS_ROOT_TREE (Tree) returns a boolean value which indicates whether a tree is the root tree in a Tree hierarchy, while GTK_TREE_ROOT_TREE (Tree) returns the root tree, an object of type GtkTree (so, remember to cast it using GTK_WIDGET (Tree) if you want to use one of the gtk_widget_*() functions on it).

Instead of directly accessing the children field of a Tree widget, it's probably best to cast it using >tt/GTK_CONTAINER (Tree)/, and pass it to the gtk_container_children() function. This creates a duplicate of the original list, so it's advisable to free it up using g_list_free() after you're done with it, or to iterate on it destructively, like this:

    children = gtk_container_children (GTK_CONTAINER (tree));
    while (children) {
      do_something_nice (GTK_TREE_ITEM (children->data));
      children = g_list_remove_link (children, children);
}

The tree_owner field is defined only in subtrees, where it points to the TreeItem widget which holds the tree in question. The level field indicates how deeply nested a particular tree is; root trees have level 0, and each successive level of subtrees has a level one greater than the parent level. This field is set only after a Tree widget is actually mapped (i.e. drawn on the screen).

Signals

void selection_changed( GtkTree *tree );

This signal will be emitted whenever the selection field of a Tree has changed. This happens when a child of the Tree is selected or deselected.

void select_child( GtkTree   *tree,
                   GtkWidget *child );

This signal is emitted when a child of the Tree is about to get selected. This happens on calls to gtk_tree_select_item(), gtk_tree_select_child(), on all button presses and calls to gtk_tree_item_toggle() and gtk_item_toggle(). It may sometimes be indirectly triggered on other occasions where children get added to or removed from the Tree.

void unselect_child (GtkTree   *tree,
                     GtkWidget *child);

This signal is emitted when a child of the Tree is about to get deselected. As of GTK 1.0.4, this seems to only occur on calls to gtk_tree_unselect_item() or gtk_tree_unselect_child(), and perhaps on other occasions, but not when a button press deselects a child, nor on emission of the "toggle" signal by gtk_item_toggle().

Functions and Macros

guint gtk_tree_get_type( void );

Returns the "GtkTree" type identifier.

GtkWidget* gtk_tree_new( void );

Create a new Tree object. The new widget is returned as a pointer to a GtkWidget object. NULL is returned on failure.

void gtk_tree_append( GtkTree   *tree,
                      GtkWidget *tree_item );

Append a tree item to a Tree.

void gtk_tree_prepend( GtkTree   *tree,
                       GtkWidget *tree_item );

Prepend a tree item to a Tree.

void gtk_tree_insert( GtkTree   *tree,
                      GtkWidget *tree_item,
                      gint       position );

Insert a tree item into a Tree at the position in the list specified by position.

void gtk_tree_remove_items( GtkTree *tree,
                            GList   *items );

Remove a list of items (in the form of a GList *) from a Tree. Note that removing an item from a tree dereferences (and thus usually) destroys it and its subtree, if it has one, and all subtrees in that subtree. If you want to remove only one item, you can use gtk_container_remove().

void gtk_tree_clear_items( GtkTree *tree,
                           gint     start,
                           gint     end );

Remove the items from position start to position end from a Tree. The same warning about dereferencing applies here, as gtk_tree_clear_items() simply constructs a list and passes it to gtk_tree_remove_items().

void gtk_tree_select_item( GtkTree *tree,
                           gint     item );

Emits the "select_item" signal for the child at position item, thus selecting the child (unless you unselect it in a signal handler).

void gtk_tree_unselect_item( GtkTree *tree,
                             gint     item );

Emits the "unselect_item" signal for the child at position item, thus unselecting the child.

void gtk_tree_select_child( GtkTree   *tree,
                            GtkWidget *tree_item );

Emits the "select_item" signal for the child tree_item, thus selecting it.

void gtk_tree_unselect_child( GtkTree   *tree,
                              GtkWidget *tree_item );

Emits the "unselect_item" signal for the child tree_item, thus unselecting it.

gint gtk_tree_child_position( GtkTree   *tree,
                              GtkWidget *child );

Returns the position in the tree of child, unless child is not in the tree, in which case it returns -1.

void gtk_tree_set_selection_mode( GtkTree          *tree,
                                  GtkSelectionMode  mode );

Sets the selection mode, which can be one of GTK_SELECTION_SINGLE (the default), GTK_SELECTION_BROWSE, GTK_SELECTION_MULTIPLE, or GTK_SELECTION_EXTENDED. This is only defined for root trees, which makes sense, since the root tree "owns" the selection. Setting it for subtrees has no effect at all; the value is simply ignored.

void gtk_tree_set_view_mode( GtkTree         *tree,
                             GtkTreeViewMode  mode ); 

Sets the "view mode", which can be either GTK_TREE_VIEW_LINE (the default) or GTK_TREE_VIEW_ITEM. The view mode propagates from a tree to its subtrees, and can't be set exclusively to a subtree (this is not exactly true - see the example code comments).

The term "view mode" is rather ambiguous - basically, it controls the way the highlight is drawn when one of a tree's children is selected. If it's GTK_TREE_VIEW_LINE, the entire TreeItem widget is highlighted, while for GTK_TREE_VIEW_ITEM, only the child widget (i.e., usually the label) is highlighted.

void gtk_tree_set_view_lines( GtkTree *tree,
                              guint    flag );

Controls whether connecting lines between tree items are drawn. flag is either TRUE, in which case they are, or FALSE, in which case they aren't.

GtkTree *GTK_TREE (gpointer obj);

Cast a generic pointer to "GtkTree *".

GtkTreeClass *GTK_TREE_CLASS (gpointer class);

Cast a generic pointer to "GtkTreeClass *".

gint GTK_IS_TREE (gpointer obj);

Determine if a generic pointer refers to a "GtkTree" object.

gint GTK_IS_ROOT_TREE (gpointer obj)

Determine if a generic pointer refers to a "GtkTree" object and is a root tree. Though this will accept any pointer, the results of passing it a pointer that does not refer to a Tree are undefined and possibly harmful.

GtkTree *GTK_TREE_ROOT_TREE (gpointer obj)

Return the root tree of a pointer to a "GtkTree" object. The above warning applies.

GList *GTK_TREE_SELECTION( gpointer obj)

Return the selection list of the root tree of a "GtkTree" object. The above warning applies here, too.

12.5 Tree Item Widget

The TreeItem widget, like CListItem, is derived from Item, which in turn is derived from Bin. Therefore, the item itself is a generic container holding exactly one child widget, which can be of any type. The TreeItem widget has a number of extra fields, but the only one we need be concerned with is the subtree field.

The definition for the TreeItem struct looks like this:

struct _GtkTreeItem
{
  GtkItem item;

  GtkWidget *subtree;
  GtkWidget *pixmaps_box;
  GtkWidget *plus_pix_widget, *minus_pix_widget;

  GList *pixmaps;               /* pixmap node for this items color depth */

  guint expanded : 1;
};

The pixmaps_box field is an EventBox which catches clicks on the plus/minus symbol which controls expansion and collapsing. The pixmaps field points to an internal data structure. Since you can always obtain the subtree of a TreeItem in a (relatively) type-safe manner with the GTK_TREE_ITEM_SUBTREE (Item) macro, it's probably advisable never to touch the insides of a TreeItem unless you really know what you're doing.

Since it is directly derived from an Item it can be treated as such by using the GTK_ITEM (TreeItem) macro. A TreeItem usually holds a label, so the convenience function gtk_list_item_new_with_label() is provided. The same effect can be achieved using code like the following, which is actually copied verbatim from gtk_tree_item_new_with_label():

tree_item = gtk_tree_item_new ();
label_widget = gtk_label_new (label);
gtk_misc_set_alignment (GTK_MISC (label_widget), 0.0, 0.5);

gtk_container_add (GTK_CONTAINER (tree_item), label_widget);
gtk_widget_show (label_widget);

As one is not forced to add a Label to a TreeItem, you could also add an HBox or an Arrow, or even a Notebook (though your app will likely be quite unpopular in this case) to the TreeItem.

If you remove all the items from a subtree, it will be destroyed and unparented, unless you reference it beforehand, and the TreeItem which owns it will be collapsed. So, if you want it to stick around, do something like the following:

gtk_widget_ref (tree);
owner = GTK_TREE(tree)->tree_owner;
gtk_container_remove (GTK_CONTAINER(tree), item);
if (tree->parent == NULL){
  gtk_tree_item_expand (GTK_TREE_ITEM(owner));
  gtk_tree_item_set_subtree (GTK_TREE_ITEM(owner), tree);
}
else
  gtk_widget_unref (tree);

Finally, drag-n-drop does work with TreeItems. You just have to make sure that the TreeItem you want to make into a drag item or a drop site has not only been added to a Tree, but that each successive parent widget has a parent itself, all the way back to a toplevel or dialog window, when you call gtk_widget_dnd_drag_set() or gtk_widget_dnd_drop_set(). Otherwise, strange things will happen.

Signals

TreeItem inherits the "select", "deselect", and "toggle" signals from Item. In addition, it adds two signals of its own, "expand" and "collapse".

void select( GtkItem *tree_item );

This signal is emitted when an item is about to be selected, either after it has been clicked on by the user, or when the program calls gtk_tree_item_select(), gtk_item_select(), or gtk_tree_select_child().

void deselect( GtkItem *tree_item );

This signal is emitted when an item is about to be unselected, either after it has been clicked on by the user, or when the program calls gtk_tree_item_deselect() or gtk_item_deselect(). In the case of TreeItems, it is also emitted by gtk_tree_unselect_child(), and sometimes gtk_tree_select_child().

void toggle( GtkItem *tree_item );

This signal is emitted when the program calls gtk_item_toggle(). The effect it has when emitted on a TreeItem is to call gtk_tree_select_child() (and never gtk_tree_unselect_child()) on the item's parent tree, if the item has a parent tree. If it doesn't, then the highlight is reversed on the item.

void expand( GtkTreeItem *tree_item );

This signal is emitted when the tree item's subtree is about to be expanded, that is, when the user clicks on the plus sign next to the item, or when the program calls gtk_tree_item_expand().

void collapse( GtkTreeItem *tree_item );

This signal is emitted when the tree item's subtree is about to be collapsed, that is, when the user clicks on the minus sign next to the item, or when the program calls gtk_tree_item_collapse().

Functions and Macros

guint gtk_tree_item_get_type( void );

Returns the "GtkTreeItem" type identifier.

GtkWidget* gtk_tree_item_new( void );

Create a new TreeItem object. The new widget is returned as a pointer to a GtkWidget object. NULL is returned on failure.

GtkWidget* gtk_tree_item_new_with_label (gchar       *label);

Create a new TreeItem object, having a single GtkLabel as the sole child. The new widget is returned as a pointer to a GtkWidget object. NULL is returned on failure.

void gtk_tree_item_select( GtkTreeItem *tree_item );

This function is basically a wrapper around a call to gtk_item_select (GTK_ITEM (tree_item)) which will emit the select signal.

void gtk_tree_item_deselect( GtkTreeItem *tree_item );

This function is basically a wrapper around a call to gtk_item_deselect (GTK_ITEM (tree_item)) which will emit the deselect signal.

void gtk_tree_item_set_subtree( GtkTreeItem *tree_item,
                                GtkWidget   *subtree );

This function adds a subtree to tree_item, showing it if tree_item is expanded, or hiding it if tree_item is collapsed. Again, remember that the tree_item must have already been added to a tree for this to work.

void gtk_tree_item_remove_subtree( GtkTreeItem *tree_item );

This removes all of tree_item's subtree's children (thus unreferencing and destroying it, any of its children's subtrees, and so on...), then removes the subtree itself, and hides the plus/minus sign.

void gtk_tree_item_expand( GtkTreeItem *tree_item );

This emits the "expand" signal on tree_item, which expands it.

void gtk_tree_item_collapse( GtkTreeItem *tree_item );

This emits the "collapse" signal on tree_item, which collapses it.

GtkTreeItem *GTK_TREE_ITEM (gpointer obj)

Cast a generic pointer to "GtkTreeItem *".

GtkTreeItemClass *GTK_TREE_ITEM_CLASS (gpointer obj)

Cast a generic pointer to "GtkTreeItemClass".

gint GTK_IS_TREE_ITEM (gpointer obj)

Determine if a generic pointer refers to a "GtkTreeItem" object.

GtkWidget GTK_TREE_ITEM_SUBTREE (gpointer obj)

Returns a tree item's subtree (obj should point to a "GtkTreeItem" object).

12.6 Tree Example

This is somewhat like the tree example in testgtk.c, but a lot less complete (although much better commented). It puts up a window with a tree, and connects all the signals for the relevant objects, so you can see when they are emitted.

/* example-start tree tree.c */

#include <gtk/gtk.h>

/* for all the GtkItem:: and GtkTreeItem:: signals */
static void cb_itemsignal( GtkWidget *item,
                           gchar     *signame )
{
  gchar *name;
  GtkLabel *label;

  /* It's a Bin, so it has one child, which we know to be a
     label, so get that */
  label = GTK_LABEL (GTK_BIN (item)->child);
  /* Get the text of the label */
  gtk_label_get (label, &name);
  /* Get the level of the tree which the item is in */
  g_print ("%s called for item %s->%p, level %d\n", signame, name,
           item, GTK_TREE (item->parent)->level);
}

/* Note that this is never called */
static void cb_unselect_child( GtkWidget *root_tree,
                               GtkWidget *child,
                               GtkWidget *subtree )
{
  g_print ("unselect_child called for root tree %p, subtree %p, child %p\n",
           root_tree, subtree, child);
}

/* Note that this is called every time the user clicks on an item,
   whether it is already selected or not. */
static void cb_select_child (GtkWidget *root_tree, GtkWidget *child,
                             GtkWidget *subtree)
{
  g_print ("select_child called for root tree %p, subtree %p, child %p\n",
           root_tree, subtree, child);
}

static void cb_selection_changed( GtkWidget *tree )
{
  GList *i;
  
  g_print ("selection_change called for tree %p\n", tree);
  g_print ("selected objects are:\n");

  i = GTK_TREE_SELECTION(tree);
  while (i){
    gchar *name;
    GtkLabel *label;
    GtkWidget *item;

    /* Get a GtkWidget pointer from the list node */
    item = GTK_WIDGET (i->data);
    label = GTK_LABEL (GTK_BIN (item)->child);
    gtk_label_get (label, &name);
    g_print ("\t%s on level %d\n", name, GTK_TREE
             (item->parent)->level);
    i = i->next;
  }
}

int main( int   argc,
          char *argv[] )
{
  GtkWidget *window, *scrolled_win, *tree;
  static gchar *itemnames[] = {"Foo", "Bar", "Baz", "Quux",
                               "Maurice"};
  gint i;

  gtk_init (&argc, &argv);

  /* a generic toplevel window */
  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  gtk_signal_connect (GTK_OBJECT(window), "delete_event",
                      GTK_SIGNAL_FUNC (gtk_main_quit), NULL);
  gtk_container_set_border_width (GTK_CONTAINER(window), 5);

  /* A generic scrolled window */
  scrolled_win = gtk_scrolled_window_new (NULL, NULL);
  gtk_scrolled_window_set_policy (GTK_SCROLLED_WINDOW (scrolled_win),
                                  GTK_POLICY_AUTOMATIC,
                                  GTK_POLICY_AUTOMATIC);
  gtk_widget_set_usize (scrolled_win, 150, 200);
  gtk_container_add (GTK_CONTAINER(window), scrolled_win);
  gtk_widget_show (scrolled_win);
  
  /* Create the root tree */
  tree = gtk_tree_new();
  g_print ("root tree is %p\n", tree);
  /* connect all GtkTree:: signals */
  gtk_signal_connect (GTK_OBJECT(tree), "select_child",
                      GTK_SIGNAL_FUNC(cb_select_child), tree);
  gtk_signal_connect (GTK_OBJECT(tree), "unselect_child",
                      GTK_SIGNAL_FUNC(cb_unselect_child), tree);
  gtk_signal_connect (GTK_OBJECT(tree), "selection_changed",
                      GTK_SIGNAL_FUNC(cb_selection_changed), tree);
  /* Add it to the scrolled window */
  gtk_scrolled_window_add_with_viewport (GTK_SCROLLED_WINDOW(scrolled_win),
                                         tree);
  /* Set the selection mode */
  gtk_tree_set_selection_mode (GTK_TREE(tree),
                               GTK_SELECTION_MULTIPLE);
  /* Show it */
  gtk_widget_show (tree);

  for (i = 0; i < 5; i++){
    GtkWidget *subtree, *item;
    gint j;

    /* Create a tree item */
    item = gtk_tree_item_new_with_label (itemnames[i]);
    /* Connect all GtkItem:: and GtkTreeItem:: signals */
    gtk_signal_connect (GTK_OBJECT(item), "select",
                        GTK_SIGNAL_FUNC(cb_itemsignal), "select");
    gtk_signal_connect (GTK_OBJECT(item), "deselect",
                        GTK_SIGNAL_FUNC(cb_itemsignal), "deselect");
    gtk_signal_connect (GTK_OBJECT(item), "toggle",
                        GTK_SIGNAL_FUNC(cb_itemsignal), "toggle");
    gtk_signal_connect (GTK_OBJECT(item), "expand",
                        GTK_SIGNAL_FUNC(cb_itemsignal), "expand");
    gtk_signal_connect (GTK_OBJECT(item), "collapse",
                        GTK_SIGNAL_FUNC(cb_itemsignal), "collapse");
    /* Add it to the parent tree */
    gtk_tree_append (GTK_TREE(tree), item);
    /* Show it - this can be done at any time */
    gtk_widget_show (item);
    /* Create this item's subtree */
    subtree = gtk_tree_new();
    g_print ("-> item %s->%p, subtree %p\n", itemnames[i], item,
             subtree);

    /* This is still necessary if you want these signals to be called
       for the subtree's children.  Note that selection_change will be 
       signalled for the root tree regardless. */
    gtk_signal_connect (GTK_OBJECT(subtree), "select_child",
                        GTK_SIGNAL_FUNC(cb_select_child), subtree);
    gtk_signal_connect (GTK_OBJECT(subtree), "unselect_child",
                        GTK_SIGNAL_FUNC(cb_unselect_child), subtree);
    /* This has absolutely no effect, because it is completely ignored 
       in subtrees */
    gtk_tree_set_selection_mode (GTK_TREE(subtree),
                                 GTK_SELECTION_SINGLE);
    /* Neither does this, but for a rather different reason - the
       view_mode and view_line values of a tree are propagated to
       subtrees when they are mapped.  So, setting it later on would
       actually have a (somewhat unpredictable) effect */
    gtk_tree_set_view_mode (GTK_TREE(subtree), GTK_TREE_VIEW_ITEM);
    /* Set this item's subtree - note that you cannot do this until
       AFTER the item has been added to its parent tree! */
    gtk_tree_item_set_subtree (GTK_TREE_ITEM(item), subtree);

    for (j = 0; j < 5; j++){
      GtkWidget *subitem;

      /* Create a subtree item, in much the same way */
      subitem = gtk_tree_item_new_with_label (itemnames[j]);
      /* Connect all GtkItem:: and GtkTreeItem:: signals */
      gtk_signal_connect (GTK_OBJECT(subitem), "select",
                          GTK_SIGNAL_FUNC(cb_itemsignal), "select");
      gtk_signal_connect (GTK_OBJECT(subitem), "deselect",
                          GTK_SIGNAL_FUNC(cb_itemsignal), "deselect");
      gtk_signal_connect (GTK_OBJECT(subitem), "toggle",
                          GTK_SIGNAL_FUNC(cb_itemsignal), "toggle");
      gtk_signal_connect (GTK_OBJECT(subitem), "expand",
                          GTK_SIGNAL_FUNC(cb_itemsignal), "expand");
      gtk_signal_connect (GTK_OBJECT(subitem), "collapse",
                          GTK_SIGNAL_FUNC(cb_itemsignal), "collapse");
      g_print ("-> -> item %s->%p\n", itemnames[j], subitem);
      /* Add it to its parent tree */
      gtk_tree_append (GTK_TREE(subtree), subitem);
      /* Show it */
      gtk_widget_show (subitem);
    }
  }

  /* Show the window and loop endlessly */
  gtk_widget_show (window);
  gtk_main();
  return 0;
}
/* example-end */