[Tile-dev] Tree API -- selection and focus

[Joe E. <jen...@fl...>]

For consistency with other parts of Tk, it would make sense
to just clone the listbox widget selection command.

On the other hand, that's not a very good match for the tree
widget: the 'set' and 'clear' subcommands operate on ranges,
and tree items don't have a natural linear ordering.
(There are several reasonable linear orders for items, but
it's not always obvious what's in the range "between"
two items, especially if $first and $last don't have the
same parent).  Besides that, I've always found the listbox
selection command somewhat clunky.

I propose the following instead:

    $tree selection --
	Returns the list of selected items.  The list is sorted
	according to the preorder traversal order (that is,
	it doesn't matter in which order items were added
	to the selection).

    $tree selection add $items --
    	Add $items to the selection.

    $tree selection remove $items --
    	Remove $items from the selection.

    $tree selection set $items --
    	$items becomes the new selection.

    $tree selection toggle $items --
    	Toggle the selected state of each item in $items.

And possibly:

    $tree selection clear --
    	Deselect everything.
	This is the same as '$tree selection set {}'.

A <<TreeviewSelect>> event is generated in all cases
(except for [$treeview selection]), even if the selection
hasn't actually changed (e.g., adding an item that was
already selected still generates the event).


Notable differences from [$listbox selection]:

    + There's no separate [$tree curselection] command;
      [$tree selection] with no additional arguments does the same thing.

    + Unlike [$listbox selection set $first $last], [$tree selection set $l]
      *does* affect the selection state of items not mentioned in $l.
      [$listbox selection set] is closer to [$tree selection add].

    + There's no selection anchor.

    + [$tree selection toggle] is included so the common operation
      of toggling the selection state of a single item can be
      done atomically.


There is also a distinguished "focus" item; this is where the
location cursor is drawn, and is the target of most keyboard
operations.  [$treeview focus] returns the current focus item,
and [$treeview focus $item] sets it.

The listbox widget uses an "activate" command and an "active"
index for this concept.  I'm trying to be more consistent with
terminology in the Tile package, and "active" means "the thing
under the mouse pointer" in other contexts; hence "focus" instead
of "active".

The widget ought to support both "single" and "multiple"
selection modes ("browse" and "extended" seem like unnecessarily
subtle distinctions).  I'd rather not add a -selectmode option
if it can be avoided; another way to support this is with distinct
sets of bindtags (which in turn can be selected by setting
the -class option).  Will try both and see how it works out...


--Joe English

  jen...@fl...