Version 2 of Stacking Order

Stacking Order (Geometry Management)

Each Tk widget of a GUI application belongs to two hierarchies: one whose levels are ordered by the master/slave relationship, and another whose levels are ordered by the parent/child relationship.

Parent/Child Hierarchy and Stacking Order

A widget's place in the parent/child hierarchy can be determined from the widget's name. The levels of the hierarchy are separated by dots in the widget name. For example, a widget .w1 is the parent of the widgets .w1.a, .w1.b, .w1.anything

The command

  winfo children $widgetname

will return a list of the children of the widget $widgetname

The parent/child hierarchy is a convenient way to organise the naming of widgets; but it also determines the stacking order, which is discussed at the end of this page.

Master/Slave Hierarchy and Geometry Management

The master/slave hierarchy is defined by geometry management: if widget $A manages the geometry of widget $B, then $B is the slave of $A. Working out the hierarchy is a little complicated, because there is more than one geometry manager.

The command

  winfo manager $B

returns the name of the geometry manager for $B - in plain Tcl/Tk the possible values are pack, grid, place, text and canvas. pack, grid, and place are full-featured geometry managers; text and canvas are classes of widget that have a limited capability to manage the geometry of widgets that are embedded in them. Add-ons to Tcl/Tk may provide alternative geometry managers, or indeed alternative widgets with geometry-management capability.

If $A uses the packer to manage $B, then the command

  pack info $B

will return a list that begins

  -in $A

followed by the other parameters used by the packer to manage $B

The command

  pack slaves $A

will return a list of widgets, including $B, that are packed in $A

(insert note on canvas and text items)

Very often, the master/slave and parent/child hierarchies are the same: this is the case if an application has only one toplevel window ("."), and always uses the default master (the parent) in geometry management: for example

  pack .frame.text

does not specify a master widget, and so the widget's parent, .frame, is used as the master.

If the programmer requires a different master, this may be specified with the pack option -in, or (indirectly) with the pack options -before or -after.

The master/slave hierarchy is an essential part of geometry management, and is the major factor that determines the layout of the widgets in a GUI application.

Stacking Order - Rules

The parent/child hierarchy influences the widget layout in a more subtle way: the widget's place in the parent/child hierarchy influences its position in the stacking order of widgets.

The stacking order determines which widget is "on top of" which, and therefore which widget is visible when two or more widgets overlap.

In Tk, the stacking order of widgets in a toplevel window is determined by these rules:

childen are always above their parents
if $B is above its sibling $C, then every descendant of $B is above every descendant of $C
the stacking order of siblings may be inspected using the command

  winfo children $widgetname

which returns a list of the children of the widget $widgetname, in stacking order with the lowest first (except that toplevel windows are not returned in stacking order).
the stacking order of siblings may be altered using the "raise" and "lower" commands

Problems with stacking order

Where you expect to see a widget, you may see only a grey rectangle. This happens if the widget is lower in the stacking order than its master.
A geometry manager may refuse to let a widget be the master of its parent or other ancestor, and will throw an error. This is because children are always above their parents in the stacking order, and so it would be impossible to make the slave widget visible.

(first draft by KJN)

Category GUI