'''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 [winfo children] 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''' For more on Geometry Management, see * [Geometry Managers] * [Comparing Geometry Managers] 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 [winfo manager] 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 default stacking order of sibling widgets is the order in which the widgets were created, '''''not''''' the order in which they were packed (or otherwise geometry-managed) * 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. * Where you expect to see a widget, you may see nothing. This can happen if it is covered by another widget (typically one that is cropped by geometry management) and the widget you expected to see is lower in the stacking order than the one that covers it. * A geometry manager may refuse to let a widget be the master of its parent or other ancestor, and will throw an error. It does this because children are always above their parents in the stacking order, and so it would be impossible to make the slave widget visible. Problems with stacking order can usually be solved by * choosing the name of each widget to reflect its intended place in the stacking order (e.g. name a widget so that it is the child of its master) * creating sibling widgets in the intended stacking order, with the lowest first; or, if this is not possible, using the [raise] and [lower] commands to change the stacking order ---- [KJN] - I'm not sure why Tk manages stacking order this way. It seems wasteful and confusing to maintain two independent hierarchies, and can give rise to obscure bugs. Is there any situation in which the following rules would be inadequate? * the parent/child hierarchy is only a naming convention, and has no effect on stacking order or geometry management * the stacking order is determined by the master/slave hierarchy * a slave is always above its master * if $B and $C are slaves of the same master, and $B is above $C, then every slave of $B is above every slave of $C * the default stacking order of slaves of the same master is the order in which the widgets were packed (or otherwise geometry-managed) * the stacking order of slaves of the same master may be altered using the [raise] and [lower] commands ---- original version by [KJN] ---- [Category GUI]