NAME

       ttk::intro - Introduction to the Tk theme engine
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OVERVIEW

       The  Tk  themed  widget  set  is  based  on  a  revised  and  enhanced  version of TIP #48
       (https://tip.tcl-lang.org/48) specified style engine.  The  main  concepts  are  described
       below.   The  basic  idea  is to separate, to the extent possible, the code implementing a
       widget's behavior from the code implementing its appearance.  Widget  class  bindings  are
       primarily responsible for maintaining the widget state and invoking callbacks; all aspects
       of the widget's appearance are controlled by the style of the widget (i.e.  the  style  of
       the elements of the widget).

THEMES

       A  theme  is  a  collection of elements and styles that determine the look and feel of the
       widget set.  Themes can be used to:

       •      isolate platform differences (X11 vs. classic Windows vs. XP vs. Aqua ...)

       •      adapt to display limitations (low-color, grayscale, monochrome, tiny screens)

       •      accessibility (high contrast, large type)

       •      application suite branding

       •      blend in with the rest of the desktop (Gnome, KDE, Java)

       •      and, of course: eye candy.

ELEMENTS

       An element displays an individual part of a widget.  For  example,  a  vertical  scrollbar
       widget contains uparrow, downarrow, trough and slider elements.

       Element  names use a recursive dotted notation.  For example, uparrow identifies a generic
       arrow  element,  and  Scrollbar.uparrow  and  Combobox.uparrow  identify   widget-specific
       elements.   When  looking  for  an  element,  the style engine looks for the specific name
       first, and if an element of that name is not  found  it  looks  for  generic  elements  by
       stripping off successive leading components of the element name.

       Like  widgets,  elements have options which specify what to display and how to display it.
       For  example,  the  text  element  (which  displays  a  text  string)  has  -text,  -font,
       -foreground,  -background, -underline, and -width options.  The value of an element option
       is taken from (in precedence order):

       •      an option of the same name and type in the widget containing the element;

       •      a dynamic setting specified by style map and the current state;

       •      the default setting specified by style configure; or

       •      the element's built-in default value for the option.

LAYOUTS

       A layout specifies which elements make up a widget and how they are arranged.  The  layout
       engine  uses  a  simplified version of the pack algorithm: starting with an initial cavity
       equal to the size of the widget, elements are allocated a parcel within the  cavity  along
       the  side  specified  by  the  -side option, and placed within the parcel according to the
       -sticky option.  For example, the layout for a horizontal scrollbar is:

              ttk::style layout Horizontal.TScrollbar {
                  Scrollbar.trough -children {
                Scrollbar.leftarrow -side left -sticky w
                Scrollbar.rightarrow -side right -sticky e
                Scrollbar.thumb -sticky ew
                  }
              }

       By default, the layout for a widget is the same as  its  class  name.   Some  widgets  may
       override  this  (for example, the ttk::scrollbar widget chooses different layouts based on
       the -orient option).

STATES

       In standard Tk, many widgets have a -state option which (in most cases) is  either  normal
       or disabled.  Some widgets support additional states, such as the entry widget which has a
       readonly state and the various flavors of buttons which have active state.

       The themed Tk widgets generalizes this idea: every widget  has  a  bitmap  of  independent
       state  flags.   Widget  state  flags  include active, disabled, pressed, focus, etc., (see
       ttk::widget(3tk) for the full list of state flags).

       Instead of a -state option, every widget now has a state widget command which is  used  to
       set  or  query  the  state.   A  state  specification  is  a  list of symbolic state names
       indicating which bits  are  set,  each  optionally  prefixed  with  an  exclamation  point
       indicating that the bit is cleared instead.

       For example, the class bindings for the ttk::button widget are:

              bind TButton <Enter>{ %W state active }
              bind TButton <Leave>{ %W state !active }
              bind TButton <ButtonPress-1>{ %W state pressed }
              bind TButton <Button1-Leave>{ %W state !pressed }
              bind TButton <Button1-Enter>{ %W state pressed }
              bind TButton <ButtonRelease-1>\
                  { %W instate {pressed} { %W state !pressed ; %W invoke } }

       This  specifies  that  the  widget  becomes active when the pointer enters the widget, and
       inactive when it leaves.  Similarly it becomes pressed when the mouse button  is  pressed,
       and  !pressed on the ButtonRelease event.  In addition, the button unpresses if pointer is
       dragged outside the widget while Button-1 is held down, and represses if it's dragged back
       in.   Finally,  when  the  mouse button is released, the widget's -command is invoked, but
       only if the button is currently in the pressed state.  (The actual bindings are  a  little
       more complicated than the above, but not by much).

STYLES

       Each widget is associated with a style, which specifies values for element options.  Style
       names use a recursive dotted notation like layouts and elements; by default,  widgets  use
       the class name to look up a style in the current theme.  For example:

              ttk::style configure TButton \
                -background #d9d9d9 \
                -foreground black \
                -relief raised \
                ;

       Many  elements  are  displayed  differently  depending  on the widget state.  For example,
       buttons have a different background when they are  active,  a  different  foreground  when
       disabled,  and  a  different relief when pressed.  The style map command specifies dynamic
       option settings for a particular style:

              ttk::style map TButton \
                -background [list disabled #d9d9d9  active #ececec] \
                -foreground [list disabled #a3a3a3] \
                -relief [list {pressed !disabled} sunken] \
                ;

SEE ALSO

       ttk::widget(3tk), ttk::style(3tk)