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Practical Programming in Tcl & Tk, Third Edition
By Brent B. Welch

Table of Contents
Chapter 29.  Simple Tk Widgets

The Scale Widget

The scale widget displays a slider in a trough. The trough represents a range of numeric values, and the slider position represents the current value. The scale can have an associated label, and it can display its current value next to the slider. The value of the scale can be used in three different ways:

  • Explicitly get and set the value with widget commands.

  • Associate the scale with a Tcl variable. The variable is kept in sync with the value of the scale, and changing the variable affects the scale.

  • Register a Tcl command to be executed after the scale value changes. You specify the initial part of the Tcl command, and the scale implementation adds the current value as another argument to the command.

Example 29-5 A scale widget.


scale .scale -from -10 -to 20 -length 200 -variable x \
   -orient horizontal -label "The value of X" \
   -tickinterval 5 -showvalue true
pack .scale

Example 29-5 shows a scale for a variable that ranges in value from -10 to +20. The variable x is defined at the global scope. The tickinterval option results in the labels across the bottom, and the showvalue option causes the current value to be displayed. The length of the scale is in screen units (i.e., pixels).

Scale Bindings

Table 29-4 lists the bindings for scale widgets. You must direct focus to a scale explicitly for the key bindings like <Up> and <Down> to take effect.

Table 29-4. Bindings for scale widgets.
<Button-1>Clicking on the trough moves the slider by one unit of resolution toward the mouse click.
<Control-Button-1>Clicking on the trough moves the slider all the way to the end of the trough toward the mouse click.
<Left> <Up>Moves the slider toward the left (top) by one unit.



Moves the slider toward the left (top) by the value of the bigIncrement attribute.
<Right> <Down>Moves the slider toward the right (bottom) one unit.



Moves the slider toward the right (bottom) by the value of the bigIncrement attribute.
<Home>Moves the slider all the way to the left (top).
<End>Moves the slider all the way to the right (bottom).

Scale Attributes

Table 29-5 lists the scale widget attributes. The table uses the resource name, which has capitals at internal word boundaries. In Tcl commands the attributes are specified with a dash and all lowercase.

Table 29-5. Attributes for scale widgets.
activeBackgroundBackground color when the mouse is over the slider.
backgroundThe background color (also bg in commands).
bigIncrementCoarse grain slider adjustment value.
borderWidthExtra space around the edge of the widget.
commandCommand to invoke when the value changes. The current value is appended as another argument
cursorCursor to display when mouse is over the widget.
digitsNumber of significant digits in scale value.
fromMinimum value. The left or top end of the scale.
fontFont for the label.
foregroundForeground color (also fg).
highlightBackgroundFocus highlight color when widget does not have focus.
highlightColorFocus highlight color when widget has focus.
highlightThicknessThickness of focus highlight rectangle.
labelA string to display with the scale.
lengthThe length, in screen units, of the long axis of the scale.
orienthorizontal or vertical.
reliefflat, sunken, raised, groove, solid or ridge.
repeatDelayDelay before keyboard auto-repeat starts. Auto-repeat is used when pressing <Button-1> on the trough.
repeatIntervalTime period between auto-repeat events.
resolutionThe value is rounded to a multiple of this value.
showValueIf true, value is displayed next to the slider.
sliderLengthThe length, in screen units, of the slider.
sliderReliefThe relief of the slider.
statenormal, active, or disabled.
takeFocusControls focus changes from keyboard traversal.
tickIntervalSpacing between tick marks. Zero means no marks.
toMaximum value. Right or bottom end of the scale.
troughColorThe color of the bar on which the slider sits.
variableName of Tcl variable. Changes to the scale widget are reflected in the Tcl variable value, and changes in the Tcl variable are reflected in the scale display.
widthWidth of the trough, or slider bar.

Programming Scales

The scale operations are primarily used by the default bindings and you do not need to program the scale directly. Table 29-6 lists the operations supported by the scale. In the table, $w is a scale widget.

Table 29-6. Operations on the scale widget.
$w cget optionReturns the value of the configuration option.
$w configure ...Queries or modifies the widget configuration.
$w coords ?value?Returns the coordinates of the point in the trough that corresponds to value, or the scale's value.
$w get ?x y?Returns the value of the scale, or the value that corresponds to the position given by x and y.
$w identify x yReturns trough1, slider, or trough2 to indicate what is under the position given by x and y.
$w set valueSets the value of the scale.

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