TIP #324: A Standard Dialog For Font Selection

Abstract

This TIP proposes the addition of a font selection dialog to the set of common dialogs already available in Tk. Where possible, this dialog will be implemented using the host platform's standard dialogs.

Rationale

A number of platforms (including Windows and Mac OS X) have standard dialogs for common user-oriented tasks, for which Tk provides an interface via commands such as tk_getOpenFile and tk_chooseColor. Another dialog that these platforms provide and which some Tk programs would find useful is a standard font selector. This TIP proposes adding a command to Tk to allow access to the platform font selection dialog where available, or to a dialog implemented in Tcl otherwise.

Unlike the existing dialog commands, the new font dialog command will not return a result, as on some platforms (Mac OS X) the standard font dialog is modeless while on others (Windows) it is modal. To accommodate this difference, all user interaction with the dialog will be communicated to the caller via callbacks or virtual events. To emphasize this difference in operation between the new command and the existing Tk dialog commands the proposed name is tk fontchooser instead of e.g. tk_chooseFont.

Proposal

Tk shall have a new command ensemble tk fontchooser with the following syntax:

tk fontchooser configure ?-option value -option value ...?

set or query one or more of the configurations options below (analogous to Tk widget configuration).

tk fontchooser show

show the font selection dialog. Depending on the platform, may return immediately or only once the dialog has been withdrawn.

tk fontchooser hide

hide the font selection dialog if it is visible and cause any pending tk fontchooser show command to return.

Configuration Options

-parent

specifies/returns the parent window of the font selection dialog (similar to the -parent option to other dialogs).

-title

specifies/returns the title of the dialog. Has no effect on platforms where the font selection dialog does not support titles.

-font

specifies/returns the font that is currently selected in the dialog if it is visible, or that will be initially selected when the dialog is shown (if supported by the platform). Can be set to the empty string to indicate that no font should be selected. Fonts can be specified in any form given by the "FONT DESCRIPTION" section in font(n).

-command

specifies/returns the command prefix to be called when a font selection has been made by the user. The command prefix is evaluated at the global level after having the specification of the selected font appended. On platforms where the font selection dialog offers the user control of further font attributes (such as color), additional key/value pairs may be appended before evaluation. Can be set to the empty string to indicate that no callback should be invoked. Fonts are specified by a list of form [3] of the "FONT DESCRIPTION" section in font(n) (i.e. a list of the form {family size style ?style ...?}).

-visible

read-only option that returns a boolean indicating whether the font selection dialog is currently visible. Attempting to set this option results in an error.

Events

<<TkFontchooserVisibility>>

virtual event sent to the dialog parent whenever the visibility of the font selection dialog changes, both as a result of user action (e.g. disposing of the dialog via OK/Cancel button or close box) and of the tk fontchooser show/hide commands being called. Binding scripts can determine the current visibility of the dialog by querying the -visible configuration option.

<<TkFontchooserFontChanged>>

virtual event sent to the dialog parent whenever the font selection dialog is visible and the selected font changes, both as a result of user action and of the -font configuration option being set. Binding scripts can determine the currently selected font by querying the -font configuration option.

Notes

Whenever a platform provides a suitable font dialog in its API, Tk shall not use a script-based alternative, even if this means missing out on features like the title or dynamic updating of the font during the selection process. This will help to ensure Tk applications meet platform-specific user-interface design guidelines.

Callers should not expect a result from tk fontchooser show and may not assume that the dialog has been withdrawn or closed when the command returns. All user interaction with the dialog is communicated to the caller via the -command callback and the <<TkFontchooser*>> virtual events. It is implementation dependent which exact user actions result in the callback being called resp. the virtual events being sent. Where an Apply or OK button is present in the dialog, it is expected for that button to trigger the -command callback and <<TkFontchooserFontChanged>> virtual event. However, on some implementations other user actions may also have that effect; on Mac OS X for instance, the standard font selection dialog immediately reflects all user choices to the caller.

In the presence of multiple widgets intended to be influenced by the font selection dialog, care needs to be taken to correctly handle focus changes: the font selected in the dialog should always match the current font of the widget with the focus, and the -command callback should only act on the widget with the focus. The recommended practice is to set font dialog -font and -command configuration options in per-widget <FocusIn> handlers (and if necessary to unset them - i.e. set to the empty string - in corresponding <FocusOut> handlers). This is particularly important for implementors of library code using the font selection dialog, to avoid conflicting with application code that may also want to use the dialog.

Because the font selection dialog is application-global, in the presence of multiple interpreters calling tk fontchooser, only the -command callback set by the interpreter that most recently called tk fontchooser configure or tk fontchooser show will be invoked in response to user action and only the -parent set by that interpreter will receive <<TkFontchooser*>> virtual events.

The font dialog implementation may only store (and return) font actual data as the value of the -font configuration option. This can be an issue when -font is set to a named font, if that font is subsequently changed, the font dialog -font option needs to be set again to ensure its selected font matches the new value of the named font.

Example

The following example demonstrates the recommended practice of updating the font dialog configuration with the focus:

 proc tip324demo {} {
     wm title . "TIP324 Demo"
     tk fontchooser configure -parent .
     button .b -command fcToggle -takefocus 0; fcVisibility .b
     bind . <<TkFontchooserVisibility>> [list fcVisibility .b]
     foreach w {.t1 .t2} {
         text $w -width 20 -height 4 -borderwidth 1 -relief solid
         bind $w <FocusIn> [list fcFocus $w]
         $w insert end "Text Widget $w"
     }
     .t1 configure -font {Courier 14}
     .t2 configure -font {Times 16}
     pack .b .t1 .t2; focus .t1
 }
 proc fcToggle {} {
     tk fontchooser [expr {[tk fontchooser configure -visible] ?
             "hide" : "show"}]
 }
 proc fcVisibility {w} {
     $w configure -text [expr {[tk fontchooser configure -visible] ?
             "Hide Font Dialog" : "Show Font Dialog"}]
 }
 proc fcFocus {w} {
     tk fontchooser configure -font [$w cget -font] -command [list fcFont $w]
 }
 proc fcFont {w font args} {
     $w configure -font [font actual $font]
 }
 tip324demo

Alternatives

TIP #213 was an earlier proposal for a font selection dialog API, it was withdrawn when it became clear that it was incompatible with platforms where the standard font dialog is modeless.

Alternative names for the new command have been proposed: tk::choosefont, tk::chooseFont, tk::fontChooser, tk_chooseFont.

Reference Implementation

Tk Patch 1477426 [1]

Copyright

This document has been placed in the public domain.

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