TIP 321: Add a [tk busy] Command

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by Aug 21.
Author:         Jos Decoster <jos.decoster@gmail.com>
State:          Final
Type:           Project
Vote:           Done
Created:        26-Jun-2008
Keywords:       Tk,BLT,busy
Tcl-Version:    8.6


The blt::busy commands can be used to make Tk widget busy, with all user interaction blocked and the cursor can be changed to e.g. a clock. This TIP proposes to add this useful feature to Tk.


BLT has a lot of very useful commands: bgexec, busy, vector, graph widget, barchart widget, ... But getting BLT to work with the latest releases of Tcl and Tk becomes more and more difficult. Some of the problems I experienced are:

Discussions on CLT and #tcl indicated that extracting functionality from BLT and add it to Tcl and Tk might be a good way to make the blt commands available for every Tcl programmer.

This TIP proposes a way to add the blt::busy command to Tk, based on the code as found in BLT2.4z and the code as found in busy.kit. While adding the code to Tk, it was rewritten to use the Tcl_Obj interface and the new option interface. The interface has also been simplified. The blt::release command was not withheld. Because often the same window is made busy again and again, using release in stead of forget might be faster. But when making an application busy, it's because it'll have to wait for another operation and/or process to finish which will typically take a much longer time than the time needed to (re)create the transparent window. The names and isbusy commands were replaced by the new current command. The confusing difference between a window which is busy, a window which was busy but still has the associated transparent window allocated and a window which was busy but no longer has its associated transparent window allocated was removed.

The name of this new Tk command as currently implemented is tk busy, part of the tk command ensemble. Adding it as an option to the grab command might cause confusion as the tk busy command has the opposite functionality of the grab command. It blocks all user interaction rather than redirecting it to one widget.


The tk busy command is an ensemble with a special feature that any unrecognized subcommand that starts with a period is treated as an invokation of the hold subcommand upon the widget with that name.

Hold Subcommand

tk busy window ?option value?

tk busy hold window ?option value?

Makes the window (and its descendants in the Tk window hierarchy) appear busy by ignoring all events sent to the window and its descendants. Window must be a valid path name of a Tk widget. A transparent window is put in front of the specified window. This transparent window is mapped the next time idle tasks are processed, and the specified window and its descendants will be blocked from user interactions. Normally update should be called immediately afterward to insure that the hold operation is in effect before the application starts its processing. The following configuration options are valid:

-cursor cursorName

Specifies the cursor to be displayed when the widget is made busy. CursorName can be in any form accepted by Tk_GetCursor. The default cursor is watch.

Forget Subcommand

tk busy forget window

Releases resources allocated by the tk busy command for window, including the transparent window. User events will again be received by the specified window. Resources are also released when the specified window is destroyed. Window must be the name of a widget previously specified in a hold operation, otherwise an error is reported.

Current Subcommand

tk busy current ?pattern?

Returns the pathnames of all windows that are currently made busy. If a pattern is given, only the path names of busy windows matching pattern are returned.

Status Subcommand

tk busy status window

Returns the busy status of a window. If window presently can not receive user interactions, 1 is returned, otherwise 0.

Configure Subcommand

tk busy configure window ?option? ?value ...?

Queries or modifies the tk busy command configuration options for window. Window must be the path name of a widget previously made busy by the hold operation. If no options are specified, a list describing all of the available options for window (see Tk_ConfigureInfo for information on the format of this list) is returned. If option is specified with no value, then the command returns a list describing the one named option (this list will be identical to the corresponding sublist of the value returned if no option is specified). If one or more option-value pairs are specified, then the command modifies the given widget option(s) to have the given value(s); in this case the command returns the empty string. Options may have any of the values accepted by the hold operation.

Please note that the option database is referenced through window. For example, if the widget .frame is to be made busy, the busy cursor can be specified for it by either option command:

        option add *frame.busyCursor gumby
        option add *Frame.BusyCursor gumby

Cget Subcommand

tk busy cget window option

Queries the tk busy command configuration options for window. Window must be the path name of a widget previously made busy by the hold operation. The command returns the present value of the specified option. Option may have any of the values accepted by the hold operation.

Reference Implementation

See SourceForge patch #1997907 https://sourceforge.net/support/tracker.php?aid=1997907 . There is no support for Mac + Aqua in this patch. It compiles on Mac + Aqua and the command will not return errors but will not resort in a busy effect. Mac + X11 does work.


Because the command as proposed above has the same interface and behavior as the blt::busy command, replacing blt::busy with tk busy is all that's needed to switch to the Tk version of the busy command.

The tk busy command is not one-by-one compatible with the blt::busy command, but typical use (hold and forget) will not suffer a lot. Aliases or an ensemble could be used to make transition easier.


The busy command is available as starkit from http://tcl.tk/starkits/busy.kit

An alternative would have been to keep the interface from BLT. This would have made transition easier, but would have kept the confusing difference between forgotten and released busy windows.


This document has been placed in the public domain.