A recurring question on comp.lang.tcl is, ``How can I center a toplevel window on the screen?''
The usual answer is to use the update command to wait for the window to be configured, and then use the [wm] command to adjust its geometry.
There is an alternative way to do it that doesn't involve the many pitfalls of update: bind to the <Configure> event and use that to notify the process that the window has been configured and it's time to compute the geometry. The code below gives the idea:
# KBK - 12 February 2001
proc make_the_window {} {
# Create the toplevel window
toplevel .t
grid [label .t.l -text "This window\nshould be centered."]
grid [button .t.d -text Dismiss -command {destroy .t}]
# Set things up to position the window when it is
# configured.
bind .t <Configure> {center_the_toplevel %W}
return
}
proc center_the_toplevel { w } {
# Callback on the <Configure> event for a toplevel
# that should be centered on the screen
# Make sure that we aren't configuring a child window
if { [string equal $w [winfo toplevel $w]] } {
# Calculate the desired geometry
set width [winfo reqwidth $w]
set height [winfo reqheight $w]
set x [expr { ( [winfo vrootwidth $w] - $width ) / 2 }]
set y [expr { ( [winfo vrootheight $w] - $height ) / 2 }]
# Hand the geometry off to the window manager
wm geometry $w ${width}x${height}+${x}+${y}
# Unbind <Configure> so that this procedure is
# not called again when the window manager finishes
# centering the window
bind $w <Configure> {}
}
return
}
grid [button .b -text "Test" -command make_the_window] \
[button .q -text "Quit" -command exit]