The placer is a geometry manager for Tk.
It provides simple fixed placement of windows, where you specify
the exact size and location of one window, called the slave,
within another window, called the $master.
The placer also provides rubber-sheet placement, where you specify the
size and location of the slave in terms of the dimensions of
the master, so that the slave changes size and location
in response to changes in the size of the master.
Lastly, the placer allows you to mix these styles of placement so
that, for example, the slave has a fixed width and height but is
centered inside the master.
The place method arranges for the placer
to manage the geometry of $slave.
The remaining arguments consist of one or more -option=>value
pairs that specify the way in which $slave's
geometry is managed.
If the placer is already managing $slave, then the
-option=>value pairs modify the configuration for $slave.
The place method returns an empty string as result.
The following -option=>value pairs are supported:
-in => $master
$master is the reference to the window relative
to which $slave is to be placed.
$master must either be $slave's parent or a descendant
of $slave's parent.
In addition, $master and $slave must both be descendants
of the same top-level window.
These restrictions are necessary to guarantee
that $slave is visible whenever $master is visible.
If this option isn't specified then the master defaults to
$slave's parent.
-x => location
Location specifies the x-coordinate within the master window
of the anchor point for $slave widget.
The location is specified in screen units (i.e. any of the forms
accepted by Tk_GetPixels) and need not lie within the bounds
of the master window.
-relx => location
Location specifies the x-coordinate within the master window
of the anchor point for $slave widget.
In this case the location is specified in a relative fashion
as a floating-point number: 0.0 corresponds to the left edge
of the master and 1.0 corresponds to the right edge of the master.
Location need not be in the range 0.0-1.0.
If both -x and -relx are specified for a slave
then their values are summed. For example, "-relx=>0.5, -x=-2"
positions the left edge of the slave 2 pixels to the left of the
center of its master.
-y => location
Location specifies the y-coordinate within the master window
of the anchor point for $slave widget.
The location is specified in screen units (i.e. any of the forms
accepted by Tk_GetPixels) and need not lie within the bounds
of the master window.
-rely => location
Location specifies the y-coordinate within the master window
of the anchor point for $slave widget.
In this case the value is specified in a relative fashion
as a floating-point number: 0.0 corresponds to the top edge
of the master and 1.0 corresponds to the bottom edge of the master.
Location need not be in the range 0.0-1.0.
If both -y and -rely are specified for a slave
then their values are summed. For example, -rely=>0.5, -x=>3
positions the top edge of the slave 3 pixels below the
center of its master.
-anchor => where
Where specifies which point of $slave is to be positioned
at the (x,y) location selected by the -x, -y,
-relx, and -rely options.
The anchor point is in terms of the outer area of $slave
including its border, if any.
Thus if where is se then the lower-right corner of
$slave's border will appear at the given (x,y) location
in the master.
The anchor position defaults to nw.
-width => size
Size specifies the width for $slave in screen units
(i.e. any of the forms accepted by Tk_GetPixels).
The width will be the outer width of $slave including its
border, if any.
If size is an empty string, or if no -width
or -relwidth option is specified, then the width requested
internally by the window will be used.
-relwidth => size
Size specifies the width for $slave.
In this case the width is specified as a floating-point number
relative to the width of the master: 0.5 means $slave will
be half as wide as the master, 1.0 means $slave will have
the same width as the master, and so on.
If both -width and -relwidth are specified for a slave,
their values are summed. For example, -relwidth=>1.0, -width=>5
makes the slave 5 pixels wider than the master.
-height => size
Size specifies the height for $slave in screen units
(i.e. any of the forms accepted by Tk_GetPixels).
The height will be the outer dimension of $slave including its
border, if any.
If size is an empty string, or if no -height or
-relheight option is specified, then the height requested
internally by the window will be used.
-relheight => size
Size specifies the height for $slave.
In this case the height is specified as a floating-point number
relative to the height of the master: 0.5 means $slave will
be half as high as the master, 1.0 means $slave will have
the same height as the master, and so on.
If both -height and -relheight are specified for a slave,
their values are summed. For example, -relheight=>1.0, -height=>-2
makes the slave 2 pixels shorter than the master.
-bordermode => mode
Mode determines the degree to which borders within the
master are used in determining the placement of the slave.
The default and most common value is inside.
In this case the placer considers the area of the master to
be the innermost area of the master, inside any border:
an option of -x=>0 corresponds to an x-coordinate just
inside the border and an option of -relwidth=>1.0
means $slave will fill the area inside the master's
border.
If mode is outside then the placer considers
the area of the master to include its border;
this mode is typically used when placing $slave
outside its master, as with the options
-x=>0, -y=>0, -anchor=>ne.
Lastly, mode may be specified as ignore, in which
case borders are ignored: the area of the master is considered
to be its official X area, which includes any internal border but
no external border. A bordermode of ignore is probably
not very useful.
If the same value is specified separately with
two different options, such as -x and -relx, then
the most recent option is used and the older one is ignored.
$slave->placeSlaves
The placeSlaves method returns a list of all the slave
windows for which $master is the master.
If there are no slaves for $master then an empty list is
returned.
$slave->placeForget
The placeForget method causes the placer to stop managing
the geometry of $slave. As a side effect of this method call
$slave will be unmapped so that it doesn't appear on the
screen.
If $slave isn't currently managed by the placer then the
method call has no effect.
placeForget returns an empty string as result.
$slave->placeInfo
The placeInfo method returns a list giving the current
configuration of $slave.
The list consists of -option=>value pairs in exactly the
same form as might be specified to the place
method.
If the configuration of a window has been retrieved with
placeInfo, that configuration can be restored later by
first using placeForget to erase any existing information
for the window and then invoking place with
the saved information.
FINE POINTS
It is not necessary for the master window to be the parent
of the slave window.
This feature is useful in at least two situations.
First, for complex window layouts it means you can create a
hierarchy of subwindows whose only purpose
is to assist in the layout of the parent.
The ``real children'' of the parent (i.e. the windows that
are significant for the application's user interface) can be
children of the parent yet be placed inside the windows
of the geometry-management hierarchy.
This means that the path names of the ``real children''
don't reflect the geometry-management hierarchy and users
can specify options for the real children
without being aware of the structure of the geometry-management
hierarchy.
A second reason for having a master different than the slave's
parent is to tie two siblings together.
For example, the placer can be used to force a window always to
be positioned centered just below one of its
siblings by specifying the configuration
Whenever the $sibling widget is repositioned in the future, the slave
will be repositioned as well.
Unlike many other geometry managers (such as the packer)
the placer does not make any attempt to manipulate the geometry of
the master windows or the parents of slave windows (i.e. it doesn't
set their requested sizes).
To control the sizes of these windows, make them windows like
frames and canvases that provide configuration options for this purpose.
