GTK uses a height-for-width (and width-for-height) geometry management
system. Height-for-width means that a widget can change how much
vertical space it needs, depending on the amount of horizontal space
that it is given (and similar for width-for-height). The most common
example is a label that reflows to fill up the available width, wraps
to fewer lines, and therefore needs less height.
Height-for-width geometry management is implemented in GTK by way
of two virtual methods:
vfunc@Gtk.Widget.get_request_mode
vfunc@Gtk.Widget.measure
There are some important things to keep in mind when implementing
height-for-width and when using it in widget implementations.
If you implement a direct gtk.widget.Widget subclass that supports
height-for-width or width-for-height geometry management for itself
or its child widgets, the vfunc@Gtk.Widget.get_request_mode virtual
function must be implemented as well and return the widget's preferred
request mode. The default implementation of this virtual function
returns gtk.types.SizeRequestMode.ConstantSize, which means that the widget will
only ever get -1 passed as the for_size value to its
vfunc@Gtk.Widget.measure implementation.
The geometry management system will query a widget hierarchy in
only one orientation at a time. When widgets are initially queried
for their minimum sizes it is generally done in two initial passes
in the gtk.types.SizeRequestMode chosen by the toplevel.
First, the default minimum and natural width for each widget
in the interface will be computed using gtk.widget.Widget.measure with an
orientation of gtk.types.Orientation.Horizontal and a for_size of -1.
Because the preferred widths for each widget depend on the preferred
widths of their children, this information propagates up the hierarchy,
and finally a minimum and natural width is determined for the entire
toplevel. Next, the toplevel will use the minimum width to query for the
minimum height contextual to that width using gtk.widget.Widget.measure with an
orientation of gtk.types.Orientation.Vertical and a for_size of the just computed
width. This will also be a highly recursive operation. The minimum height
for the minimum width is normally used to set the minimum size constraint
on the toplevel.
After the toplevel window has initially requested its size in both
dimensions it can go on to allocate itself a reasonable size (or a size
previously specified with gtk.window.Window.setDefaultSize). During the
recursive allocation process it’s important to note that request cycles
will be recursively executed while widgets allocate their children.
Each widget, once allocated a size, will go on to first share the
space in one orientation among its children and then request each child's
height for its target allocated width or its width for allocated height,
depending. In this way a gtk.widget.Widget will typically be requested its size
a number of times before actually being allocated a size. The size a
widget is finally allocated can of course differ from the size it has
requested. For this reason, gtk.widget.Widget caches a small number of results
to avoid re-querying for the same sizes in one allocation cycle.
If a widget does move content around to intelligently use up the
allocated size then it must support the request in both
gtk.types.SizeRequestModes even if the widget in question only
trades sizes in a single orientation.
For instance, a gtk.label.Label that does height-for-width word wrapping
will not expect to have vfunc@Gtk.Widget.measure with an orientation of
gtk.types.Orientation.Vertical called because that call is specific to a
width-for-height request. In this case the label must return the height
required for its own minimum possible width. By following this rule any
widget that handles height-for-width or width-for-height requests will
always be allocated at least enough space to fit its own content.
staticvoid
foo_widget_measure (GtkWidget *widget,
GtkOrientation orientation,
int for_size,
int *minimum_size,
int *natural_size,
int *minimum_baseline,
int *natural_baseline)
{
if (orientation == GTK_ORIENTATION_HORIZONTAL)
{
// Calculate minimum and natural width
}
else// VERTICAL
{
if (i_am_in_height_for_width_mode)
{
int min_width, dummy;
// First, get the minimum width of our widget
GTK_WIDGET_GET_CLASS (widget)->measure (widget, GTK_ORIENTATION_HORIZONTAL, -1,
&min_width, &dummy, &dummy, &dummy);
// Now use the minimum width to retrieve the minimum and natural height to display// that width.
GTK_WIDGET_GET_CLASS (widget)->measure (widget, GTK_ORIENTATION_VERTICAL, min_width,
minimum_size, natural_size, &dummy, &dummy);
}
else
{
// ... some widgets do both.
}
}
}
Often a widget needs to get its own request during size request or
allocation. For example, when computing height it may need to also
compute width. Or when deciding how to use an allocation, the widget
may need to know its natural size. In these cases, the widget should
be careful to call its virtual methods directly, like in the code
example above.
It will not work to use the wrapper function gtk.widget.Widget.measure
inside your own vfunc@Gtk.Widget.size_allocate implementation.
These return a request adjusted by gtk.size_group.SizeGroup, the widget's
align and expand flags, as well as its CSS style.
If a widget used the wrappers inside its virtual method implementations,
then the adjustments (such as widget margins) would be applied
twice. GTK therefore does not allow this and will warn if you try
to do it.
Of course if you are getting the size request for another widget, such
as a child widget, you must use gtk.widget.Widget.measure; otherwise, you
would not properly consider widget margins, gtk.size_group.SizeGroup, and
so forth.
GTK also supports baseline vertical alignment of widgets. This
means that widgets are positioned such that the typographical baseline of
widgets in the same row are aligned. This happens if a widget supports
baselines, has a vertical alignment using baselines, and is inside
a widget that supports baselines and has a natural “row” that it aligns to
the baseline, or a baseline assigned to it by the grandparent.
Baseline alignment support for a widget is also done by the
vfunc@Gtk.Widget.measure virtual function. It allows you to report
both a minimum and natural size.
If a widget ends up baseline aligned it will be allocated all the space in
the parent as if it was gtk.types.Align.Fill, but the selected baseline can be
found via gtk.widget.Widget.getBaseline. If the baseline has a
value other than -1 you need to align the widget such that the baseline
appears at the position.
The gtk.widget.Widget implementation of the gtk.buildable.Buildable interface
supports various custom elements to specify additional aspects of widgets
that are not directly expressed as properties.
The interface description semantics expected in composite template descriptions
is slightly different from regular gtk.builder.Builder XML.
Unlike regular interface descriptions, gtk.widget_class.WidgetClass.setTemplate
will expect a <template> tag as a direct child of the toplevel
<interface> tag. The <template> tag must specify the “class” attribute
which must be the type name of the widget. Optionally, the “parent”
attribute may be specified to specify the direct parent type of the widget
type; this is ignored by gtk.builder.Builder but can be used by UI design tools to
introspect what kind of properties and internal children exist for a given
type when the actual type does not exist.
The XML which is contained inside the <template> tag behaves as if it were
added to the <object> tag defining the widget itself. You may set properties
on a widget by inserting <property> tags into the <template> tag, and also
add <child> tags to add children and extend a widget in the normal way you
would with <object> tags.
Additionally, <object> tags can also be added before and after the initial
<template> tag in the normal way, allowing one to define auxiliary objects
which might be referenced by other widgets declared as children of the
<template> tag.
Since, unlike the <object> tag, the <template> tag does not contain an
“id” attribute, if you need to refer to the instance of the object itself that
the template will create, simply refer to the template class name in an
applicable element content.
Here is an example of a template definition, which includes an example of
this in the <signal> tag:
staticvoid
foo_widget_dispose (GObject *gobject)
{
FooWidget *self = FOO_WIDGET (gobject);
// Dispose objects for which you have a reference...// Clear the template children for this widget type
gtk_widget_dispose_template (GTK_WIDGET (self), FOO_TYPE_WIDGET);
G_OBJECT_CLASS (foo_widget_parent_class)->dispose (gobject);
}
You can access widgets defined in the template using the
gtk.widget.Widget.getTemplateChild function, but you will typically declare
a pointer in the instance private data structure of your type using the same
name as the widget in the template definition, and call
gtk.widget_class.WidgetClass.bindTemplateChildFull (or one of its wrapper macros
func@Gtk.widget_class_bind_template_child and func@Gtk.widget_class_bind_template_child_private)
with that name, e.g.
You can also use gtk.widget_class.WidgetClass.bindTemplateCallbackFull (or
is wrapper macro func@Gtk.widget_class_bind_template_callback) to connect
a signal callback defined in the template with a function visible in the
scope of the class, e.g.
// the signal handler has the instance and user data swapped// because of the swapped="yes" attribute in the template XMLstaticvoid
hello_button_clicked (FooWidget *self,
GtkButton *button)
{
g_print ("Hello, world!\n");
}
staticvoid
foo_widget_class_init (FooWidgetClass *klass)
{
// ...
gtk_widget_class_set_template_from_resource (GTK_WIDGET_CLASS (klass),
"/com/example/ui/foowidget.ui");
gtk_widget_class_bind_template_callback (GTK_WIDGET_CLASS (klass), hello_button_clicked);
}
The base class for all widgets.
gtk.widget.Widget is the base class all widgets in GTK derive from. It manages the widget lifecycle, layout, states and style.
Height-for-width Geometry Management
GTK uses a height-for-width (and width-for-height) geometry management system. Height-for-width means that a widget can change how much vertical space it needs, depending on the amount of horizontal space that it is given (and similar for width-for-height). The most common example is a label that reflows to fill up the available width, wraps to fewer lines, and therefore needs less height.
Height-for-width geometry management is implemented in GTK by way of two virtual methods:
There are some important things to keep in mind when implementing height-for-width and when using it in widget implementations.
If you implement a direct gtk.widget.Widget subclass that supports height-for-width or width-for-height geometry management for itself or its child widgets, the vfunc@Gtk.Widget.get_request_mode virtual function must be implemented as well and return the widget's preferred request mode. The default implementation of this virtual function returns gtk.types.SizeRequestMode.ConstantSize, which means that the widget will only ever get -1 passed as the for_size value to its vfunc@Gtk.Widget.measure implementation.
The geometry management system will query a widget hierarchy in only one orientation at a time. When widgets are initially queried for their minimum sizes it is generally done in two initial passes in the gtk.types.SizeRequestMode chosen by the toplevel.
For example, when queried in the normal gtk.types.SizeRequestMode.HeightForWidth mode:
First, the default minimum and natural width for each widget in the interface will be computed using gtk.widget.Widget.measure with an orientation of gtk.types.Orientation.Horizontal and a for_size of -1. Because the preferred widths for each widget depend on the preferred widths of their children, this information propagates up the hierarchy, and finally a minimum and natural width is determined for the entire toplevel. Next, the toplevel will use the minimum width to query for the minimum height contextual to that width using gtk.widget.Widget.measure with an orientation of gtk.types.Orientation.Vertical and a for_size of the just computed width. This will also be a highly recursive operation. The minimum height for the minimum width is normally used to set the minimum size constraint on the toplevel.
After the toplevel window has initially requested its size in both dimensions it can go on to allocate itself a reasonable size (or a size previously specified with gtk.window.Window.setDefaultSize). During the recursive allocation process it’s important to note that request cycles will be recursively executed while widgets allocate their children. Each widget, once allocated a size, will go on to first share the space in one orientation among its children and then request each child's height for its target allocated width or its width for allocated height, depending. In this way a gtk.widget.Widget will typically be requested its size a number of times before actually being allocated a size. The size a widget is finally allocated can of course differ from the size it has requested. For this reason, gtk.widget.Widget caches a small number of results to avoid re-querying for the same sizes in one allocation cycle.
If a widget does move content around to intelligently use up the allocated size then it must support the request in both gtk.types.SizeRequestModes even if the widget in question only trades sizes in a single orientation.
For instance, a gtk.label.Label that does height-for-width word wrapping will not expect to have vfunc@Gtk.Widget.measure with an orientation of gtk.types.Orientation.Vertical called because that call is specific to a width-for-height request. In this case the label must return the height required for its own minimum possible width. By following this rule any widget that handles height-for-width or width-for-height requests will always be allocated at least enough space to fit its own content.
Here are some examples of how a gtk.types.SizeRequestMode.HeightForWidth widget generally deals with width-for-height requests:
Often a widget needs to get its own request during size request or allocation. For example, when computing height it may need to also compute width. Or when deciding how to use an allocation, the widget may need to know its natural size. In these cases, the widget should be careful to call its virtual methods directly, like in the code example above.
It will not work to use the wrapper function gtk.widget.Widget.measure inside your own vfunc@Gtk.Widget.size_allocate implementation. These return a request adjusted by gtk.size_group.SizeGroup, the widget's align and expand flags, as well as its CSS style.
If a widget used the wrappers inside its virtual method implementations, then the adjustments (such as widget margins) would be applied twice. GTK therefore does not allow this and will warn if you try to do it.
Of course if you are getting the size request for another widget, such as a child widget, you must use gtk.widget.Widget.measure; otherwise, you would not properly consider widget margins, gtk.size_group.SizeGroup, and so forth.
GTK also supports baseline vertical alignment of widgets. This means that widgets are positioned such that the typographical baseline of widgets in the same row are aligned. This happens if a widget supports baselines, has a vertical alignment using baselines, and is inside a widget that supports baselines and has a natural “row” that it aligns to the baseline, or a baseline assigned to it by the grandparent.
Baseline alignment support for a widget is also done by the vfunc@Gtk.Widget.measure virtual function. It allows you to report both a minimum and natural size.
If a widget ends up baseline aligned it will be allocated all the space in the parent as if it was gtk.types.Align.Fill, but the selected baseline can be found via gtk.widget.Widget.getBaseline. If the baseline has a value other than -1 you need to align the widget such that the baseline appears at the position.
GtkWidget as GtkBuildable
The gtk.widget.Widget implementation of the gtk.buildable.Buildable interface supports various custom elements to specify additional aspects of widgets that are not directly expressed as properties.
If the widget uses a gtk.layout_manager.LayoutManager, gtk.widget.Widget supports a custom <layout> element, used to define layout properties:
gtk.widget.Widget allows style information such as style classes to be associated with widgets, using the custom <style> element:
gtk.widget.Widget allows defining accessibility information, such as properties, relations, and states, using the custom <accessibility> element:
Building composite widgets from template XML
GtkWidget exposes some facilities to automate the procedure of creating composite widgets using "templates".
To create composite widgets with gtk.builder.Builder XML, one must associate the interface description with the widget class at class initialization time using gtk.widget_class.WidgetClass.setTemplate.
The interface description semantics expected in composite template descriptions is slightly different from regular gtk.builder.Builder XML.
Unlike regular interface descriptions, gtk.widget_class.WidgetClass.setTemplate will expect a <template> tag as a direct child of the toplevel <interface> tag. The <template> tag must specify the “class” attribute which must be the type name of the widget. Optionally, the “parent” attribute may be specified to specify the direct parent type of the widget type; this is ignored by gtk.builder.Builder but can be used by UI design tools to introspect what kind of properties and internal children exist for a given type when the actual type does not exist.
The XML which is contained inside the <template> tag behaves as if it were added to the <object> tag defining the widget itself. You may set properties on a widget by inserting <property> tags into the <template> tag, and also add <child> tags to add children and extend a widget in the normal way you would with <object> tags.
Additionally, <object> tags can also be added before and after the initial <template> tag in the normal way, allowing one to define auxiliary objects which might be referenced by other widgets declared as children of the <template> tag.
Since, unlike the <object> tag, the <template> tag does not contain an “id” attribute, if you need to refer to the instance of the object itself that the template will create, simply refer to the template class name in an applicable element content.
Here is an example of a template definition, which includes an example of this in the <signal> tag:
Typically, you'll place the template fragment into a file that is bundled with your project, using gio.resource.Resource. In order to load the template, you need to call gtk.widget_class.WidgetClass.setTemplateFromResource from the class initialization of your gtk.widget.Widget type:
You will also need to call gtk.widget.Widget.initTemplate from the instance initialization function:
as well as calling gtk.widget.Widget.disposeTemplate from the dispose function:
You can access widgets defined in the template using the gtk.widget.Widget.getTemplateChild function, but you will typically declare a pointer in the instance private data structure of your type using the same name as the widget in the template definition, and call gtk.widget_class.WidgetClass.bindTemplateChildFull (or one of its wrapper macros func@Gtk.widget_class_bind_template_child and func@Gtk.widget_class_bind_template_child_private) with that name, e.g.
You can also use gtk.widget_class.WidgetClass.bindTemplateCallbackFull (or is wrapper macro func@Gtk.widget_class_bind_template_callback) to connect a signal callback defined in the template with a function visible in the scope of the class, e.g.