Commit 86b3c1b6 authored by Tristan Van Berkom's avatar Tristan Van Berkom

Enhanced GtkWidget documentation with regards to height-for-width geometry management.

parent 5e8baa5f
......@@ -72,73 +72,190 @@
* <refsect2 id="geometry-management">
* <title>Height-for-width Geometry Management</title>
* <para>
* 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.
*
* GTK+'s traditional two-pass <link linkend="size-allocation">size-allocation</link>
* algorithm does not allow this flexibility. #GtkWidget provides a default
* implementation of the height-for-width methods for existing widgets,
* which always requests the same height, regardless of the available width.
*
* Some important things to keep in mind when implementing
* height-for-width and when using it in container
* implementations.
*
* The geometry management system will query a logical hierarchy in
* 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 five virtual methods:
* <variablelist>
* <varlistentry>
* <term>#GtkWidgetClass.get_request_mode()</term>
* <listitem>
* This allows a widget to tell it's parent container whether
* it preferrs to be allocated in %GTK_SIZE_REQUEST_HEIGHT_FOR_WIDTH
* or %GTK_SIZE_REQUEST_WIDTH_FOR_HEIGHT mode.
*
* %GTK_SIZE_REQUEST_HEIGHT_FOR_WIDTH means the widget preferrs to
* have #GtkWidgetClass.get_preferred_width() called and then
* #GtkWidgetClass.get_preferred_height_for_width() and is the
* default return for unimplemented cases.
*
* However it's important to note (as described below) that any
* widget which trades height-for-width must respond properly to
* both #GtkSizeRequestModes since it might be queried in either
* orientation by it's parent container.
* </listitem>
* </varlistentry>
* <varlistentry>
* <term>#GtkWidgetClass.get_preferred_width()</term>
* <listitem>
* This is called by containers to obtain the minimum and
* natural width of a widget. A widget will never be allocated
* a width less than it's minimum and will only ever be allocated
* a width greater than the natural width once all of the said
* widget's siblings have received their natural widths.
*
* Furthermore a widget will only ever be allocated a width greater
* than it's natural width if it was configured to receive extra
* expand space from it's parent container.
* </listitem>
* </varlistentry>
* <varlistentry>
* <term>#GtkWidgetClass.get_preferred_height()</term>
* <listitem>
* This is called by containers to obtain the minimum and
* natural height of a widget.
*
* A widget that does not actually trade any height for width
* or width for height only has to implement these two virtual
* methods (#GtkWidgetClass.get_preferred_width() and
* #GtkWidgetClass.get_preferred_height()).
* </listitem>
* </varlistentry>
* <varlistentry>
* <term>#GtkWidgetClass.get_preferred_height_for_width()</term>
* <listitem>
* This is similar to #GtkWidgetClass.get_preferred_height() except
* that it is passed a contextual width to request height for. By
* implementing this virtual method it is possible for a #GtkLabel
* to tell it's parent how much height would be required if the
* label were to be allocated a said width.
* </listitem>
* </varlistentry>
* <varlistentry>
* <term>#GtkWidgetClass.get_preferred_width_for_height()</term>
* <listitem>
* This is analogous to #GtkWidgetClass.get_preferred_height_for_width()
* except that it operates in the oposite orientation. It's rare that
* a widget actually does %GTK_SIZE_REQUEST_WIDTH_FOR_HEIGHT requests
* but can happen when for example; a widget or container gets additional
* columns to compensate for a smaller allocated height.
* </listitem>
* </varlistentry>
* </variablelist>
*
* There are some important things to keep in mind when implementing
* height-for-width and when using it in container implementations.
*
* 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 a dual pass
* in the direction chosen by the toplevel.
*
* For instance when queried in the normal height-for-width mode:
* First the default minimum and natural width for each widget
* in the interface will computed and collectively returned to
* the toplevel by way of gtk_widget_get_preferred_width().
* Next, the toplevel will use the minimum width to query for the
* minimum height contextual to that width using
* gtk_widget_get_preferred_height_for_width(), which will also
* be a highly recursive operation. This minimum-for-minimum size can
* be used to set the minimum size constraint on the toplevel.
*
* When allocating, each container can use the minimum and natural
* sizes reported by their children to allocate natural sizes and
* expose as much content as possible with the given allocation.
*
* That means that the request operation at allocation time will
* usually fire again in contexts of different allocated sizes than
* the ones originally queried for. #GtkWidget caches a
* small number of results to avoid re-querying for the same
* allocated size in one allocation cycle.
*
* A widget that does not actually do height-for-width
* or width-for-height size negotiations only has to implement
* #GtkWidgetClass.get_preferred_width() and
* #GtkWidgetClass.get_preferred_height().
* for their minimum sizes it is generally done in two initial passes
* in the #GtkSizeRequestMode chosen by the toplevel.
*
* For example, when queried in the normal
* %GTK_SIZE_REQUEST_HEIGHT_FOR_WIDTH mode:
* First, the default minimum and natural width for each widget
* in the interface will be computed using gtk_width_get_preferred_width().
* Because the preferred widths for each container depends on the preferred
* widths of thier 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_get_preferred_height_for_width(), which 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
* (unless gtk_window_set_geometry_hints() is explicitly used instead).
*
* After the toplevel window has initially requested it's size in both
* dimensions it can go on to allocate itself a reasonable size (or a size
* previously specified with gtk_window_set_default_size()). During the
* recursive allocation process it's important to note that request cycles
* will be recursively executed while container widgets allocate their children.
* Each container 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 their target allocated width or width for allocated height
* depending. In this way a #GtkWidget 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
* requested. For this reason; #GtkWidget caches a small number of results
* to avoid re-querying for the same sizes in one allocation cycle.
*
* See <link linkend="container-geometry-management">GtkContainer's
* geometry management section</link>
* to learn more about how height-for-width allocations are performed
* by container widgets.
*
* If a widget does move content around to smartly use up the
* allocated size, then it must support the request properly in
* both orientations; even if the request only makes sense in
* one orientation.
* allocated size, then it must support the request in both
* #GtkSizeRequestModes even if the widget in question only
* trades sizes in a single orientation.
*
* For instance, a #GtkLabel that does height-for-width word wrapping
* will not expect to have #GtkWidgetClass.get_preferred_height() called because that
* call is specific to a width-for-height request. In this case the
* label must return the heights contextual to its 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.
* will not expect to have #GtkWidgetClass.get_preferred_height() called
* because that call is specific to a width-for-height request. In this
* case the label must return the heights required for it's 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_SIZE_REQUEST_HEIGHT_FOR_WIDTH widget
* generally deals with width-for-height requests, for #GtkWidgetClass.get_preferred_height()
* it will do:
* <programlisting><![CDATA[
* static void
* foo_widget_get_preferred_height (GtkWidget *widget, gint *min_height, gint *nat_height)
* {
* if (i_am_in_height_for_width_mode)
* {
* gint min_width;
*
* GTK_WIDGET_GET_CLASS (widget)->get_preferred_width (widget, &min_width, NULL);
* GTK_WIDGET_GET_CLASS (widget)->get_preferred_height_for_width (widget, min_width,
* min_height, nat_height);
* }
* else
* {
* ... some widgets do both, for instance if a GtkLabel is rotated to 90 degrees
* it will return the minimum and natural height for the rotated label here.
* }
* }
* ]]></programlisting>
*
* And in #GtkWidgetClass.get_preferred_width_for_height() it will simply return
* the minimum and natural width:
*
* <programlisting><![CDATA[
* static void
* foo_widget_get_preferred_width_for_height (GtkWidget *widget, gint for_height,
* gint *min_width, gint *nat_width)
* {
* if (i_am_in_height_for_width_mode)
* {
* GTK_WIDGET_GET_CLASS (widget)->get_preferred_width (widget, min_width, nat_width);
* }
* else
* {
* ... again if a widget is sometimes operating in width-for-height mode
* (like a rotated GtkLabel) it can go ahead and do it's real width for
* height calculation here.
* }
* }
* ]]></programlisting>
*
* 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 this:
* 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 this:
* <example>
* <title>Widget calling its own size request method.</title>
* <programlisting>
* GTK_WIDGET_GET_CLASS(widget)-&gt;get_preferred_width (widget), &min, &natural);
* GTK_WIDGET_GET_CLASS(widget)-&gt;get_preferred_width (widget),
* &min, &natural);
* </programlisting>
* </example>
*
......
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