Uses constraints to describe relations between widgets.
GtkConstraintLayout is a layout manager that uses relations between widget attributes, expressed via [classGtk.Constraint] instances, to measure and allocate widgets.
### How do constraints work
Constraints are objects defining the relationship between attributes of a widget; you can read the description of the [classGtk.Constraint] class to have a more in depth definition.
By taking multiple constraints and applying them to the children of a widget using GtkConstraintLayout, it's possible to describe complex layout policies; each constraint applied to a child or to the parent widgets contributes to the full description of the layout, in terms of parameters for resolving the value of each attribute.
It is important to note that a layout is defined by the totality of constraints; removing a child, or a constraint, from an existing layout without changing the remaining constraints may result in an unstable or unsolvable layout.
Constraints have an implicit "reading order"; you should start describing each edge of each child, as well as their relationship with the parent container, from the top left (or top right, in RTL languages), horizontally first, and then vertically.
A constraint-based layout with too few constraints can become "unstable", that is: have more than one solution. The behavior of an unstable layout is undefined.
A constraint-based layout with conflicting constraints may be unsolvable, and lead to an unstable layout. You can use the [propertyGtk.Constraint:strength] property of [classGtk.Constraint] to "nudge" the layout towards a solution.
### GtkConstraintLayout as GtkBuildable
GtkConstraintLayout implements the [ifaceGtk.Buildable] interface and has a custom "constraints" element which allows describing constraints in a [classGtk.Builder] UI file.
An example of a UI definition fragment specifying a constraint:
target="button" target-attribute="start" relation="eq" source="super"
source-attribute="start" constant="12" strength="required" /> <constraint
target="button" target-attribute="width" relation="ge" constant="250"
strength="strong" /> </constraints> </object> ```
The definition above will add two constraints to the GtkConstraintLayout:
- a required constraint between the leading edge of "button" and the
leading edge of the widget using the constraint layout, plus 12 pixels - a
strong, constant constraint making the width of "button" greater than, or
equal to 250 pixels
The "target" and "target-attribute" attributes are required.
The "source" and "source-attribute" attributes of the "constraint" element
are optional; if they are not specified, the constraint is assumed to be a
constant.
The "relation" attribute is optional; if not specified, the constraint is
assumed to be an equality.
The "strength" attribute is optional; if not specified, the constraint is
assumed to be required.
The "source" and "target" attributes can be set to "super" to indicate
that the constraint target is the widget using the GtkConstraintLayout.
There can be "constant" and "multiplier" attributes.
Additionally, the "constraints" element can also contain a description of
the `GtkConstraintGuides` used by the layout:
```xml <constraints> <guide min-width="100" max-width="500"
name="hspace"/> <guide min-height="64" nat-height="128" name="vspace"
strength="strong"/> </constraints> ```
The "guide" element has the following optional attributes:
- "min-width", "nat-width", and "max-width", describe the minimum,
natural, and maximum width of the guide, respectively - "min-height",
"nat-height", and "max-height", describe the minimum, natural, and maximum
height of the guide, respectively - "strength" describes the strength of
the constraint on the natural size of the guide; if not specified, the
constraint is assumed to have a medium strength - "name" describes a name
for the guide, useful when debugging
### Using the Visual Format Language
Complex constraints can be described using a compact syntax called VFL, or
*Visual Format Language*.
The Visual Format Language describes all the constraints on a row or
column, typically starting from the leading edge towards the trailing one.
Each element of the layout is composed by "views", which identify a
[ifaceGtk.ConstraintTarget].
For instance:
``` [button]-[textField] ```
Describes a constraint that binds the trailing edge of "button" to the
leading edge of "textField", leaving a default space between the two.
Using VFL is also possible to specify predicates that describe constraints
on attributes like width and height:
``` // Width must be greater than, or equal to 50 [button(>=50)]
// Width of button1 must be equal to width of button2 [button1(==button2)]
The default orientation for a VFL description is horizontal, unless otherwise specified:
// vertical orientation, default attribute: height V:[button1(==button2)]
It's also possible to specify multiple predicates, as well as their strength:
be 250 [button(>=150Required, ==250Medium)] ```
Finally, it's also possible to use simple arithmetic operators:
``` // width of button1 must be equal to width of button2 // divided by 2
plus 12 [button1(button2 / 2 + 12)] ```
<group>Layout containers</group>
function "+"
(Widget : access Gtk_Constraint_Layout_Record'Class)
return Gtk.Buildable.Gtk_Buildable
function "-"
(Interf : Gtk.Buildable.Gtk_Buildable)
return Gtk_Constraint_Layout
function Constraint_Layout_Child_Get_Type return Glib.GType
function Get_Type return Glib.GType
type Gtk_Constraint_Layout is access all Gtk_Constraint_Layout_Record'Class;
type Gtk_Constraint_Layout_Child is access all Gtk_Constraint_Layout_Child_Record'Class;
type Gtk_Constraint_Layout_Child_Record is new Gtk_Layout_Child_Record with null record;
function Gtk_Constraint_Layout_New return Gtk_Constraint_Layout
Creates a new GtkConstraintLayout layout manager.
type Gtk_Constraint_Layout_Record is new Gtk_Layout_Manager_Record with null record;
procedure Gtk_New (Layout : out Gtk_Constraint_Layout)
Creates a new GtkConstraintLayout layout manager. Initialize does nothing if the object was already created with another call to Initialize* or G_New.
package Implements_Gtk_Buildable is new Glib.Types.Implements
(Gtk.Buildable.Gtk_Buildable, Gtk_Constraint_Layout_Record, Gtk_Constraint_Layout);
procedure Initialize
(Layout : not null access Gtk_Constraint_Layout_Record'Class)
Creates a new GtkConstraintLayout layout manager. Initialize does nothing if the object was already created with another call to Initialize* or G_New.
procedure Remove_All_Constraints
(Layout : not null access Gtk_Constraint_Layout_Record)
Removes all constraints from the layout manager.