How do constraints work

Constraints are objects defining the relationship between attributes of a widget; you can read the description of the GtkConstraint 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 “strength” property of GtkConstraint to "nudge" the layout towards a solution.

GtkConstraintLayout as GtkBuildable

GtkConstraintLayout implements the GtkBuildable interface and has a custom "constraints" element which allows describing constraints in a GtkBuilder UI file.

An example of a UI definition fragment specifying a constraint:

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<object class="GtkConstraintLayout">
  <constraints>
    <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:

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:

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<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:

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 GtkConstraintTarget.

For instance:

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[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:

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// 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:

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// horizontal orientation, default attribute: width
H:[button(>=150)]

// vertical orientation, default attribute: height
V:[button1(==button2)]

It's also possible to specify multiple predicates, as well as their strength:

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// minimum width of button must be 150
// natural width of button can be 250
[button(>=150@required, ==250@medium)]

Finally, it's also possible to use simple arithmetic operators:

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// width of button1 must be equal to width of button2
// divided by 2 plus 12
[button1(button2 / 2 + 12)]

gtk_constraint_layout_new ()

GtkLayoutManager *
gtk_constraint_layout_new (void);

Creates a new GtkConstraintLayout layout manager.

gtk_constraint_layout_add_constraint ()

void
gtk_constraint_layout_add_constraint (GtkConstraintLayout *layout,
                                      GtkConstraint *constraint);

Adds a GtkConstraint to the layout manager.

The “source” and “target” properties of constraint can be:

The layout acquires the ownership of constraint after calling this function.

gtk_constraint_layout_remove_constraint ()

void
gtk_constraint_layout_remove_constraint
                               (GtkConstraintLayout *layout,
                                GtkConstraint *constraint);

Removes constraint from the layout manager, so that it no longer influences the layout.

gtk_constraint_layout_remove_all_constraints ()

void
gtk_constraint_layout_remove_all_constraints
                               (GtkConstraintLayout *layout);

Removes all constraints from the layout manager.

gtk_constraint_layout_add_guide ()

void
gtk_constraint_layout_add_guide (GtkConstraintLayout *layout,
                                 GtkConstraintGuide *guide);

Adds a guide to layout . A guide can be used as the source or target of constraints, like a widget, but it is not visible.

The layout acquires the ownership of guide after calling this function.

gtk_constraint_layout_remove_guide ()

void
gtk_constraint_layout_remove_guide (GtkConstraintLayout *layout,
                                    GtkConstraintGuide *guide);

Removes guide from the layout manager, so that it no longer influences the layout.

gtk_constraint_layout_add_constraints_from_description ()

GList *
gtk_constraint_layout_add_constraints_from_description
                               (GtkConstraintLayout *layout,
                                const char * const lines[],
                                gsize n_lines,
                                int hspacing,
                                int vspacing,
                                GError **error,
                                const char *first_view,
                                ...);

Creates a list of constraints they formal description using a compact description syntax called VFL, or "Visual Format Language".

This function is a convenience wrapper around gtk_constraint_layout_add_constraints_from_descriptionv(), using variadic arguments to populate the view/target map.

gtk_constraint_layout_add_constraints_from_descriptionv ()

GList *
gtk_constraint_layout_add_constraints_from_descriptionv
                               (GtkConstraintLayout *layout,
                                const char * const lines[],
                                gsize n_lines,
                                int hspacing,
                                int vspacing,
                                GHashTable *views,
                                GError **error);

Creates a list of constraints from a formal description using a compact description syntax called VFL, or "Visual Format Language".

The Visual Format Language is based on Apple's AutoLayout VFL.

The views dictionary is used to match GtkConstraintTargets to the symbolic view name inside the VFL.

The VFL grammar is:

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<visualFormatString> = (<orientation>)?
                       (<superview><connection>)?
                       <view>(<connection><view>)*
                       (<connection><superview>)?
       <orientation> = 'H' | 'V'
         <superview> = '|'
        <connection> = '' | '-' <predicateList> '-' | '-'
     <predicateList> = <simplePredicate> | <predicateListWithParens>
   <simplePredicate> = <metricName> | <positiveNumber>
  <predicateListWithParens> = '(' <predicate> (',' <predicate>)* ')'
         <predicate> = (<relation>)? <objectOfPredicate> (<operatorList>)? ('@' <priority>)?
          <relation> = '==' | '<=' | '>='
 <objectOfPredicate> = <constant> | <viewName> | ('.' <attributeName>)?
          <priority> = <positiveNumber> | 'required' | 'strong' | 'medium' | 'weak'
          <constant> = <number>
      <operatorList> = (<multiplyOperator>)? (<addOperator>)?
  <multiplyOperator> = [ '*' | '/' ] <positiveNumber>
       <addOperator> = [ '+' | '-' ] <positiveNumber>
          <viewName> = [A-Za-z_]([A-Za-z0-9_]*) // A C identifier
        <metricName> = [A-Za-z_]([A-Za-z0-9_]*) // A C identifier
     <attributeName> = 'top' | 'bottom' | 'left' | 'right' | 'width' | 'height' |
                       'start' | 'end' | 'centerX' | 'centerY' | 'baseline'
    <positiveNumber> // A positive real number parseable by g_ascii_strtod()
            <number> // A real number parseable by g_ascii_strtod()

**Note**: The VFL grammar used by GTK is slightly different than the one defined by Apple, as it can use symbolic values for the constraint's strength instead of numeric values; additionally, GTK allows adding simple arithmetic operations inside predicates.

Examples of VFL descriptions are:

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// Default spacing
[button]-[textField]

// Width constraint
[button(>=50)]

// Connection to super view
|-50-[purpleBox]-50-|

// Vertical layout
V:[topField]-10-[bottomField]

// Flush views
[maroonView][blueView]

// Priority
[button(100@strong)]

// Equal widths
[button1(==button2)]

// Multiple predicates
[flexibleButton(>=70,<=100)]

// A complete line of layout
|-[find]-[findNext]-[findField(>=20)]-|

// Operators
[button1(button2 / 3 + 50)]

// Named attributes
[button1(==button2.height)]

[rename-to gtk_constraint_layout_add_constraints_from_description]

gtk_constraint_layout_observe_constraints ()

GListModel *
gtk_constraint_layout_observe_constraints
                               (GtkConstraintLayout *layout);

Returns a GListModel to track the constraints that are part of layout .

Calling this function will enable extra internal bookkeeping to track constraints and emit signals on the returned listmodel. It may slow down operations a lot.

Applications should try hard to avoid calling this function because of the slowdowns.

gtk_constraint_layout_observe_guides ()

GListModel *
gtk_constraint_layout_observe_guides (GtkConstraintLayout *layout);

Returns a GListModel to track the guides that are part of layout .

Calling this function will enable extra internal bookkeeping to track guides and emit signals on the returned listmodel. It may slow down operations a lot.

Applications should try hard to avoid calling this function because of the slowdowns.

GtkConstraintLayout

typedef struct _GtkConstraintLayout GtkConstraintLayout;

A layout manager using GtkConstraint to describe relations between widgets.

GtkConstraintLayoutChild

typedef struct _GtkConstraintLayoutChild GtkConstraintLayoutChild;

A GtkLayoutChild in a GtkConstraintLayout.

enum GtkConstraintVflParserError

Domain for VFL parsing errors.