ConstraintLayout

class ConstraintLayout(**properties: Any)

Superclasses: LayoutManager, Object

Implemented Interfaces: Buildable

A layout manager using constraints to describe relations between widgets.

GtkConstraintLayout is a layout manager that uses relations between widget attributes, expressed via 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 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 strength property of Constraint to “nudge” the layout towards a solution.

GtkConstraintLayout as GtkBuildable

GtkConstraintLayout implements the Buildable interface and has a custom “constraints” element which allows describing constraints in a Builder UI file.

An example of a UI definition fragment specifying a constraint:

<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:

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:

<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 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:

// 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:

// 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:

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

Constructors

class ConstraintLayout

classmethod new() → LayoutManager: Creates a new GtkConstraintLayout layout manager.

Methods

class ConstraintLayout

add_constraint(constraint: Constraint) → None

Adds a constraint to the layout manager.

The source and target properties of constraint can be:

set to NULL to indicate that the constraint refers to the widget using layout

set to the Widget using layout

set to a child of the Widget using layout

set to a ConstraintGuide that is part of layout

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

Parameters:: constraint – a Constraint

add_constraints_from_description(lines: Sequence[str], hspacing: int, vspacing: int, views: dict[str, ConstraintTarget]) → list[Constraint]

Creates a list of constraints from a VFL description.

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

Parameters:

lines – an array of Visual Format Language lines defining a set of constraints
hspacing – default horizontal spacing value, or -1 for the fallback value
vspacing – default vertical spacing value, or -1 for the fallback value
views

add_guide(guide: ConstraintGuide) → None

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.

Parameters:: guide – a ConstraintGuide object

observe_constraints() → ListModel

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

observe_guides() → ListModel

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

remove_all_constraints() → None: Removes all constraints from the layout manager.

remove_constraint(constraint: Constraint) → None

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

Parameters:: constraint – a Constraint

remove_guide(guide: ConstraintGuide) → None

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

Parameters:: guide – a ConstraintGuide object