ClutterConstraint — Abstract class for constraints on position or size

Types and Values

Object Hierarchy

    ╰── GInitiallyUnowned
        ╰── ClutterActorMeta
            ╰── ClutterConstraint
                ├── ClutterAlignConstraint
                ├── ClutterBindConstraint
                ├── ClutterPathConstraint
                ╰── ClutterSnapConstraint


ClutterConstraint is a base abstract class for modifiers of a ClutterActor position or size.

A ClutterConstraint sub-class should contain the logic for modifying the position or size of the ClutterActor to which it is applied, by updating the actor's allocation. Each ClutterConstraint can change the allocation of the actor to which they are applied by overriding the ClutterConstraintClass.update_allocation() virtual function.

ClutterConstraint is available since Clutter 1.4

Using Constraints

Constraints can be used with fixed layout managers, like ClutterFixedLayout, or with actors implicitly using a fixed layout manager, like ClutterGroup and ClutterStage.

Constraints provide a way to build user interfaces by using relations between ClutterActors, without explicit fixed positioning and sizing, similarly to how fluid layout managers like ClutterBoxLayout and ClutterTableLayout lay out their children.

Constraints are attached to a ClutterActor, and are available for inspection using clutter_actor_get_constraints().

Clutter provides different implementation of the ClutterConstraint abstract class, for instance:

  • ClutterAlignConstraint, a constraint that can be used to align an actor to another one on either the horizontal or the vertical axis, using a normalized value between 0 and 1.

  • ClutterBindConstraint, a constraint binds the X, Y, width or height of an actor to the corresponding position or size of a source actor, with or without an offset.

  • ClutterSnapConstraint, a constraint that "snaps" together the edges of two ClutterActors; if an actor uses two constraints on both its horizontal or vertical edges then it can also expand to fit the empty space.

The constraints example uses various types of ClutterConstraints to lay out three actors on a resizable stage. Only the central actor has an explicit size, and no actor has an explicit position.

  • The ClutterActor with “name” layerA is explicitly sized to 100 pixels by 25 pixels, and it's added to the ClutterStage

  • two ClutterAlignConstraints are used to anchor layerA to the center of the stage, by using 0.5 as the alignment “factor” on both the X and Y axis

  • the ClutterActor with “name” layerB is added to the ClutterStage with no explicit size

  • the “x” and “width” of layerB are bound to the same properties of layerA using two ClutterBindConstraint objects, thus keeping layerB aligned to layerA

  • the top edge of layerB is snapped together with the bottom edge of layerA; the bottom edge of layerB is also snapped together with the bottom edge of the ClutterStage; an offset is given to the two ClutterSnapConstraintss to allow for some padding; since layerB is snapped between two different ClutterActors, its height is stretched to match the gap

  • the ClutterActor with “name” layerC mirrors layerB, snapping the top edge of the ClutterStage to the top edge of layerC and the top edge of layerA to the bottom edge of layerC

You can try resizing interactively the ClutterStage and verify that the three ClutterActors maintain the same position and size relative to each other, and to the ClutterStage.

It is important to note that Clutter does not avoid loops or competing constraints; if two or more ClutterConstraints are operating on the same positional or dimensional attributes of an actor, or if the constraints on two different actors depend on each other, then the behavior is undefined.

Implementing a ClutterConstraint

Creating a sub-class of ClutterConstraint requires the implementation of the ClutterConstraintClass.update_allocation() virtual function.

The update_allocation() virtual function is called during the allocation sequence of a ClutterActor, and allows any ClutterConstraint attached to that actor to modify the allocation before it is passed to the actor's ClutterActorClass.allocate() implementation.

The ClutterActorBox passed to the update_allocation() implementation contains the original allocation of the ClutterActor, plus the eventual modifications applied by the other ClutterConstraints, in the same order the constraints have been applied to the actor.

It is not necessary for a ClutterConstraint sub-class to chain up to the parent's implementation.

If a ClutterConstraint is parametrized - i.e. if it contains properties that affect the way the constraint is implemented - it should call clutter_actor_queue_relayout() on the actor to which it is attached to whenever any parameter is changed. The actor to which it is attached can be recovered at any point using clutter_actor_meta_get_actor().


Types and Values


typedef struct _ClutterConstraint ClutterConstraint;

The ClutterConstraint structure contains only private data and should be accessed using the provided API

Since: 1.4

struct ClutterConstraintClass

struct ClutterConstraintClass {
  void (* update_allocation) (ClutterConstraint *constraint,
                              ClutterActor      *actor,
                              ClutterActorBox   *allocation);

  void (* update_preferred_size) (ClutterConstraint  *constraint,
                                  ClutterActor       *actor,
                                  ClutterOrientation  direction,
                                  float               for_size,
                                  float              *minimum_size,
                                  float              *natural_size);

The ClutterConstraintClass structure contains only private data


update_allocation ()

virtual function used to update the allocation of the ClutterActor using the ClutterConstraint


update_preferred_size ()

virtual function used to update the preferred size of the ClutterActor using the ClutterConstraint; optional, since 1.22


Since: 1.4

See Also