### Class

# Clutter Constraint

#### Description [src]

    abstract class Clutter.Constraint : Clutter.ActorMeta {
      /* No available fields */
    }

Abstract class for constraints on position or size

`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.

## 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` 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 `ClutterActor: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 `ClutterAlignConstraint:factor` on both the X and Y axis
- the `ClutterActor` with `ClutterActor:name` `layerB` is added to the `ClutterStage` with no explicit size
- the `ClutterActor:x` and `ClutterActor: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 `ClutterActor: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().

#### Ancestors

- ClutterActorMeta
- GInitiallyUnowned
- GObject

#### Descendants

- ClutterAlignConstraint
- ClutterBindConstraint
- ClutterPathConstraint
- ClutterSnapConstraint

#### Instance methods

###### clutter_constraint_update_preferred_size

Asks the `constraint` to update the size request of a `ClutterActor`.

##### Methods inherited from ClutterActorMeta (5)

###### clutter_actor_meta_get_actor

Retrieves a pointer to the `ClutterActor` that owns `meta`.

###### clutter_actor_meta_get_enabled

Retrieves whether `meta` is enabled.

###### clutter_actor_meta_get_name

Retrieves the name set using `clutter_actor_meta_set_name()`

###### clutter_actor_meta_set_enabled

Sets whether `meta` should be enabled or not.

###### clutter_actor_meta_set_name

Sets the name of `meta`.

##### Methods inherited from GObject (43)

Please see GObject for a full list of methods.

#### Properties

##### Properties inherited from ClutterActorMeta (3)

###### Clutter.ActorMeta:actor

The `ClutterActor` attached to the `ClutterActorMeta` instance.

###### Clutter.ActorMeta:enabled

Whether or not the `ClutterActorMeta` is enabled.

###### Clutter.ActorMeta:name

The unique name to access the `ClutterActorMeta`.

#### Signals

##### Signals inherited from GObject (1)

###### GObject::notify

The notify signal is emitted on an object when one of its properties has its value set through g_object_set_property(), g_object_set(), et al.

#### Class structure

    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
      );
      void (* _clutter_constraint1) (
    void
      );
      void (* _clutter_constraint2) (
    void
      );
      void (* _clutter_constraint3) (
    void
      );
      void (* _clutter_constraint4) (
    void
      );
      void (* _clutter_constraint5) (
    void
      );
      void (* _clutter_constraint6) (
    void
      );
      void (* _clutter_constraint7) (
    void
      );

    }

The `ClutterConstraintClass` structure contains only private data.

###### Class members

`update_allocation: void (* update_allocation) ( ClutterConstraint* constraint, ClutterActor* actor, ClutterActorBox* allocation )`
  No description available.
`update_preferred_size: void (* update_preferred_size) ( ClutterConstraint* constraint, ClutterActor* actor, ClutterOrientation direction, float for_size, float* minimum_size, float* natural_size )`
  No description available.
`_clutter_constraint1: void (* _clutter_constraint1) ( void )`
  No description available.
`_clutter_constraint2: void (* _clutter_constraint2) ( void )`
  No description available.
`_clutter_constraint3: void (* _clutter_constraint3) ( void )`
  No description available.
`_clutter_constraint4: void (* _clutter_constraint4) ( void )`
  No description available.
`_clutter_constraint5: void (* _clutter_constraint5) ( void )`
  No description available.
`_clutter_constraint6: void (* _clutter_constraint6) ( void )`
  No description available.
`_clutter_constraint7: void (* _clutter_constraint7) ( void )`
  No description available.

#### Virtual methods

###### Clutter.ConstraintClass.update_allocation

No description available.

###### Clutter.ConstraintClass.update_preferred_size

Asks the `constraint` to update the size request of a `ClutterActor`.