## ClutterConstraint

ClutterConstraint — Abstract class for constraints on position or size

## Synopsis

                        ClutterConstraint;
    struct              ClutterConstraintClass;

## Object Hierarchy

      GObject
       +----GInitiallyUnowned
             +----ClutterActorMeta
                   +----ClutterConstraint
                         +----ClutterAlignConstraint
                         +----ClutterBindConstraint
                         +----ClutterPathConstraint
                         +----ClutterSnapConstraint

## Description

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 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 | this constraint can be used to align an actor to another one, on either the horizontal or the vertical axis; the ClutterAlignConstraint uses a normalized offset between 0.0 (the top or the left of the source actor, depending on the axis) and 1.0 (the bottom or the right of the source actor, depending on the axis).
ClutterBindConstraint | this constraint binds the X, Y, width or height of an actor to the corresponding position or size of a source actor; it can also apply an offset.
ClutterSnapConstraint | this constraint "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.

Example 2. Usage of constraints

The example below uses various 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.

1. The ClutterRectangle with "name" layerA is explicitly sized to 100 pixels by 25 pixels, and it's added to the ClutterStage;
2. 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.
3. the ClutterRectangle with "name" layerB is added to the ClutterStage with no explicit size;
4. the "x" and "width" of layerB are bound to the same properties of layerA using two ClutterBindConstraint objects, thus keeping layerB aligned to layerA;
5. 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 ClutterSnapConstraints to allow for some padding; since layerB is snapped between two different ClutterActors, its height is stretched to match the gap;
6. the ClutterRectangle 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;

Figure 3. Constraints

    #include <stdlib.h>
    #include <clutter/clutter.h>

    int
    main (int argc, char *argv[])
    {
      ClutterActor *stage, *layer_a, *layer_b, *layer_c;

      if (clutter_init (&argc, &argv) != CLUTTER_INIT_SUCCESS)
        return 1;

      /* the main container */
      stage = clutter_stage_new ();
      clutter_actor_set_name (stage, "stage");
      clutter_stage_set_title (CLUTTER_STAGE (stage), "Snap Constraint");
      clutter_actor_set_background_color (stage, CLUTTER_COLOR_Aluminium1);
      clutter_stage_set_user_resizable (CLUTTER_STAGE (stage), TRUE);
      g_signal_connect (stage, "destroy", G_CALLBACK (clutter_main_quit), NULL);

      /* first layer, with a fixed (100, 25) size */
      layer_a = clutter_actor_new ();
      clutter_actor_set_background_color (layer_a, CLUTTER_COLOR_ScarletRed);
      clutter_actor_set_name (layer_a, "layerA");
      clutter_actor_set_size (layer_a, 100.0, 25.0);
      clutter_actor_add_child (stage, layer_a);

      /* the first layer is anchored to the middle of the stage */
      clutter_actor_add_constraint (layer_a, clutter_align_constraint_new (stage, CLUTTER_ALIGN_BOTH, 0.5));

      /* second layer, with no implicit size */
      layer_b = clutter_actor_new ();
      clutter_actor_set_background_color (layer_b, CLUTTER_COLOR_DarkButter);
      clutter_actor_set_name (layer_b, "layerB");
      clutter_actor_add_child (stage, layer_b);

      /* the second layer tracks the X coordinate and the width of
       * the first layer
       */
      clutter_actor_add_constraint (layer_b, clutter_bind_constraint_new (layer_a, CLUTTER_BIND_X, 0.0));
      clutter_actor_add_constraint (layer_b, clutter_bind_constraint_new (layer_a, CLUTTER_BIND_WIDTH, 0.0));

      /* the second layer is snapped between the bottom edge of
       * the first layer, and the bottom edge of the stage; a
       * spacing of 10 pixels in each direction is added for padding
       */
      clutter_actor_add_constraint (layer_b,
                                    clutter_snap_constraint_new (layer_a,
                                                                 CLUTTER_SNAP_EDGE_TOP,
                                                                 CLUTTER_SNAP_EDGE_BOTTOM,
                                                                 10.0));

      clutter_actor_add_constraint (layer_b,
                                    clutter_snap_constraint_new (stage,
                                                                 CLUTTER_SNAP_EDGE_BOTTOM,
                                                                 CLUTTER_SNAP_EDGE_BOTTOM,
                                                                 -10.0));

      /* the third layer, with no implicit size */
      layer_c = clutter_actor_new ();
      clutter_actor_set_background_color (layer_c, CLUTTER_COLOR_LightChameleon);
      clutter_actor_set_name (layer_c, "layerC");
      clutter_actor_add_child (stage, layer_c);

      /* as for the second layer, the third layer tracks the X
       * coordinate and width of the first layer
       */
      clutter_actor_add_constraint (layer_c, clutter_bind_constraint_new (layer_a, CLUTTER_BIND_X, 0.0));
      clutter_actor_add_constraint (layer_c, clutter_bind_constraint_new (layer_a, CLUTTER_BIND_WIDTH, 0.0));

      /* the third layer is snapped between the top edge of the stage
       * and the top edge of the first layer; again, a spacing of
       * 10 pixels in each direction is added for padding
       */
      clutter_actor_add_constraint (layer_c,
                                    clutter_snap_constraint_new (layer_a,
                                                                 CLUTTER_SNAP_EDGE_BOTTOM,
                                                                 CLUTTER_SNAP_EDGE_TOP,
                                                                 -10.0));
      clutter_actor_add_constraint (layer_c,
                                    clutter_snap_constraint_new (stage,
                                                                 CLUTTER_SNAP_EDGE_TOP,
                                                                 CLUTTER_SNAP_EDGE_TOP,
                                                                 10.0));

      clutter_actor_show (stage);

      clutter_main ();

      return EXIT_SUCCESS;
    }

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.

### Warning

It's 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 ``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 ``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.

### Note

Constraints are queried in the same order as they were applied using `clutter_actor_add_constraint()` or `clutter_actor_add_constraint_with_name()`.

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()`.

ClutterConstraint is available since Clutter 1.4

## Details

### ClutterConstraint

    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 {
    };

The ClutterConstraintClass structure contains only private data

Since 1.4

## See Also

ClutterAction