3. Binding the size of one actor to the size of another
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## 3. Binding the size of one actor to the size of another

### 3.1. Problem

You want one actor (the "target") to automatically change its width or height (or both) when the size of another actor (the "source") changes.

Example use cases:

- Making an actor adjust itself to the size of the stage (particularly when the stage is resizable).

- Putting one actor on top of another and keeping their sizes in sync.

### 3.2. Solution

Create a ClutterBindConstraint bound to the width and/or height of one actor (the "source"). Add that constraint to an actor (the "target") whose size should follow the size of the source.

This short example shows how to create and add a constraint; `source` and `target` can be any two ClutterActors:

    ClutterConstraint *width_constraint;

    /* create a constraint which binds a target actor's width to 100px less than
     * the width of the source actor (use CLUTTER_BIND_HEIGHT to create a
     * constraint based on an actor's height)
     *
     * the third argument is a positive or negative offset from the actor's
     * dimension, in pixels; this is added to the height or width of the source
     * actor before the constraint is applied to the target actor
     */
    width_constraint = clutter_bind_constraint_new (source, CLUTTER_BIND_WIDTH, -100);

    /* add the constraint to an actor */
    clutter_actor_add_constraint (target, width_constraint);

Below is a full example, showing how to incorporate a constraint into a Clutter application.

Example 7.3. Constraining the size of a texture to the size of the stage using ClutterBindConstraint

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

    static const ClutterColor stage_color = { 0x33, 0x33, 0x55, 0xff };

    int
    main (int argc, char *argv[])
    {
      /* the stage is the "source" for constraints on the texture */
      ClutterActor *stage;

      /* the "target" actor which will be bound by the constraints */
      ClutterActor *texture;

      ClutterConstraint *width_binding;
      ClutterConstraint *height_binding;

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

      stage = clutter_stage_new ();
      clutter_actor_set_size (stage, 400, 400);
      clutter_stage_set_color (CLUTTER_STAGE (stage), &stage_color);
      g_signal_connect (stage, "destroy", G_CALLBACK (clutter_main_quit), NULL);

      /* make the stage resizable */
      clutter_stage_set_user_resizable (CLUTTER_STAGE (stage), TRUE);

      texture = clutter_texture_new ();
      clutter_actor_set_opacity (texture, 50);
      clutter_texture_set_repeat (CLUTTER_TEXTURE (texture), TRUE, TRUE);
      clutter_texture_set_from_file (CLUTTER_TEXTURE (texture), "smiley.png", NULL);

      /* the texture's width will be 100px less than the stage's */
      width_binding = clutter_bind_constraint_new (stage, CLUTTER_BIND_WIDTH, -100);

      /* the texture's height will be 100px less than the stage's */
      height_binding = clutter_bind_constraint_new (stage, CLUTTER_BIND_HEIGHT, -100);

      /* add the constraints to the texture */
      clutter_actor_add_constraint (texture, width_binding);
      clutter_actor_add_constraint (texture, height_binding);

      /* add some alignment constraints */
      clutter_actor_add_constraint (texture, clutter_align_constraint_new (stage, CLUTTER_ALIGN_X_AXIS, 0.5));
      clutter_actor_add_constraint (texture, clutter_align_constraint_new (stage, CLUTTER_ALIGN_Y_AXIS, 0.5));

      clutter_container_add_actor (CLUTTER_CONTAINER (stage), texture);

      clutter_actor_show (stage);

      clutter_main ();

      return EXIT_SUCCESS;
    }

The texture in this example is 100px smaller than the stage, leaving a border of visible stage around the texture; and the texture has a tiled image on it. The tiling changes as the texture changes size. Also note that two ClutterAlignConstraints are added to center the actor on the stage.

The result looks like this:

### 3.3. Discussion

Sizing constraints are a good solution in these cases:

- Where you can't use a layout manager. For example, you can't apply a layout manager to the stage directly; so if you want to control the size of an actor based on the size of the stage (as in the example above), constraints are a good substitute for a layout manager .

- Where the layout of a UI is fairly simple (perhaps up to half a dozen actors) and fairly static. An example might be something like a text editor, where the arrangement of the UI (menu bar, toolbar, editing panel, footer) changes infrequently. Of course, it is possible to arrange top-level components using constraints, but still use layout managers inside individual components (e.g. a flow layout manager to manage buttons in the toolbar).

- Where you have an actor whose size can change erratically, but you still want to be able to track its size to control another actor's size. An example might be an application like a drawing program, where a user can create their own actors: you might want the user to be able to describe loose, custom constraints between actors like "keep these actors at the same width", then allow those actors to be moved around and resized in a free-form way as a group. In this situation, a layout manager is too rigid and not appropriate; but adding ClutterConstraints to actors in response to user actions could work well.

  The sample code in the appendix is the kind of thing you might include in a drawing program: you can resize a texture with a key press (`+` to increase size, `-` to decrease), and click on the actor to select/deselect it (a semi-transparent overlay is toggled on the texture). The size of the overlay is bound and aligned to the texture, so that it covers and slightly overlaps the texture regardless of its size.

### Note

You can bind an actor to a single dimension (just height or depth) of another actor: you don't have to bind both height and width. Also, you don't have to bind both dimensions of the target to the same source: for example, you could bind the target's height to one source (actor A) and its width to another source (actor B).

A ClutterBindConstraint can also be used to constrain a target actor's position on the `x` and `y` axes to the position of a source actor. This is covered in another recipe.

#### 3.3.1. Another way to bind actors' sizes together

There is another way to control the size of a target actor, based on the size of a source: you can create a handler for the `allocation-changed` signal of the source, emitted when its size and/or position changes. This signal includes all the data about the source's new allocation (height, width, x and y coordindates), which the handler function can then use to resize the target.

Alternatively, if you're only interested in a change to width or height, you can create a handler for the `notify::width` or `notify::height` signal (respectively), and modify the target's width/height in the handler.

This approach may be useful if you need a type of control over alignment and size which is not possible using constraints alone (e.g. one actor's size should be a proportion of another's). See the code in this section for an example where the size of one actor is dynamically set to 10% more than the size of another.

### Note

This recipe explains more about monitoring changes to an actor's size.

### 3.4. Full examples

Example 7.4. Creating an automatically-resizing overlay for a texture using ClutterBindConstraint

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

    #define STAGE_SIDE 400
    #define RECTANGLE_SIDE STAGE_SIDE * 0.5
    #define TEXTURE_SIZE_MAX STAGE_SIDE * 0.9
    #define TEXTURE_SIZE_MIN STAGE_SIDE * 0.1
    #define TEXTURE_SIZE_STEP 0.2
    #define OVERLAY_OPACITY_OFF 0
    #define OVERLAY_OPACITY_ON 100

    static const ClutterColor stage_color = { 0x33, 0x33, 0x55, 0xff };
    static const ClutterColor overlay_color = { 0xaa, 0x99, 0x00, 0xff };

    /* change the texture size with +/- */
    static gboolean
    key_press_cb (ClutterActor *actor,
                  ClutterEvent *event,
                  gpointer      user_data)
    {
      ClutterActor *texture;
      gfloat texture_width, texture_height;
      guint key_pressed;

      texture = CLUTTER_ACTOR (user_data);
      clutter_actor_get_size (texture, &texture_width, &texture_height);

      key_pressed = clutter_event_get_key_symbol (event);

      if (key_pressed == CLUTTER_KEY_plus)
        {
          texture_width *= 1.0 + TEXTURE_SIZE_STEP;
          texture_height *= 1.0 + TEXTURE_SIZE_STEP;
        }
      else if (key_pressed == CLUTTER_KEY_minus)
        {
          texture_width *= 1.0 - TEXTURE_SIZE_STEP;
          texture_height *= 1.0 - TEXTURE_SIZE_STEP;
        }

      if (texture_width <= TEXTURE_SIZE_MAX && texture_width >= TEXTURE_SIZE_MIN)
        clutter_actor_animate (texture, CLUTTER_EASE_OUT_CUBIC, 500,
                               "width", texture_width,
                               "height", texture_height,
                               NULL);

      return TRUE;
    }

    /* turn overlay opacity on/off */
    static void
    click_cb (ClutterClickAction *action,
              ClutterActor       *actor,
              gpointer            user_data)
    {
      ClutterActor *overlay = CLUTTER_ACTOR (user_data);
      guint8 opacity = clutter_actor_get_opacity (overlay);

      if (opacity < OVERLAY_OPACITY_ON)
        opacity = OVERLAY_OPACITY_ON;
      else
        opacity = OVERLAY_OPACITY_OFF;

      clutter_actor_set_opacity (overlay, opacity);
    }

    int
    main (int argc, char *argv[])
    {
      ClutterActor *stage;
      ClutterActor *texture;
      ClutterActor *overlay;
      ClutterAction *click;
      GError *error = NULL;

      const gchar *filename = "redhand.png";

      if (argc > 1)
        filename = argv[1];

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

      stage = clutter_stage_new ();
      clutter_actor_set_size (stage, STAGE_SIDE, STAGE_SIDE);
      clutter_stage_set_color (CLUTTER_STAGE (stage), &stage_color);
      g_signal_connect (stage, "destroy", G_CALLBACK (clutter_main_quit), NULL);

      texture = clutter_texture_new ();
      clutter_texture_set_keep_aspect_ratio (CLUTTER_TEXTURE (texture), TRUE);
      clutter_actor_set_reactive (texture, TRUE);
      clutter_actor_set_size (texture, RECTANGLE_SIDE, RECTANGLE_SIDE);
      clutter_actor_add_constraint (texture, clutter_align_constraint_new (stage, CLUTTER_ALIGN_X_AXIS, 0.5));
      clutter_actor_add_constraint (texture, clutter_align_constraint_new (stage, CLUTTER_ALIGN_Y_AXIS, 0.5));

      clutter_texture_set_from_file (CLUTTER_TEXTURE (texture),
                                     filename,
                                     &error);

      if (error != NULL)
        {
          g_warning ("Error loading %s\n%s", filename, error->message);
          g_error_free (error);
          exit (EXIT_FAILURE);
        }

      /* overlay is 10px wider and taller than the texture, and centered on it;
       * initially, it is transparent; but it is made semi-opaque when the
       * texture is clicked
       */
      overlay = clutter_rectangle_new_with_color (&overlay_color);
      clutter_actor_set_opacity (overlay, OVERLAY_OPACITY_OFF);
      clutter_actor_add_constraint (overlay, clutter_bind_constraint_new (texture, CLUTTER_BIND_WIDTH, 10));
      clutter_actor_add_constraint (overlay, clutter_bind_constraint_new (texture, CLUTTER_BIND_HEIGHT, 10));
      clutter_actor_add_constraint (overlay, clutter_align_constraint_new (texture, CLUTTER_ALIGN_X_AXIS, 0.5));
      clutter_actor_add_constraint (overlay, clutter_align_constraint_new (texture, CLUTTER_ALIGN_Y_AXIS, 0.5));

      click = clutter_click_action_new ();
      clutter_actor_add_action (texture, click);

      clutter_container_add (CLUTTER_CONTAINER (stage), texture, overlay, NULL);
      clutter_actor_raise_top (overlay);

      g_signal_connect (click, "clicked", G_CALLBACK (click_cb), overlay);

      g_signal_connect (stage,
                        "key-press-event",
                        G_CALLBACK (key_press_cb),
                        texture);

      clutter_actor_show (stage);

      clutter_main ();

      return EXIT_SUCCESS;
    }

Example 7.5. Using the `allocation-changed` signal of one actor to trigger proportional size changes in another

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

    #define OVERLAY_FACTOR 1.1

    static const ClutterColor stage_color = { 0x33, 0x33, 0x55, 0xff };

    void
    allocation_changed_cb (ClutterActor           *actor,
                           const ClutterActorBox  *allocation,
                           ClutterAllocationFlags  flags,
                           gpointer                user_data)
    {
      ClutterActor *overlay = CLUTTER_ACTOR (user_data);

      gfloat width, height, x, y;
      clutter_actor_box_get_size (allocation, &width, &height);
      clutter_actor_box_get_origin (allocation, &x, &y);

      clutter_actor_set_size (overlay,
                              width * OVERLAY_FACTOR,
                              height * OVERLAY_FACTOR);

      clutter_actor_set_position (overlay,
                                  x - ((OVERLAY_FACTOR - 1) * width * 0.5),
                                  y - ((OVERLAY_FACTOR - 1) * width * 0.5));
    }

    int
    main (int argc, char *argv[])
    {
      ClutterActor *stage;
      ClutterActor *actor;
      ClutterActor *overlay;

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

      stage = clutter_stage_new ();
      clutter_actor_set_size (stage, 400, 400);
      clutter_stage_set_color (CLUTTER_STAGE (stage), &stage_color);
      g_signal_connect (stage, "destroy", G_CALLBACK (clutter_main_quit), NULL);

      actor = clutter_actor_new ();
      clutter_actor_set_background_color (actor, CLUTTER_COLOR_Red);
      clutter_actor_set_size (actor, 100, 100);
      clutter_actor_set_position (actor, 150, 150);
      clutter_actor_add_child (stage, actor);

      overlay = clutter_actor_new ();
      clutter_actor_set_background_color (overlay, CLUTTER_COLOR_Blue);
      clutter_actor_set_opacity (overlay, 128);

      g_signal_connect (actor,
                        "allocation-changed",
                        G_CALLBACK (allocation_changed_cb),
                        overlay);

      clutter_actor_add_child (stage, overlay);

      clutter_actor_animate (actor, CLUTTER_LINEAR, 2000,
                             "width", 300.0,
                             "height", 300.0,
                             "x", 50.0,
                             "y", 50.0,
                             NULL);

      clutter_actor_show (stage);

      clutter_main ();

      return EXIT_SUCCESS;
    }

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