ClutterActor Invariants
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ClutterActor behaviour has invariants that will be kept with the same API and
ABI guarantees as the whole Clutter library.

Sections:

   i. Flags
        a. Public ClutterActor Flags
        b. Private ClutterActor Flags
        c. Private Pick Modes
  ii. Invariants
 iii. State changes
  iv. Responsibilities of a ClutterActor
        a. Adding to a container
        b. Removing from a container
        c. Initial state

 i. Flags
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This section describes the various flags and enumerations used by
ClutterActor.

 a. Public ClutterActor Flags

CLUTTER_ACTOR_REALIZED
        Set by clutter_actor_realize(), unset by clutter_actor_unrealize().
        Means: the actor has memory associated to its paint cycle.

CLUTTER_ACTOR_MAPPED
        Set by clutter_actor_show(), unset by clutter_actor_hide()
        May only be set if CLUTTER_ACTOR_IS_REALIZED (actor).
        Means: the actor has been set as visible

CLUTTER_ACTOR_VISIBLE
        Implies: CLUTTER_ACTOR_IS_REALIZED (actor)
                 &&
                 CLUTTER_ACTOR_IS_MAPPED (actor)

CLUTTER_ACTOR_REACTIVE
        Set and unset by clutter_actor_set_reactive()
        Means: the actor is now reactive to events.

 b. Private ClutterActor flags

CLUTTER_ACTOR_IN_DESTRUCTION
        Set internally by clutter_actor_destroy()

CLUTTER_ACTOR_IS_TOPLEVEL
        Set internally by the initialization of ClutterStage

CLUTTER_ACTOR_IN_REPARENT
        Set internally by clutter_actor_reparent()

CLUTTER_ACTOR_SYNC_MATRICES
        Set internally by ClutterStage implementations
        Means: the size of the stage changed and the viewport must be
        synchronized to the new size

CLUTTER_ACTOR_IN_PAINT:
        Set internally by clutter_actor_paint()

CLUTTER_ACTOR_IN_RELAYOUT
        Set internally by clutter_relayout()

 c. Private Pick Modes

CLUTTER_PICK_NONE
        No pick operation is performed during the paint

CLUTTER_PICK_REACTIVE
        Only reactive actors will be picked

CLUTTER_PICK_ALL
        All visible actors will be picked

 ii. Invariants
-------------------------------------------------------------------------------

This section describes the various constraints and invariants on ClutterActor.

In the following

        A  => B         means   if A is true then B is true
        A <=> B         means   A is true if and only if B is true
                                (equivalent to A => B && A <= B)

1) CLUTTER_ACTOR_IN_DESTRUCTION => !CLUTTER_ACTOR_IS_MAPPED (actor) &&
                                   !CLUTTER_ACTOR_IS_REALIZED (actor)

        clutter_actor_destroy() will cause an actor to be hidden
        and unrealized.

2) CLUTTER_ACTOR_IS_MAPPED (actor) => CLUTTER_ACTOR_IS_REALIZED (actor)

        calling clutter_actor_show() on an unrealized actor will cause
        a realization to happen.

3) if clutter_actor_set_parent (actor, parent):
     CLUTTER_ACTOR_IS_REALIZED (parent) => CLUTTER_ACTOR_IS_REALIZED (actor)

        calling clutter_actor_set_parent() on an actor and a realized
        parent will cause a realization on the actor.

4) if clutter_actor_unparent (actor):
     CLUTTER_ACTOR_IS_MAPPED (actor) <=> CLUTTER_ACTOR_IN_REPARENT

        calling clutter_actor_unparent() on an actor will hide the actor;
        calling clutter_actor_reparent() on an actor will leave the actor
        in the same state.

 iii. State changes
-------------------------------------------------------------------------------

clutter_actor_show:
        1. if !CLUTTER_ACTOR_REALIZED calls clutter_actor_realize
        2. sets CLUTTER_ACTOR_MAPPED

clutter_actor_hide:
        1. sets !CLUTTER_ACTOR_MAPPED

clutter_actor_destroy:
        1. sets CLUTTER_ACTOR_IN_DESTRUCTION

clutter_actor_realize:
        sets CLUTTER_ACTOR_REALIZED

clutter_actor_unrealized:
        1. if CLUTTER_ACTOR_MAPPED calls clutter_actor_hide
        2. sets !CLUTTER_ACTOR_REALIZED

clutter_actor_set_parent:
        1. sets actor->parent
        2. if parent is CLUTTER_ACTOR_REALIZED calls clutter_actor_realize
        3. if actor->show_on_set_parent is TRUE calls clutter_actor_show

clutter_actor_unset_parent:
        1. unsets actor->parent
        2. if !CLUTTER_ACTOR_IN_REPARENT calls clutter_actor_hide

clutter_actor_reparent:
        sets CLUTTER_ACTOR_IN_REPARENT
        equivalent to:
                clutter_actor_unparent
                clutter_actor_set_parent

 iv. Responsibilities of a ClutterActor
-------------------------------------------------------------------------------

 a. Adding to a container

When adding an actor to a container, the container must:

 1. call clutter_actor_set_parent (actor, container)
 2. call clutter_actor_queue_relayout (container)

 b. Removing from a container

When removing an actor from a container, the container must:

 1. call clutter_actor_unparent (actor)
 2. call clutter_actor_queue_relayout (container)

Notes:

* here a container actor is any actor that contains children actors; it
  does not imply the implementation of the ClutterContainer interface.

* clutter_actor_unparent() will hide the actor except in the special case
  when CLUTTER_ACTOR_IN_REPARENT is set.

 c. Initial state

When creating an actor, the initial state is:

 1. !CLUTTER_ACTOR_REALIZED
 2. !CLUTTER_ACTOR_MAPPED

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