The layout manager reference should have some documentation on how
to use a LayoutManager object inside a container and how to implement
a LayoutManager sub-class correctly.
The LayoutManager implementation might opt to take a back pointer
to the Container that is using the layout instance; this allows
direct access to the container itself from within the implementation.
Instead of overloading ClutterChildMeta with both container and layout
metadata and delegate to every LayoutManager implementation to keep a
backpointer to the layout manager instance, we can simply subclass
ChildMeta into LayoutMeta and presto! everything works out pretty well
for everyone.
The ChildMeta object is a storage for child-container properties,
that is properties that exist only when an actor is inside a specific
container. The LayoutManager delegate class should also have
layout-specific properties -- so, for this job, we can "recycle"
ChildMeta as the storage.
If a sub-class of LayoutManager wishes to implement a parametrized
layout policy it also needs a way to notify the container using the
layout manager that the layout has changed. We cannot do it directly
and automatically from the LayoutManager because a) it has no back
link to the actor that it is using it and b) it can be attached to
multiple actors.
This is a job for <cue raising dramatic music> signals!
By adding ClutterLayoutManager::layout-changed (and its relative
emitted function) we can notify actors using the layout manager that
the layout parameters have been changed, and thus they should queue
a relayout.
A layout manager instance makes only sense if it's owned by a
container. For this reason, it should have a floating reference
instead of a full reference on construction; this allows constructing
Boxes like:
box = clutter_box_new (clutter_fixed_layout_new ());
without leaking the layout manager instance.
The LayoutManager class is an abstract proxy for the size requesition
and size allocation process in ClutterActor.
A ClutterLayoutManager sub-class must implement get_preferred_width(),
get_preferred_height() and allocate(); a ClutterContainer using the
LayoutManager API will then proxy the corresponding Actor virtual
functions to the LayoutManager instance. This allows having a generic
"blank" ClutterActor sub-class, implementing the ClutterContainer
interface, which leaves only the layout management implementation to
the application developers.
