When removing the last Action, Constraint, or Effect, we should also be
clearing the corresponding MetaGroup: code inside ClutterActor relies on
NULL checks, and changing them all to check for NULL && n_items == 0
would not be fun.
We need to remove the transition only if the current repeat is equal to
the number of repeats, and if the transition was marked as remove on
complete. Otherwise, the transition has to remain where it is.
The opacity internal setter will do it for us, and it will take into
consideration any eventual flatten effect applied to the actor.
This unbreaks the actor-offscreen-redirect conformance test.
We were using g_list_foreach() prior to the first Apocalypse, and that
function is resilient against changes to the list while iterating it;
since we are not using a GList any more, we need handle this case
ourselves.
When the easing state has a duration of zero milliseconds we can skip
the entire create_transition() call inside set_width() and set_height(),
to avoid what may be a costly call to get_preferred_*.
If we update a transition that is currently playing, we need to check
the current easing state, and look at the eventual duration, in case
the user wants to cancel the transition.
Instead of checking the duration of the current easing state we should
check if there's a transition in progress, and update it
unconditionally.
If there is no easing state, or the easing state has a duration of zero
milliseconds, then create_transition() should bail out early and set the
requested final state.
This allows us to write:
clutter_actor_save_easing_state (actor);
clutter_actor_set_x (actor, 200);
clutter_actor_restore_easing_state (actor);
[...]
clutter_actor_set_x (actor, 100);
and have the second set_x() update the easing in progress, instead of
being ignored.
https://bugzilla.gnome.org/show_bug.cgi?id=672945
Commit 80626e7584 removed an
IN_DESTRUCTION check from within the add_child_internal() method,
outlining an option for bringing it back. It was too late for the 1.10
cycle to do it, and eventually pick up the pieces, but now that we're
at the beginning of the 1.11 cycle we can restore it, and add checks
elsewhere to balance it.
Should not have been there in the first place: the animatable will be
set either using ClutterTransition API, or when adding the transition
to a ClutterActor.
When adding a transition to a ClutterActor, the actor should hold a
reference on it, and release it only when we remove it. This makes
transitions just like other objects held by ClutterActor.
While you can get a per-transition notification of completion, it can be
convenient to also have a way to notify that all the transitions
involving an actor are complete. A simple signal triggered by the
removal of the last transition fits the bill pretty neatly.
If restore_easing_state() is called on the last easing state on the
stack, clean up the stack, so that we don't leave stale pointers
around to later segfault on.
When setting the easing mode, duration, or delay without having ever
called clutter_actor_save_easing_state(). It's confusing, and not
really nice.
In the future, we'll have a default easing state implicitly created by
the actor itself, but for the time being explicitly opting in is
preferrable.
Yes, it's not really the proper GL name for a linear-on-every-axis of a
texture plus linear-between-mipmap-levels minification filter, but it
has three redeeming qualities as a name:
- LINEAR_MIPMAP_LINEAR sucks, as it introduces GL concepts like
mipmaps in the API naming, while we're trying to avoid that;
- people using GL already know what 'trilinear' means in this context
without going all Khronos on their asses;
- we're using 2D textures anyway, so 'linear on two axes and linear
between mipmap levels' can be effectively approximated to
'trilinear'.
I mean, if even the OpenGL official wiki says:
Unfortunately, what most people think of as "trilinear" is not linear
filtering of a 3D texture, but what in OpenGL terms is GL_LINEAR mag
filter and GL_LINEAR_MIPMAP_LINEAR in the min filter in a 2D texture.
That is, it is bilinear filtering of each appropriate mipmap level,
and doing a third linear filter between the adjacent mipmap levels.
Hence the term "trilinear".
-- http://www.opengl.org/wiki/Texture
then the horse has already been flogged to death, and I don't intend to
be accused of necrophilia and sadism by flogging it some more.
Prior art: every single GL tutorial in the history of ever;
CoreAnimation's scaling filter enumerations.
If people want to start using 1D or 3D textures they they are probably
going to be using Cogl API directly, and that has the GL naming scheme
for minification and magnification filters anyway.
It's a bit late in the game for changing the emission of the paint
signal with actors that use paint nodes - mostly because we have both
implicit paint nodes (background color, content) and explicit paint
nodes (the paint_node virtual).
When we branch for 1.12 we can revert this change.
The ::paint signal is the old way to paint an actor; the paint_node()
virtual function is the new way. It's still not possible to traverse the
whole scene graph and build a render tree of PaintNode instances, but
with this change we simultaneously cut out the ::paint signal emission
from the critical path for actors that are using the new PaintNode-based
API, and we retain backward compatibility in the interim period between
1.10 and 2.0.
ClutterContent is an interface for creating delegate objects that handle
what an actor is going to paint.
Since they are a newly added type, they only hook into the new PaintNode
based API.
The position and size of the content is controlled in part by the
content's own preferred size, and by the ClutterContentGravity
enumeration.
The ::paint-node virtual inside ClutterActor is what we want people to
use when painting their actors.
Right now, it's a new code path, that gets called while painting; the
paint_node() implementation should only paint the actor itself, and not
its children — they will get their own paint_node() called when needed.
Internally, ClutterActor will automatically create a dummy PaintNode and
paint the background color; then control will be handed out to the
implementation on the class. This is required to maintain compatibility
with the old ::paint signal emission.
Once we are able to get rid of the paint (and pick) sequences, we'll
switch to a fully retained render tree.
As it turns out, we do end up recursing inside the ::paint signal
emission - especially inside the conformance test suite.
This thoroughly sucks - and we'll only be able to fix it properly
when we bump API for 2.0.
ClutterActor should be able to hold all transitions, even the ones that
have been explicitly created.
This will allow to add new transitions types in the future, like the
keyframe-based one, or the transition group.
It should be possible to set up the delay of a transition, but since
we start the Transition instance before returning control to the caller,
we cannot use clutter_actor_get_transition() to do it without something
extra-awkward, like:
transition = clutter_actor_get_transition (actor, "width");
clutter_timeline_stop (transition);
clutter_timeline_set_delay (transition, 1000);
clutter_timeline_start (transition);
for each property involved. It's much easier to add a delay to the
easing state of an actor.
Clutter is meant to be, and I quote from the README, a toolkit:
for creating fast, compelling, portable, and dynamic graphical
user interfaces
and yet the default mode of operation for setting an actor's state on
the scene graph (position, size, opacity, rotation, scaling, depth,
etc.) is *not* dynamic. We assume a static UI, and then animate it.
This is the wrong way to design an API for a toolkit meant to be used to
create animated user interfaces. The default mode of operation should be
to implicitly animate every state transition, and only allow skipping
the animation if the user consciously decides to do so — i.e. the design
tenet of the API should be to make The Right Thing™ by default, and make
it really hard (or even impossible) to do The Wrong Thing™.
So we should identify "animatable" properties, i.e. those properties
that should be implicitly animated by ClutterActor, and use the
animation framework we provide to tween the transitions between the
current state and the desired state; the implicit animation should
happen when setting these properties using the public accessors, and not
through some added functionality. For instance, the following:
clutter_actor_set_position (actor, newX, newY);
should not make the actor jump to the (newX, newY) point; it should
tween the actor's position between the current point and the desired
point.
Since we have to maintain backward compatibility with existing
applications, we still need to mark the transitions explicitly, but we
can be smart about it, and treat transition states as a stack that can
be pushed and popped, e.g.:
clutter_actor_save_easing_state (actor);
clutter_actor_set_easing_duration (actor, 500);
clutter_actor_set_position (actor, newX, newY);
clutter_actor_set_opacity (actor, newOpacity);
clutter_actor_restore_easing_state (actor);
And we can even start stacking animations, e.g.:
clutter_actor_save_easing_state (actor);
clutter_actor_set_easing_duration (actor, 500);
clutter_actor_set_position (actor, newX, newY);
clutter_actor_save_easing_state (actor);
clutter_actor_set_easing_duration (actor, 500);
clutter_actor_set_easing_mode (actor, CLUTTER_LINEAR);
clutter_actor_set_opacity (actor, newOpacity);
clutter_actor_set_depth (actor, newDepth);
clutter_actor_restore_easing_state (actor);
clutter_actor_restore_easing_state (actor);
And so on, and so forth.
The implementation takes advantage of the newly added Transition API,
which uses only ClutterTimeline sub-classes and ClutterInterval, to cut
down the amount of signal emissions and memory management of object
instances; as well of using the ClutterAnimatable interface for custom
properties and interpolation of values.
The ::paint, ::queue-redraw, and ::queue-relayout signals should be
marked as no-recurse and no-hooks; these signals are emitted *a lot*
during each frame, and since GLib has a bunch of optimizations for
signals with no closures, we should try and squeeze every single CPU
cycle we can.
In theory, handlers connected to the ::allocation-changed signal may be
able to modify the actor's real allocation and allocation flags,
especially now that we use STATIC_SCOPE; let's avoid this, so that we
don't regret it later.
The ActorBox passed to the ::allocation-changed signal should be
annotated as STATIC_SCOPE, given that it's a pointer to a structure
inside ClutterActorPrivate - hence there's no risk of it actually being
freed from a signal handler. This allows the GSignal machinery to avoid
a costly copy/free for each signal emission.
If the redraw entry is not cleared, queueing a redraw from a signal
handler could reinsert the same object in the stage redraw list,
causing the segfault later (as the object is immediately freed)
https://bugzilla.gnome.org/show_bug.cgi?id=671173
We currently check for the IN_DESTRUCTION flag inside the
add_child_internal() function.
This check disallows calling methods that change the stacking order
within the destruction sequence, by triggering a critical warning first,
and leaving the actor in an undefined state, which then ends up being
caught by an assertion.
The reproducible sequence is:
- actor gets destroyed;
- another actor, linked to the first, will try to change the
stacking order of the first actor;
- changing the stacking order is a composite operation composed
by the following steps:
1. ref() the child;
2. remove_child_internal(), which removes the reference;
3. add_child_internal(), which adds a reference;
- the state of the actor is not changed between (2) and (3), as
it could be an expensive recomputation;
- if (3) bails out, then the actor is in an undefined state, but
still alive;
- the destruction sequence terminates, but the actor is unparented
while its state indicates being parented instead.
- assertion failure.
The obvious fix would be to decompose each set_child_*_sibling() method
into proper remove_child()/add_child(), with state validation; this may
cause excessive work, though, and trigger a cascade of other bugs in
code that assumes that a change in the stacking order is an atomic
operation.
Another potential fix is to just remove this check here, and let code
doing stacking order changes inside the destruction sequence of an actor
continue doing the work.
The third fix is to silently bail out early from every
set_child_*_sibling() and set_child_at_index() method, and avoid doing
work.
I have a preference for the second solution, since it involves the least
amount of work, and the least amount of code duplication.
See bug: https://bugzilla.gnome.org/show_bug.cgi?id=670647
Now that ClutterActor has a default paint volume, subclasses may wish
to retrieve it without chaining up to the parent's implementation of
the get_paint_volume() function.
The get_default_paint_volume() returns a ClutterPaintVolume pointer
to the paint volume as computed by the default implementation of the
get_paint_volume() virtual function; it can only be used immediately,
as it's not guaranteed to survive across multiple frames.
The experimental cogl_pipeline_new() api was recently changed so it
explicitly takes a CoglContext. This updates all calls to
cogl_pipeline_new() in clutter accordingly.
If we have N children and the user passes N (or a number beyond N) to
clutter_actor_insert_child_at_index, we should respond by adding the
child at the end, not silently doing nothing.
This should avoid trying to fix the origin of a paint volume set from
the allocation's origin, and thus breaking everything.
A PaintVolume for an actor is defined to be relative to the actor's
modelview unless specifically modified by internal functions; the origin
of an actor's allocation is, on the other hand, parent-relative.
There are times when we don't want to remove all children and count of
the reference count to drop to 0 to ensure destruction; there are cases,
such as managed environments, where it's preferable to ensure that the
children of an actor get actually destroyed.
A bunch of private symbols have escaped into the SO; let's rectify this
situation by using the '_' private prefix, or making them static as they
should have been.
Some of Cogl's experimental apis have changed so that the buffer apis
now need to be passed a context argument and some drawing apis have been
replaced with cogl_framebuffer_ drawing apis that take explicit
framebuffer and pipeline arguments.
These changes were made as part of Cogl moving towards a more stateless
api that doesn't rely on a global context.
This patch updates Clutter to work with the latest Cogl api and bumps
the required Cogl version to 1.9.5.
Reviewed-by: Emmanuele Bassi <ebassi@linux.intel.com>
Reviewed-by: Neil Roberts <neil@linux.intel.com>
Similar to the clutter_actor_iter_remove(), but it'll call destroy()
instead of remove_child().
We can also reimplement the ::destroy default handler using it, and make
it more compact.
There is a typo in the check for a negative index: the index variable
should be index_, not index - unfortunately, the latter can still be
resolved to index(3), so compiler and linker are perfectly happy.
https://bugzilla.gnome.org/show_bug.cgi?id=669730
ClutterActor stopped requiring to override the map and unmap virtual
functions some time ago.
Now that ClutterActor implements the Container interface, overriding map
and unmap to control the MAPPED state of the children is pretty much
going to be a source of bugs and misunderstandings.
Plus, the ordering of the unmap, destroy, dispose, and finalize calls
should be be documented properly.
The documentation should clarify all that.
When calling clutter_actor_destroy(), ClutterActor calls
update_map_state() on itself to unset the REALIZED and MAPPED states,
prior to running the dispose() implementation.
The default dispose() will call remove_child() (either directly or
through the Container implementation), which will check for the MAPPED
state and then run update_map_state() again. We use the previously set
MAPPED state to decide whether or not the parent should queue for a
relayout/redraw when removing a visible children.
If the MAPPED flag was cleared prior to remove_child(), though, it'll
always be unset by the time we get to remove_child(), and this will
cause missing redraws/relayouts; we were ignoring this prior the
post-First Apocalypse changes because we were doing:
if (was_mapped)
clutter_actor_queue_relayout (parent);
clutter_actor_queue_redraw (parent);
which is obviously wrong. Once I removed that glaring brain damage from
the remove_child() implementation, bugs started appearing — bugs that
were probably the reason why we introduced that brain damage in the
first place, instead of checking the source of those bugs.
The obvious fix is to avoid clearing up the actor's state on destroy()
until we remove the actor from its parent. This also reduces the amount
of work we do, and the code paths that can potentially go wrong.
Since the code dealing with ClutterShader is pretty self-contained, now,
we can safely move it outside of the main ClutterActor source file and
into its own. This will allow us to just drop a bunch of files when
branching for 2.0.
Iterating over children and ancestors of an actor is a relatively common
operation. Currently, you only have one option: start a for() loop, get
the first child of the actor, and advance to the next sibling for the
list of children; or start a for() loop and advance to the parent of the
actor.
These operations can be easily done through the ClutterActor API, but
they all require going through the public API, and performing multiple
type checks on the arguments.
Along with the DOM API, it would be nice to have an ancillary, utility
API that uses an iterator structure to hold the state, and can be
advanced in a loop.
https://bugzilla.gnome.org/show_bug.cgi?id=668669
Now that we reinstated Group to its "former glory", we need to ensure
that applications using the deprecated containers with the new DOM API
in ClutterActor can actually work - or, at least, not break horribly.
This actually means making sure that ClutterStage and ClutterGroup can
cope with the DOM, while retaining their old implementations, as well as
their bizarre idiosyncrasies and their utter, utter brokenness.
Making Group just a proxy to Actor broke some behaviour that application
and toolkit code was relying on. Let's keep Group around to fight
another day.
This commit fixes gnome-shell as far as I can test it.
The usual way to implement a container actor is to override the
allocate() virtual function, chain up, and then allocate the actor's
children.
Clutter now has the ability to delegate layout management to
ClutterLayoutManager directly; in the allocation, this is done by
checking whether the actor has children, and then call
clutter_layout_manager_allocate() from within the default implementation
of the ClutterActor::allocate() vfunc. The same vfunc that everyone, has
been chaining up to.
Whoopsie.
Well, we can check if there's a layout manager, and if it's NULL, we
bail out. Except that there's a default layout manager, and it's the
fixed layout manager, so that classes like Group and Stage work by
default.
Double whoopsie.
The fix for this scenario is a bit nasty; we have to check if the actor
class has overridden the allocate() vfunc or not, before actually
looking at the layout manager. This means that classes that override the
allocate() vfunc are expected to do everything that ClutterActor's
default implementation does - which I think it's a fair requirement to
have.
For newly written code, though, it would probably be best if we just
provided a function that does the right thing by default, and that
you're supposed to be calling from within the allocate() vfunc
implementation, if you ever chose to override it. This new function,
clutter_actor_set_allocation(), should come with a warning the size of
Texas, to avoid people thinking it's a way to override the whole "call
allocate() on each child" mechanism. Plus, it should check if we're
inside an allocation sequence, and bail out if not.
If we want to set a default layout manager, we need to do so inside
init(), as it's not guaranteed that people subclassing Actor and
overriding ::constructed will actually chain up as they should.
The default pick() behaviour does not take into consideration the
children of a ClutterActor because the existing containter actors
usually override pick(), chain up, and then paint their children.
With ClutterActor now a concrete class, though, we need a way to pick
its children without requiring a sub-class; we could simply iterate over
the children inside the default pick() implementation, but this would
lead to double painting, which is not acceptable.
A moderately gross hack is to check if the Actor instance did override
the pick() implementation, and if it is not the case, paint the children
in pick mode.
The hide_all() method is pretty much pointless, as hiding an actor will
automatically prevent its children from being painted. The show_all()
method would only be marginally useful, if actors weren't set to be
visible by default when added to another actor - which was the case when
we introduced show_all() and hide_all().
The concept of "internal child" only meant anything when we had a
separate API for containers and actors. Now that we plugged that
particular hole, we can drop all the hacks we used to have in place
to work around its design limitations.
It can be convenient to be able to set, or get, all the components of an
actor's margin at the same time; since we already have a boxed type for
storing a margin, an accessors pair based on it is not a complicated
addition to the API.
Inside the set_child_[above|below]_sibling() and set_child_at_index() we
should be using the internal API for mutating the children list, instead
of the delegate functions. This ensures that we go through a single,
well-defined code path for all operations on the list of children of
an actor.
We have a replacement in ClutterActor, now.
The old ClutterContainer API needs to be deprecated, and the raise() and
lower() virtual functions need a default implementation, so we can check
for implementations overriding them, by using the diagnostic mode like
we do for add(), remove(), and foreach().
The sort_depth_order() virtual function just doesn't do anything, as it
should have been made ages ago.
The Actor wrappers for the Container methods also need to be deprecated.
ClutterActor provides four methods for changing the paint sequence order
of its children:
raise_top()
raise()
lower()
lower_bottom()
The first and last one being just wrappers around raise() and lower(),
respectively. These methods have various issues: they omit the parent,
preferring to retrieve it from the actor passed as the first argument;
this does not match the new style of API introduced to operate on the
list of children of an actor.
Additionally, the raise() and lower() methods of ClutterActor call into
the Container interface, and are not really aptly named (raise() in
particular collides with the completely unrelated 'raise' keyword in
Python, and usually needs to be wrapped in order to be used at all).
Furthermore, we need public methods that Container can call from its
default implementation, as well as methods to port current Container
implementations.
Finally, since we have insert_child_at_index(), we should also have an
equivalent set_child_at_index() as well.
The internal versions of add_child() and remove_child() currently use
boolean arguments to control things like the ChildMeta instances and
the emissions of signals; using more than one boolean argument is an
indication that you need flags to avoid readability issues, as well as
providing a way to add new behaviours without a combinatorial explosion
of arguments, later on.
I don't feel comfortable with this feature, and its implementation
still has too many rough edges. We can safely punt it for now, and
introduce it at a later point, as it doesn't block existing features
or API.
We need to paint the background color in the default class handler for
two reasons: it's logically appropriate, and we don't want actor
subclasses overriding the ::paint class handler to change behaviour only
because somebody decided to set the background color.
Instead of making ClutterActor implement the basic add/remove/foreach
virtual functions of ClutterContainer, we can simply do that from
within the ClutterContainer implementation.
Given the size and scope of the changes in ClutterActor, we ought to
rewrite the overall description of what an actor is, what it does, and
how are you supposed to use it and subclass it.
This will make things interesting.
We have better replacements in ClutterActor, that do The Right Thing™
instead of deferring control and requiring reimplementation in every
single container actor.
The correct sequence of actions should be remove(old) → insert(new), not
insert(new) → remove(old). We can implement a simple delegate insertion
functions to insert the new child between the previous and next siblings
of the old child.
While we're at it, let's also add a unit test for replace_child().
Providing a default get_paint_volume() that takes into account the
children of an actor was a goal of the whole First Apocalypse; if we
make all the containers rely on it, and yet we return a FALSE value
(meaning: we don't have a valid paint volume) even when we do have it,
then we are going to break the whole machinery, though.
The insert_child_at_index, insert_below and insert_above messed up the
first and last child pointers in various cases. This commit fixes all
the instances of first and last child pointers being stale or set to
NULL.
Instead of requiring every consumer of the ClutterActor API that wishes
to iterate over the children of an actor to use the get_children()
method, we should provide an iteration API directly inside ClutterActor
itself.
Instead of storing the list of children, let's turn Actor inside a
proper node of a tree.
This change adds the following members to the Actor private data
structure:
first_child
last_child
prev_sibling
next_sibling
and removes the "children" GList from it; iteration is performed through
the direct pointers to the siblings and children.
This change makes removal, insertion before a sibling, and insertion
after a sibling constant time operations, but it still retains the
feature of ClutterActor.add_child() to build the list of children while
taking into account the depth set on the newly added child, to allow the
default painter's algorithm we employ inside the paint() implementation
to at least try and cope with the :depth property (albeit in a fairly
naïve way). Changes in the :depth property will not change the paint
sequence any more: this functionality will be restored later.
ClutterTransformInfo is a (private) ancillary data structure that
contains all the decomposed transformation data, i.e. rotation angles
and centers, scale factors and centers, and anchor point. This data
structure is stored in the GData of the actor instance instead of the
actor's private data. This change gives us:
• a smaller, cleaner private data structure;
• no size penalty for untransformed actors;
• static constant storage for the defaults, shared across all
instances;
• cache locality for all the decomposed transformation data,
given that the structure size is smaller.
At the end of the day, the only authoritative piece of information for
actor transformation is the CoglMatrix that we initialize in
apply_transform() from all the decomposed parameters, and that can stay
inside the private data structure of ClutterActor.
There are only two kinds of actors that allow underallocations,
according to the API contract:
• ClutterStage, as it is a viewport and it doesn't have an implicit
minimum size;
• Actors using the CLUTTER_ACTOR_NO_LAYOUT escape hatch, which allows
them to bail out from our layout management policies.
The warning about underallocations should take these two exceptions
under consideration.
ClutterActor now has all the API and capabilities for being a concrete
class:
- layout management, through delegation
- container implementation and API
- background color
This means that a simple scene can be built straight out of actors
without using subclasses except for the Stage.
This is the first step towards the deprecation of most of the Actor
subclasses provided by Clutter.
ClutterActor can do better by default than just giving up immediately.
An actor can check for the clip region, and for its children's paint
volume, for instance.
Just these two should give us a better default implementation for newly
written code.
Each actor should have a background color property, disabled by default.
This property allows us to cover 99% of the use cases for
ClutterRectangle, and brings us one step closer to being able to
instantiate ClutterActor directly.
And make sure that overriding Container and calling
clutter_actor_add_child() will result in the same sequence of operations
as the current set_parent()+queue_relayout()+signal_emit pattern.
Existing containers can continue using:
clutter_actor_set_parent (child, CLUTTER_ACTOR (container));
clutter_actor_queue_relayout (CLUTTER_ACTOR (container));
g_signal_emit_by_name (container, "actor-added", child);
and newly written containers overriding Container.add() can simply call:
clutter_actor_add_child (CLUTTER_ACTOR (container), child);
instead.
We need to queue a relayout when removing a visible child from a visible
parent.
We also need to insert the child at the right position (depending on the
depth) so that newly added actors will be painted on top.
Remove four more floats from ClutterActorPrivate.
The fixed minimum and natural sizes should be stored inside the
ClutterLayoutInfo structure, along with the fixed position.
Add a failsafe against a NULL parent, to avoid a segfault when calling
clutter_actor_allocate() on the Stage.
We also need to deal with floating point values: straight comparison is
not going to cut it.
ClutterActor has various properties controlling the allocation:
- x-align, y-align
- margin-top, margin-bottom, margin-left, margin-right
These properties should adjust the ClutterActorBox passed from the
parent actor to its children when calling clutter_actor_allocate(),
so that the child can just allocate its children at the right origin
with the right available size.
The actor class should be able to hold the margin offsets like it does
for expand and alignment flags.
Instead of filling the private data structure with data, we should be
able to use an ancillary data structure, given that all this data is
optional and might never be set in the first place.
In case no layout manager was set during construction, we fall back to a
FixedLayout. The FixedLayout has the property of making the fixed
positioning and sizing API, as well as the various Constraints, work
out of the box.
