/** \page dw-size-request-pos Size requisitions depending on positions

<div style="border: 2px solid #ffff00; margin: 1em 0;
  padding: 0.5em 1em; background-color: #ffffe0">The complex "widget
  sizes" is currently divided into three documents: \ref
  dw-widget-sizes, \ref dw-grows, and **Size requisitions depending on
  positions** (this document). Furthermore, there are some notes in
  \ref dw-miscellaneous.</div>


Motivation
==========

As described in \ref dw-out-of-flow (*The sizeRequest/sizeAllocate
problem*), the principle that the size of a widget depends only on the
sizes of its children causes some problems with floats; the current
solution is a frequent correction by calling
dw::core::Widget::queueResize. In this document, an alternative
approach is presented.

<div style="border: 2px solid #ffff00; margin: 1em 0; padding: 0.5em 1em;
  background-color: #ffffe0">This approach works very well for floats, but not
  for positioned elements, which means that calling
  dw::core::Widget::queueResize is still needed for the latter. On the other
  hand, dw::oof::OOFFloatsMgr (which is much more complex than
  dw::oof::OOFPositionedMgr) can be simplified quite much.</div>


General Idea
============

A widget size may depend on the position relative to an ancestor
widget. If a widget wants to get the size of a child widget, it should:

1. call the new methods dw::core::Widget::numSizeRequestReferences and
   dw::core::Widget::sizeRequestReference, which return all widgets
   relative to which the child's position must be calculated;
2. call dw::core::Widget::sizeRequest with the positions relative to
   these widgets.

<div style="border: 2px solid #ffff00; margin: 1em 0;
  padding: 0.5em 1em; background-color: #ffffe0">It is not sufficient
  to work with *absolute* positions, since there may be an
  interruption passing the positions so that absolute positions are
  often not known.</div>

All positions passed to dw::core::Widget::sizeRequest must constitute
the position at which this child will be allocated.

There are situations where the parent widget is unable to determine
these positions before the size is known. An example: a textblock
widget cannot determine the positions of an inline widget (like an
image, or an inline block) before the line is finished; on the other
hand, finishing the line depends on knowing the sizes of the inline
widgets.

This may result in a conflict, when the size of an inline widget
depends on positions. Generally, the only widget whose size depends on
positions is dw::Textblock (the size will depend on the positions
within its oof container, see \ref dw-out-of-flow), so this conflict
occurs with inline blocks.

This conflict is handled in different ways:

1. Fortunately, this case is irrelevat for floats: an inline block
   constitutes its own floats container, so that there is no dependence
   on a position within another widget.

2. For positioned elements, this case is relevant, since an inline
   block is in most cases not a container for positioned elements. In
   this case, a generator will call the methods
   dw::oof::OutOfFlowMgr::tellIncompletePosition1 and
   dw::oof::OutOfFlowMgr::tellIncompletePosition2, instead of
   dw::oof::OutOfFlowMgr::tellPosition and
   dw::oof::OutOfFlowMgr::tellPosition2, respectively. (Since this
   case is irrelevant for floats,
   dw::oof::OOFFloatsMgr::tellIncompletePosition1 and
   dw::oof::OOFFloatsMgr::tellIncompletePosition2 are not implemented but
   simply abort.)

(This is not (yet) considered for borders: borders are only relevant
for floats, but conflicts do not occur for floats.)


Extremes
--------
Extremes may depend on the position in an analogue way, see:

- dw::core::Widget::numGetExtremesReferences,
- dw::core::Widget::getExtremesReference, and
- dw::core::Widget::getExtremes.

Resizing
--------
Currently, the size of a widget has to be recalculated when:

1. it has called dw::core::Widget::queueResize, or
2. the size of a child widget has to be recalculated.

Since for this new approach, the size does not only depend on the size of the
children, the second condition must be modified. See beginning of
dw::core::Widget::sizeRequest.

An implementation may have to consider, this too, especially when implementing
incremental resizing (see \ref dw-widget-sizes); see
dw::Textblock::sizeRequestImpl as an example.

Regarding changes of the position is not sufficient. Consider this example,
where a float size changes as soon as the image is loaded:

\image html dw-size-request-pos-01.png

The second paragraph ("A longer paragraph" ...) stays at the same position, both
absolute and relative to the float container, but has to be rewrapped because of
the float. Instead, this is handled by dw::oof::OutOfFlowMgr::markSizeChange
(and likewise dw::oof::OutOfFlowMgr::markExtremesChange), which is called by the
implementation of `markSizeChange` (or `markExtremesChange`, respectively) of
the OOF container. (See also the end of the comment of dw::oof::OOFAwareWidget.)


Plan
====

1. General design (dw::core::Widget::sizeRequestReference, changes to
   dw::core::Widget::sizeRequest). Completed.

2. Implementation for dw::Textblock. Completed.

3. Change interface of dw::oof::OutOfFlowMgr (this affects mostly only
   comments). Completed.

   Affects methods dw::oof::OutOfFlowMgr::tellPosition1,
   dw::oof::OutOfFlowMgr::tellPosition2,
   dw::oof::OutOfFlowMgr::getLeftBorder,
   dw::oof::OutOfFlowMgr::getRightBorder,
   dw::oof::OutOfFlowMgr::hasFloatLeft,
   dw::oof::OutOfFlowMgr::hasFloatRight,
   dw::oof::OutOfFlowMgr::getLeftFloatHeight, and
   dw::oof::OutOfFlowMgr::getRightFloatHeight.

4. Apply step 3 to calls within dw::Textblock. Completed.

   <b>Attention:</b> After this step, and before completing the next steps, the
   code is inconsistent and so incorrect.

5. Implement step 3 for floats (affects dw::oof::OOFFloatsMgr). **MOSTLY
   COMPLETED.**

6. Implement step 3 for positioned elements (affects only
   dw::oof::OOFPositionedMgr). **INCOMPLETE.** (But positioned elements are
   currently deactivated.)


Issues
======

- Since the signature of dw::core::Widget::sizeRequestImpl changes quite often
  during the development of *size requisitions depending on positions*, a
  simpler option dw::core::Widget::sizeRequestSimpl has been added. May be
  removed again, after the design is stable.

- As an alternative, passing the references may be done in a new method, which
  is called *before* dw::core::Widget::sizeRequestImpl. This makes even more
  sense, after dw::core::Widget::calcExtraSpace and
  dw::core::Widget::calcExtraSpaceImpl have been extended by references.

- There may be inconsistencies for widget styles; see
  [revision f797436687fe](http://flpsed.org/hgweb/dillo_grows/rev/f797436687fe)
  as an example for a fix. Perhaps a different approach, where breaks are added,
  _if `display` has the value `block`_ (or analogue), will work better.

*/