[GSoC] Chained Text Boxes in Draw - Report 29/06/2014

[Matteo Campanelli]

Hi,
I'm trying to implement chained text boxes in Draw, i.e. linking boxes
logically so that text that cannot fit in one can overflow in its successor.

Last week I spent some time to progress on the following subgoals:
- take inspiration from how Writer does it (such a feature is already
present for Writer frames);
- figure out in which part of the code in editeng should be modified to
achieve the same result in Draw.

In general I'm looking at the code to find answers to the following
questions:
(a) how can one observe an overflow of text in a frame/box?
(b) after the point where overflow occurs is found, how to 'assign' the
remaining text to the successor?
(c) what kind of abstractions are used for text (paragraph potions, line
portions) during layouting and how to use them?

Although I found some important references in the code, my findings are
still incomplete and my few small experiments so far have failed.
Below I will try to elaborate more on some related code references I came
to know about.


In Writer
-----------

First, the way Writer carries around information about
predecessor/successor in chains is by means of a derived class of
SfxPoolItem, SwFmtChain
<http://docs.libreoffice.org/sw/html/classSwFmtChain.html>.
In formatting phase, however, this information seems to be accessed
directly from frames themselves ( SwFlyFrm
<http://docs.libreoffice.org/sw/html/classSwFlyFrm.html> ) by the methods
GetNextLink/GetPrevLink.
Then I tried to figure _how_ this information was used from formatting
routines. Breaking in GetNextLink, however, has a very high rate of hit and
has not been helpful in any way (I couldn't get any intuition from the
backtrace). Neither has been reading manually all the code this method is
referenced by (from Writer docs).
So far I assuemd this was the only point of access to that information
(successor), this hypothesis might be false or that code is too complicated
for my eyes at this stage.

In Draw
----------

Although knowing "how Writer does it" would probably be useful, what is
necessary is understanding where to modify formatting code in Draw.
Two crucial methods seems to be:
1) ImpEditEngine::FormatDoc
2) ImpEditEngine::CreateLines (called by FormatDoc for each paragraph)

CreateLines's responsibilities seem to be to restructure the lines in a
paragraph (if they changed) so that they fit in the X boundaries.

FormatDoc's responsibilities seem to include:
- calling CreateLines for each paragraph portion,
- update some view's information if the height of the paragraph changed.

It seems plausible that FormatDoc is the place where an overflow event is
observed (directly or indirectly, by invoked methods) and text should be
assigned to its next link in the chain if such an overflow occurs

This week
-------------

Current goals include understanding FormatDoc's code further trying to
answer questions (a) and (b) above.

Cheers,
Matteo
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