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https://issues.apache.org/jira/browse/PDFBOX-5263?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17410005#comment-17410005
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Christian Appl edited comment on PDFBOX-5263 at 9/4/21, 5:20 PM:
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[~lehmi] always feel free to complain! :)
This library had much time to grow and is as complex, as the standard itself.
I´d rather know, when my ideas are bad! (possibly for some reason I might not
even know about :D)
The nice thing about the COSBase classes right now is how simple they are and
how little there is, that can be broken. I can understand why you´d like to
keep it that way.
As far as I can see, there are two ways to go:
- Either the objects themselves can keep track of changes, and know about
their context, to organize this.
Which is currently the path I am following. (which btw does not mean I would
deny changing pathes.)
The COSBase instances know about the COSDocument to ask whether the document
is still being parsed, or if COSObjects are currently being dereferenced. As
long as either of those is the case, the COSBase objects must not track
changes, as the document is still being initialized! Hence currently created or
modified Objects are representing the initial state of the document. (Therefore
must not be included in increments.)
- Alternative solution: Some "StateManager/ChangeListener" is keeping track of
changes in the name of the document and is informed by the structures
accordingly. In that case the objects themselves remain "naive" and are instead
handled by some other entity.
To be honest: I dislike classes, that do something with/for other objects, I´d
always prefer the objects to solve problems by themselves. But still - if you´d
prefer this route, I have no issue switching pathes.
Both routes allow solving this problem, both have their own pitfalls,
necessities and issues.
I don´t insist on making that decision, the code implemented up to now, is
(requiring some changes) usable for both routes. (and this still is causing
more issues, than it is solving at the moment - hence rewriting stuff is
something I have to do anyway :D)
You´re welcome and thanks for "complaining"! :D
*Edit:*
Thinking about it... A birds eye view on the document might be preferable after
all.
- The manager/observer would be initialized after the document has been parsed.
Eliminating the necessity to track document states.
- The manager/observer would require a set of methods, that allowed informing
it about different updates of COSBase objects. (Object being dereferenced, at
further monitored objects, from that path.
- The manager/observer would directly know the objects, that are being
monitored (registering itself to new objects as required) - hence it would not
require to inform ancestors of nodes.
- The manager/observer could directly be asked for a full increment set at the
end, without the necessity for further analysing. (building an increment in
realtime, as the user modifies the document.)
- The manager would centralize reactions to different document events, also
allowing for a more centralized modification of it´s reactions.
Feel free to disagree... but possibly I myself would prefer the alternative
route in this case.
was (Author: capsvd):
[~lehmi] always feel free to complain! :)
This library had much time to grow and is as complex, as the standard itself.
I´d rather know, when my ideas are bad! (possibly for some reason I might not
even know about :D)
The nice thing about the COSBase classes right now is how simple they are and
how little there is, that can be broken. I can understand why you´d like to
keep it that way.
As far as I can see, there are two ways to go:
- Either the objects themselves can keep track of changes, and know about
their context, to organize this.
Which is currently the path I am following. (which btw does not mean I would
deny changing pathes.)
The COSBase instances know about the COSDocument to ask whether the document
is still being parsed, or if COSObjects are currently being dereferenced. As
long as either of those is the case, the COSBase objects must not track
changes, as the document is still being initialized! Hence currently created or
modified Objects are representing the initial state of the document. (Therefore
must not be included in increments.)
- Alternative solution: Some "StateManager/ChangeListener" is keeping track of
changes in the name of the document and is informed by the structures
accordingly. In that case the objects themselves remain "naive" and are instead
handled by some other entity.
To be honest: I dislike classes, that do something with/for other objects, I´d
always prefer the objects to solve problems by themselves. But still - if you´d
prefer this route, I have no issue switching pathes.
Both routes allow solving this problem, both have their own pitfalls,
necessities and issues.
I don´t insist on making that decision, the code implemented up to now, is
(requiring some changes) usable for both routes. (and this still is causing
more issues, than it is solving at the moment - hence rewriting stuff is
something I have to do anyway :D)
You´re welcome and thanks for "complaining"! :D
> Suggestion: Signing actual document changes - Enhancing incremental saving
> --------------------------------------------------------------------------
>
> Key: PDFBOX-5263
> URL: https://issues.apache.org/jira/browse/PDFBOX-5263
> Project: PDFBox
> Issue Type: Improvement
> Components: Parsing, PDModel, Writing
> Affects Versions: 3.0.0 PDFBox
> Reporter: Christian Appl
> Priority: Major
> Fix For: 3.0.0 PDFBox
>
> Attachments: Enhanced_incremental_saving_.patch,
> Enhanced_incremental_saving_PDFBox3.patch,
> Prototype__Document_and_reference_holder_aware_COSContext_.patch,
> Updating_context_management_.patch, image-2021-08-23-14-55-24-077.png,
> image-2021-08-26-09-52-33-567.png, image-2021-08-26-09-54-24-897.png,
> image-2021-08-26-10-00-07-383.png, image-2021-08-26-10-02-08-003.png,
> image-2021-08-26-10-03-47-940.png, image-2021-08-26-10-06-42-925.png,
> image-2021-08-26-10-09-12-698.png, image-2021-08-26-10-12-19-265.png
>
>
> *TL;DR:*
> Currently it is rather tedious to create incremental changes in between
> signatures via PDFBox. I attempted to simplify that and wrote a patch.
> This is rather a POC, than an actual suggestion for direct inclusion. (For
> reasons explained later.)
> *Signatures and incremental PDF documents:*
> A typical reason for wanting to sign a document multiple times (creating an
> incremental PDF) is , that in between signatures the document changed and the
> additional signature shall sign the new state of the document.
> If one wanted to implement such incremental changes using PDFBox, he would
> find, that most of the time made changes are completly ignored, when calling
> "saveIncremental".
> As documented for the "saveIncremental" methods and especially the matching
> constructors in "COSWriter", this would require, to identify the "path" of
> all made changes and one would need to set the "needToBeUpdated" flag of all
> elements of that path.
> *But:*
> As documented one would have to have exact understanding of what he did and
> how the PDF standard does implement this, he would have to identify said
> structures and the more complex the changes were, the more tedious this would
> become.
> *Also:*
> Because of the implementation of incremental saving in COSWriter, the whole
> path must be informed that it required an update.
> Resulting in unnecessary large increments, as not all ancestors might
> actually have changed.
> e.g. If one added an image to a preexisting page of the document - the
> contentstream, the resources of the page and the page dictionary would have
> changed. But the "pages" array and all it's ancestors would not have changed
> a bit, but still would have to be informed and included.
> *Assumptions that lead to this patch:*
> - COSWriter should not stop iterating a COSTree just because a parent element
> did not change. It's descendants still could have changed!
> - Externally managing an object´s update state is tedious and error-prone.
> Objects that implement "COSUpdateInfo" should know and manage by themselves
> whether they were freshly created or altered
> (e.g.: A COSDictionary should be able to remember, that a setter had been
> called).
> - If "COSUpdateInfo" objects were self aware and would solve this by
> themselves, it would not be necessary anymore to set update states manually.
> *Problems:*
> The first and obvious problem is, that the initial loading of a document is
> creating and altering new COS structures and we obviously don't want objects
> to observe and remember those changes. An object that is created during
> document initialization must be treated as preexisting.
> However: COSBase is not context aware - it does know it's descendants, but
> neither does it know it's parent, nor does it know it's root.
> If it was, that actually would present the optimal solution, as in that case
> the Object could ask it's root for the current load state and therefore would
> be able to ignore said changes caused by the initial loading of a document.
> But it is not. (My opinion is - it should be! But more on that later.)
> Therefore a a helper named COSUpdateInfoList was implemented, which was
> capable of finding COSUpdateInfo objects in a COS structure, and that allowed
> resetting their update state after loading was completed.
> *Description of the patch:*
> The patch implements selfaware COSUpdateInfo objects, which the COSWriter has
> been adapted to process. PDFBox therefore is capable of monitoring changes in
> realtime and to automatically include altered structures in an incremental
> save of the document, therefore creating increments (or an increment), that a
> signature would sign.
> *Result:*
> Using this patch documents could be created:
> incrementally adding pages, adding contents to pages, adding annotations,
> altering structures, removing structures.
> As far as has been initially tested the resulting documents were valid,
> viewable in a reader and the objects overwritten in increments seemed correct.
> *But -* *Caveat:*
> This patch does introduce atleast one ugly class (most likely you will be
> able to point out more, that could be optimized :)) and that is
> "COSUpdateInfoList" - as already explained: In my opinion such a class should
> not exist, the COSUpdateInfo objects should be context aware and should be
> capable of regulating their own behaviour.
> Whenever the alternatives are to either manage an object externally, or to
> "teach" an object to solve problems autonomously, I will tend to prefer the
> latter... but I did not dare to do that.
> This would require, that either further constructors or setters would have to
> be introduced for COSBase objects, that allowed setting parent/root/context
> for the object.
> Which would result in further massive changes for using applications and
> PDFBox itself - as all instantiations of COSBase objects (PDObjects) would
> have to be adapted.
> However: I would prefer if COSBase objects actually were context aware.
> But as stated... I did not dare to touch it and instead chose the ugly
> workarround, that would introduce yet another iteration over the whole
> COSDocument structure.
> Eliminating COSUpdateInfoList would be preferable!
> *Suggestion:*
> As PDFBox 3 is already changing how documents and objects are handled, I
> would suggest, that also COSBase objects should be made context and selfaware
> in PDFBox 3.
> This would allow simplifying handling COS objects using PDFBox and it would
> allow for an easier and automized handling of incremental saving.
> *Usage example:*
> The following "pseudo code" (actually using simplified Helper classes)
> demonstrates the intended usage:
> !image-2021-08-23-14-55-24-077.png!
> *As always:* Thank you very much for your work and support! I hope this
> suggestion is to your liking.
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