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Add information about helper classes to the documentation

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Jay Berkenbilt 2018-06-21 11:23:28 -04:00
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manual/qpdf-manual.xml
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@ -1751,53 +1751,54 @@ outfile.pdf</option>
</para>
<para>
In general, one should adhere strictly to a specification when
writing but be liberal in reading. This way, the product of our
writing but be liberal in reading. This way, the product of our
software will be accepted by the widest range of other programs,
and we will accept the widest range of input files. This library
and we will accept the widest range of input files. This library
attempts to conform to that philosophy whenever possible but also
aims to provide strict checking for people who want to validate
PDF files. If you don't want to see warnings and are trying to
PDF files. If you don't want to see warnings and are trying to
write something that is tolerant, you can call
<literal>setSuppressWarnings(true)</literal>. If you want to fail
<literal>setSuppressWarnings(true)</literal>. If you want to fail
on the first error, you can call
<literal>setAttemptRecovery(false)</literal>. The default
behavior is to generating warnings for recoverable problems. Note
that recovery will not always produce the desired results even if
it is able to get through the file. Unlike most other PDF files
that produce generic warnings such as &ldquo;This file is
<literal>setAttemptRecovery(false)</literal>. The default behavior
is to generating warnings for recoverable problems. Note that
recovery will not always produce the desired results even if it is
able to get through the file. Unlike most other PDF files that
produce generic warnings such as &ldquo;This file is
damaged,&rdquo;, qpdf generally issues a detailed error message
that would be most useful to a PDF developer. This is by design
as there seems to be a shortage of PDF validation tools out
there. (This was, in fact, one of the major motivations behind
the initial creation of qpdf.)
that would be most useful to a PDF developer. This is by design as
there seems to be a shortage of PDF validation tools out there.
This was, in fact, one of the major motivations behind the initial
creation of qpdf.
</para>
</sect1>
<sect1 id="ref.design-goals">
<title>Design Goals</title>
<para>
The QPDF package includes support for reading and rewriting PDF
files. It aims to hide from the user details involving object
files. It aims to hide from the user details involving object
locations, modified (appended) PDF files, the
directness/indirectness of objects, and stream filters including
encryption. It does not aim to hide knowledge of the object
hierarchy or content stream contents. Put another way, a user of
encryption. It does not aim to hide knowledge of the object
hierarchy or content stream contents. Put another way, a user of
the qpdf library is expected to have knowledge about how PDF files
work, but is not expected to have to keep track of bookkeeping
details such as file positions.
</para>
<para>
A user of the library never has to care whether an object is
direct or indirect. All access to objects deals with this
transparently. All memory management details are also handled by
the library.
direct or indirect, though it is possible to determine whether an
object is direct or not if this information is needed. All access
to objects deals with this transparently. All memory management
details are also handled by the library.
</para>
<para>
The <classname>PointerHolder</classname> object is used internally
by the library to deal with memory management. This is basically
a smart pointer object very similar in spirit to the Boost
library's <classname>shared_ptr</classname> object, but predating
it by several years. This library also makes use of a technique
for giving fine-grained access to methods in one class to other
by the library to deal with memory management. This is basically a
smart pointer object very similar in spirit to C++-11's
<classname>std::shared_ptr</classname> object, but predating it by
several years. This library also makes use of a technique for
giving fine-grained access to methods in one class to other
classes by using public subclasses with friends and only private
members that in turn call private methods of the containing class.
See <classname>QPDFObjectHandle::Factory</classname> as an
@ -1810,29 +1811,20 @@ outfile.pdf</option>
files.
</para>
<para>
<classname>QPDFObject</classname> is the basic PDF Object class.
It is an abstract base class from which are derived classes for
each type of PDF object. Clients do not interact with Objects
directly but instead interact with
<classname>QPDFObjectHandle</classname>.
</para>
<para>
<classname>QPDFObjectHandle</classname> contains
<classname>PointerHolder&lt;QPDFObject&gt;</classname> and
includes accessor methods that are type-safe proxies to the
methods of the derived object classes as well as methods for
querying object types. They can be passed around by value,
copied, stored in containers, etc. with very low overhead.
Instances of <classname>QPDFObjectHandle</classname> always
contain a reference back to the <classname>QPDF</classname> object
from which they were created. A
The primary class for interacting with PDF objects is
<classname>QPDFObjectHandle</classname>. Instances of this class
can be passed around by value, copied, stored in containers, etc.
with very low overhead. Instances of
<classname>QPDFObjectHandle</classname> created by reading from a
file will always contain a reference back to the
<classname>QPDF</classname> object from which they were created. A
<classname>QPDFObjectHandle</classname> may be direct or indirect.
If indirect, the <classname>QPDFObject</classname> the
<classname>PointerHolder</classname> initially points to is a null
pointer. In this case, the first attempt to access the underlying
pointer. In this case, the first attempt to access the underlying
<classname>QPDFObject</classname> will result in the
<classname>QPDFObject</classname> being resolved via a call to the
referenced <classname>QPDF</classname> instance. This makes it
referenced <classname>QPDF</classname> instance. This makes it
essentially impossible to make coding errors in which certain
things will work for some PDF files and not for others based on
which objects are direct and which objects are indirect.
@ -1848,48 +1840,6 @@ outfile.pdf</option>
modified in several ways. See comments in
<filename>QPDFObjectHandle.hh</filename> for details.
</para>
<para>
When the <classname>QPDF</classname> class creates a new object,
it dynamically allocates the appropriate type of
<classname>QPDFObject</classname> and immediately hands the
pointer to an instance of <classname>QPDFObjectHandle</classname>.
The parser reads a token from the current file position. If the
token is a not either a dictionary or array opener, an object is
immediately constructed from the single token and the parser
returns. Otherwise, the parser is invoked recursively in a
special mode in which it accumulates objects until it finds a
balancing closer. During this process, the
&ldquo;<literal>R</literal>&rdquo; keyword is recognized and an
indirect <classname>QPDFObjectHandle</classname> may be
constructed.
</para>
<para>
The <function>QPDF::resolve()</function> method, which is used to
resolve an indirect object, may be invoked from the
<classname>QPDFObjectHandle</classname> class. It first checks a
cache to see whether this object has already been read. If not,
it reads the object from the PDF file and caches it. It the
returns the resulting <classname>QPDFObjectHandle</classname>.
The calling object handle then replaces its
<classname>PointerHolder&lt;QDFObject&gt;</classname> with the one
from the newly returned <classname>QPDFObjectHandle</classname>.
In this way, only a single copy of any direct object need exist
and clients can access objects transparently without knowing
caring whether they are direct or indirect objects. Additionally,
no object is ever read from the file more than once. That means
that only the portions of the PDF file that are actually needed
are ever read from the input file, thus allowing the qpdf package
to take advantage of this important design goal of PDF files.
</para>
<para>
If the requested object is inside of an object stream, the object
stream itself is first read into memory. Then the tokenizer reads
objects from the memory stream based on the offset information
stored in the stream. Those individual objects are cached, after
which the temporary buffer holding the object stream contents are
discarded. In this way, the first time an object in an object
stream is requested, all objects in the stream are cached.
</para>
<para>
An instance of <classname>QPDF</classname> is constructed by using
the class's default constructor. If desired, the
@ -1934,8 +1884,206 @@ outfile.pdf</option>
<para>
There are some convenience routines for very common operations
such as walking the page tree and returning a vector of all page
objects. For full details, please see the header file
<filename>QPDF.hh</filename>.
objects. For full details, please see the header files
<filename>QPDF.hh</filename> and
<filename>QPDFObjectHandle.hh</filename>. There are also some
additional helper classes that provide higher level API functions
for certain document constructions. These are discussed in <xref
linkend="ref.helper-classes"/>.
</para>
</sect1>
<sect1 id="ref.helper-classes">
<title>Helper Classes</title>
<para>
QPDF version 8.1 introduced the concept of helper classes. Helper
classes are intended to contain higher level APIs that allow
developers to work with certain document constructs at an
abstraction level above that of
<classname>QPDFObjectHandle</classname> while staying true to
qpdf's philosophy of not hiding document structure from the
developer. As with qpdf in general, the goal is take away some of
the more tedious bookkeeping aspects of working with PDF files,
not to remove the need for the developer to understand how the PDF
construction in question works. The driving factor behind the
creation of helper classes was to allow the evolution of higher
level interfaces in qpdf without polluting the interfaces of the
main top-level classes <classname>QPDF</classname> and
<classname>QPDFObjectHandle</classname>.
</para>
<para>
There are two kinds of helper classes:
<emphasis>document</emphasis> helpers and
<emphasis>object</emphasis> helpers. Document helpers are
constructed with a reference to a <classname>QPDF</classname>
object and provide methods for working with structures that are at
the document level. Object helpers are constructed with an
instance of a <classname>QPDFObjectHandle</classname> and provide
methods for working with specific types of objects.
</para>
<para>
Examples of document helpers include
<classname>QPDFPageDocumentHelper</classname>, which contains
methods for operating on the document's page trees, such as
enumerating all pages of a document and adding and removing pages;
and <classname>QPDFAcroFormDocumentHelper</classname>, which
contains document-level methods related to interactive forms, such
as enumerating form fields and creating mappings between form
fields and annotations.
</para>
<para>
Examples of object helpers include
<classname>QPDFPageObjectHelper</classname> for performing
operations on pages such as page rotation and some operations on
content streams, <classname>QPDFFormFieldObjectHelper</classname>
for performing operations related to interactive form fields, and
<classname>QPDFAnnotationObjectHelper</classname> for working with
annotations.
</para>
<para>
It is always possible to retrieve the underlying
<classname>QPDF</classname> reference from a document helper and
the underlying <classname>QPDFObjectHandle</classname> reference
from an object helper. Helpers are designed to be helpers, not
wrappers. The intention is that, in general, it is safe to freely
intermix operations that use helpers with operations that use the
underlying objects. Document and object helpers do not attempt to
provide a complete interface for working with the things they are
helping with, nor do they attempt to encapsulate underlying
structures. They just provide a few methods to help with
error-prone, repetitive, or complex tasks. In some cases, a helper
object may cache some information that is expensive to gather. In
such cases, the helper classes are implemented so that their own
methods keep the cache consistent, and the header file will
provide a method to invalidate the cache and a description of what
kinds of operations would make the cache invalid. If in doubt, you
can always discard a helper class and create a new one with the
same underlying objects, which will ensure that you have discarded
any stale information.
</para>
<para>
By Convention, document helpers are called
<classname>QPDFSomethingDocumentHelper</classname> and are derived
from <classname>QPDFDocumentHelper</classname>, and object helpers
are called <classname>QPDFSomethingObjectHelper</classname> and
are derived from <classname>QPDFObjectHelper</classname>. For
details on specific helpers, please see their header files. You
can find them by looking at
<filename>include/qpdf/QPDF*DocumentHelper.hh</filename> and
<filename>include/qpdf/QPDF*ObjectHelper.hh</filename>.
</para>
<para>
In order to avoid creation of circular dependencies, the following
general guidelines are followed with helper classes:
<itemizedlist>
<listitem>
<para>
Core class interfaces do not know about helper classes. For
example, no methods of <classname>QPDF</classname> or
<classname>QPDFObjectHandle</classname> will include helper
classes in their interfaces.
</para>
</listitem>
<listitem>
<para>
Interfaces of object helpers will usually not use document
helpers in their interfaces. This is because it is much more
useful for document helpers to have methods that return object
helpers. Most operations in PDF files start at the document
level and go from there to the object level rather than the
other way around. It can sometimes be useful to map back from
object-level structures to document-level structures. If there
is a desire to do this, it will generally be provided by a
method in the document helper class.
</para>
</listitem>
<listitem>
<para>
Most of the time, object helpers don't know about other object
helpers. However, in some cases, one type of object may be a
container for another type of object, in which case it may make
sense for the outer object to know about the inner object. For
example, there are methods in the
<classname>QPDFPageObjectHelper</classname> that know
<classname>QPDFAnnotationObjectHelper</classname> because
references to annotations are contained in page dictionaries.
</para>
</listitem>
<listitem>
<para>
Any helper or core library class may use helpers in their
implementations.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Prior to qpdf version 8.1, higher level interfaces were added as
&ldquo;convenience functions&rdquo; in either
<classname>QPDF</classname> or
<classname>QPDFObjectHandle</classname>. For compatibility, older
convenience functions for operating with pages will remain in
those classes even as alternatives are provided in helper classes.
Going forward, new higher level interfaces will be provided using
helper classes.
</para>
</sect1>
<sect1 id="ref.implementation-notes">
<title>Implementation Notes</title>
<para>
This section contains a few notes about QPDF's internal
implementation, particularly around what it does when it first
processes a file. This section is a bit of a simplification of
what it actually does, but it could serve as a starting point to
someone trying to understand the implementation. There is nothing
in this section that you need to know to use the qpdf library.
</para>
<para>
<classname>QPDFObject</classname> is the basic PDF Object class.
It is an abstract base class from which are derived classes for
each type of PDF object. Clients do not interact with Objects
directly but instead interact with
<classname>QPDFObjectHandle</classname>.
</para>
<para>
When the <classname>QPDF</classname> class creates a new object,
it dynamically allocates the appropriate type of
<classname>QPDFObject</classname> and immediately hands the
pointer to an instance of <classname>QPDFObjectHandle</classname>.
The parser reads a token from the current file position. If the
token is a not either a dictionary or array opener, an object is
immediately constructed from the single token and the parser
returns. Otherwise, the parser iterates in a special mode in which
it accumulates objects until it finds a balancing closer. During
this process, the &ldquo;<literal>R</literal>&rdquo; keyword is
recognized and an indirect <classname>QPDFObjectHandle</classname>
may be constructed.
</para>
<para>
The <function>QPDF::resolve()</function> method, which is used to
resolve an indirect object, may be invoked from the
<classname>QPDFObjectHandle</classname> class. It first checks a
cache to see whether this object has already been read. If not,
it reads the object from the PDF file and caches it. It the
returns the resulting <classname>QPDFObjectHandle</classname>.
The calling object handle then replaces its
<classname>PointerHolder&lt;QDFObject&gt;</classname> with the one
from the newly returned <classname>QPDFObjectHandle</classname>.
In this way, only a single copy of any direct object need exist
and clients can access objects transparently without knowing
caring whether they are direct or indirect objects. Additionally,
no object is ever read from the file more than once. That means
that only the portions of the PDF file that are actually needed
are ever read from the input file, thus allowing the qpdf package
to take advantage of this important design goal of PDF files.
</para>
<para>
If the requested object is inside of an object stream, the object
stream itself is first read into memory. Then the tokenizer reads
objects from the memory stream based on the offset information
stored in the stream. Those individual objects are cached, after
which the temporary buffer holding the object stream contents are
discarded. In this way, the first time an object in an object
stream is requested, all objects in the stream are cached.
</para>
<para>
The following example should clarify how
@ -1951,12 +2099,11 @@ outfile.pdf</option>
<listitem>
<para>
The <classname>QPDF</classname> class checks the beginning of
<filename>a.pdf</filename> for
<literal>%!PDF-1.[0-9]+</literal>. It then reads the cross
reference table mentioned at the end of the file, ensuring that
it is looking before the last <literal>%%EOF</literal>. After
getting to <literal>trailer</literal> keyword, it invokes the
parser.
<filename>a.pdf</filename> for a PDF header. It then reads the
cross reference table mentioned at the end of the file,
ensuring that it is looking before the last
<literal>%%EOF</literal>. After getting to
<literal>trailer</literal> keyword, it invokes the parser.
</para>
</listitem>
<listitem>