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[dh-make-perl] / dev / arm / libarchive-zip-perl / libarchive-zip-perl-1.18 / docs / ideas.txt

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+Newsgroups: comp.lang.perl.modules
+Subject: Best form for allowing module extension?
+Reply-To: 
+Followup-To: 
+Keywords: 
+Summary: 
+
+I am writing a module (Archive::Zip) that implements the basic read and
+write functionality for Zip archive files. These files have provisions
+for extensions for specific platforms: each member has an 'extra field'
+that can contain OS-specific (or, indeed, any member-specific) data. The
+overall format of this data is specified (<Header ID>, <count>, <data>),
+but the actual contents depends on the Header ID.
+
+Because I'm only working in a couple of operating environments, and
+because I'm not trying to write a full "unzip" or "PKZIP" replacement, I
+don't really want to try to interpret all of these formats.
+
+From the PKWARE Appnote.txt file:
+
+         The current Header ID mappings defined by PKWARE are:
+
+          0x0007        AV Info
+          0x0009        OS/2
+          0x000a        NTFS 
+          0x000c        VAX/VMS
+          0x000d        Unix
+          0x000f        Patch Descriptor
+
+          Several third party mappings commonly used are:
+
+          0x4b46        FWKCS MD5 (see below)
+          0x07c8        Macintosh
+          0x4341        Acorn/SparkFS 
+          0x4453        Windows NT security descriptor (binary ACL)
+          0x4704        VM/CMS
+          0x470f        MVS
+          0x4c41        OS/2 access control list (text ACL)
+          0x4d49        Info-ZIP VMS (VAX or Alpha)
+          0x5455        extended timestamp
+          0x5855        Info-ZIP Unix (original, also OS/2, NT, etc)
+          0x6542        BeOS/BeBox
+          0x756e        ASi Unix
+          0x7855        Info-ZIP Unix (new)
+          0xfd4a        SMS/QDOS
+
+I want to make it easy for other people to provide this support without
+changing my code.
+
+Note that not all of these extensions have anything to do with file
+permissions, although it may be helpful to provide one or more hooks for
+extracting files:
+
+       * supply OS-specific filename
+       * open file for write (set permissions)
+       * after closing file (to set ownership, timestamps, etc.)
+
+I can provide generic support for these extra fields, so that each
+member can have 0 or more extra fields, each with a type tag and
+uninterpreted data.
+
+I have seen File::Spec and File::Spec::Unix, etc., and don't think that
+this scheme is appropriate, since you could have a zip file that was
+produced on one operating system being extracted by another.
+
+Also, it is possible to have multiple types of extra fields in a single
+zip file.
+
+What I have thought about is this: a user who wants to interpret the
+extended information in the zip members can include the appropriate
+extension modules:
+
+# ==================== in user's code ==================== 
+use Archive::Zip;      # basic functionality
+use Archive::Zip::Unix;        # to interpret Unix file permissions, etc.
+use Archive::Zip::MD5; # to interpret MD5 extended info
+
+my $zip = Archive::Zip->new();
+$zip->read('ZIPFILE.ZIP');
+foreach my $member ($zip->members())
+{
+       foreach my $extraField ($member->extraFields())
+       {
+               print $extraField->info() . "\n";
+       }
+
+       $member->extract();
+}
+# ==================== end user's code ====================
+
+I can make an extensible class for writers of OS-specific modules to
+inherit from:
+
+# ==================== in my code ==================== 
+package Archive::Zip::ExtraField;
+my %Handlers;
+
+# Each subclass must call this with their class name and tag ID.
+sub registerType
+{
+       my ($class, $tag) = @_;
+       $Handlers{ $tag } = $class;
+}
+
+# Overrideable methods
+sub info 
+{
+       my $self = shift;
+       ref($self) . " " . $self->{tag} . " " . $self->{dataLength};
+}
+
+# Provide OS-specific name if any or undef
+sub preferredFileName { undef }
+
+# Returns numeric arg for open() call or undef
+sub openPermissions { undef }
+
+# Hook for doing things after file is extracted
+# Called as: $extraField->afterClosingExtractedFile($fileName)
+sub afterClosingExtractedFile { }
+
+package Archive::Zip::Member;
+
+# return array of extra fields
+sub extraFields() { ... }
+
+sub extract
+{
+       my $self = shift;
+       my ($preferredFileName) = 
+               grep { $_ }
+               (map { $_->preferredFileName() } $self->extraFields());
+       my $fileName = $preferredFileName || $self->fileName();
+       # ... similar things for open permissions ...
+       my $fh = FileHandle->new($fileName, $openPermissions);
+       # ... extract data to fh ...
+       $fh->close();
+       map { $_->afterClosingExtractedFile($fileName) }
+               $self->extraFields();
+}
+# ==================== end my code ====================
+
+
+Does this seem like a good way to go? Any other suggestions?
+
+-- 
+Ned Konz
+currently: Stanwood, WA
+email:     ned@bike-nomad.com
+homepage:  http://www.bike-nomad.com