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Objective-Zip

Introduction

Objective-Zip is a small Objective-C library that wraps ZLib and MiniZip in an object-oriented friendly way.

What is contained here

The source repository contains full sources for ZLib, MiniZip and Objective-Zip, together with some unit tests. The versions included are:

  • 1.2.8 for ZLib.
  • 1.1 (as of 13/5/2017) for MiniZip.
  • latest version for Objective-Zip.

Please note that ZLib and MiniZip are included here only to provide a complete and self-contained package, but they are copyrighted by their respective authors and redistributed on respect of their software license. Please refer to their websites (linked above) for more informations.

Getting started

Objective-Zip exposes basic functionalities to read and write zip files, encapsulating both ZLib for the compression mechanism and MiniZip for the zip wrapping.

Adding Objective-Zip to your project

The library is distributed via CocoaPods, you can add a dependency in you pod file with the following line:

pod 'objective-zip', '~> 1.0'

You can then access Objective-Zip classes with the following import statement if you plan to use exception handling:

#import "Objective-Zip.h"

Alternatively you can use the following import statement if you plan to use Apple's NSError pattern:

#import "Objective-Zip+NSError.h"

More on error handling at the end of this document.

Main concepts

Objective-Zip is centered on a class called (with a lack of fantasy) OZZipFile. It can be created with the common Objective-C procedure of an alloc followed by an init, specifying in the latter if the zip file is being created, appended or unzipped:

OZZipFile *zipFile= [[OZZipFile alloc] initWithFileName:@"test.zip"
    mode:OZZipFileModeCreate];

Creating and appending are both write-only modalities, while unzipping is a read-only modality. You can not request reading operations on a write-mode zip file, nor request writing operations on a read-mode zip file.

Adding a file to a zip file

The ZipFile class has a couple of methods to add new files to a zip file, one of which keeps the file in clear and the other encrypts it with a password. Both methods return an instance of a OZZipWriteStream class, which will be used solely for the scope of writing the content of the file, and then must be closed:

OZZipWriteStream *stream= [zipFile writeFileInZipWithName:@"abc.txt"
    compressionLevel:OZZipCompressionLevelBest];

[stream writeData:abcData];
[stream finishedWriting];

Adding a file to a zip file using encryption

Objective-Zip supports only traditional PKWare encryption, which is also the format most widely supported by common unzip utilities.

To add a file with encryption, it is necessary to precompute a CRC32 of the file being added. This is needed by traditional PKWare encryption to later verify that the password provided for decryption is correct.

The library includes a handy crc32 method as an NSData category (automatically imported under the umbrella header):

NSData *fileData= // Your file data
uint32_t crc= [fileData crc32];

OZZipWriteStream *stream= [zipFile writeFileInZipWithName:@"abc.txt"
    compressionLevel:OZZipCompressionLevelBest
    password:@"password"
    crc32:crc];

[stream writeData:fileData];
[stream finishedWriting];

Note that passing 0 (or any other non-CRC32 number) as the crc32 argument will make the decryption fail, even if the correct password is specified.

Note also that if your file is too large to be stored in a single NSData, you can still compute the CRC32 progressively by using a loop:

NSFileHandle *fileHandle= // Your file handle

uint32_t crc= 0;
do {

    // Read a chunk of the file in data buffer 
    NSData *data= [fileHandle readDataOfLength:BUFFER_SIZE];
    if ([data length] == 0)
        break;

    crc= [data crc32withInitialCrc32:crc];

} while (YES);

Reading a file from a zip file

The OZZipFile class, when used in unzip mode, must be treated like a cursor: you position the instance on a file at a time, either by step-forwarding or by locating the file by name. Once you are on the correct file, you can obtain an instance of a OZZipReadStream that will let you read the content (and then must be closed).

Since the file may not fit into memory, you can read it block by block using a buffer:

OZZipFile *unzipFile= [[OZZipFile alloc] initWithFileName:@"test.zip"
    mode:OZZipFileModeUnzip];

[unzipFile goToFirstFileInZip];

OZZipReadStream *read= [unzipFile readCurrentFileInZip];
NSMutableData *data= [[NSMutableData alloc] initWithLength:BUFFER_SIZE];

do {

    // Reset buffer length
    [data setLength:BUFFER_SIZE];

    // Read bytes and check for end of file
    int bytesRead= [read readDataWithBuffer:data];
    if (bytesRead <= 0)
        break;

    [data setLength:bytesRead];

    // Do something with data

} while (YES);

[read finishedReading];

Alternatively, if you know in advance the file will fit into memory, you may preallocate a buffer big enough and read the all file at once. In the example below the buffer is preallocated with precisely the uncompressed size of the file:

OZZipFile *unzipFile= [[OZZipFile alloc] initWithFileName:@"test.zip"
    mode:OZZipFileModeUnzip];

[unzipFile goToFirstFileInZip];
OZFileInZipInfo *info= [unzipFile getCurrentFileInZipInfo];

OZZipReadStream *read= [unzipFile readCurrentFileInZip];
NSMutableData *data= [[NSMutableData alloc] initWithLength:info.length];
[read readDataWithBuffer:data];

// Do something with data

[read finishedReading];

Note that the NSMutableData instance that acts as the read buffer must have been set with a length greater than 0: the readDataWithBuffer API will use that length to know how many bytes it can fetch from the zip file.

Listing files in a zip file

When the ZipFile class is used in unzip mode, it can also list the files contained in zip by filling an NSArray with instances of FileInZipInfo class. You can then use its name property to locate the file inside the zip and expand it:

OZZipFile *unzipFile= [[OZZipFile alloc] initWithFileName:@"test.zip"
    mode:OZZipFileModeUnzip];

NSArray *infos= [unzipFile listFileInZipInfos];
for (OZFileInZipInfo *info in infos) {
    NSLog(@"- %@ %@ %llu (%d)", info.name, info.date, info.size, info.level);

    // Locate the file in the zip
    [unzipFile locateFileInZip:info.name];

    // Expand the file in memory
    OZZipReadStream *read= [unzipFile readCurrentFileInZip];
    NSMutableData *data= [[NSMutableData alloc] initWithLength:info.length];
    int bytesRead= [read readDataWithBuffer:data];
    [read finishedReading];
}

Note that the OZFileInZipInfo class provide two sizes:

  • length is the original (uncompressed) file size, while
  • size is the compressed file size.

Closing the zip file

Remember, when you are done, to close your OZZipFile instance to avoid file corruption problems:

[zipFile close];

File/folder hierarchy inide the zip

Please note that inside the zip files there is no representation of a file-folder hierarchy: it is simply embedded in file names (i.e.: a file with a name like "x/y/z/file.txt"). It is up to the program that extracts files to consider these file names as expressing a structure and rebuild it on the file system (and viceversa during creation). Common zippers/unzippers simply follow this rule.

Error handling

Objective-Zip provides two kinds of error handling:

  • standard exception handling;
  • Apple's NSError pattern.

With standard exception handling, Objective-Zip will throw an exception of class OZZipException any time an error occurs (programmer or runtime errors).

To use standard exception handling import Objective-Zip in your project with this statement:

#import "Objective-Zip.h"

With Apple's NSError pattern, Objective-Zip will expect a NSError pointer-to-pointer argument and will fill it with an NSError instance whenever a runtime error occurs. Will revert to throwing an exception (of OZZipException class) in case of programmer errors.

To use Apple's NSError pattern import Objective-Zip in your project with this statement:

#import "Objective-Zip+NSError.h"

Apple's NSError pattern is of course mandatory with Swift programming language, since it does not support exception handling.

Differences in the interface

Note that a few minor differences exist in the standard interface vs. the NSError pattern interface. Specifically:

  • [OZZipFile locateFileInZip:error:] returns a NSInteger in place of a BOOL. Here the special values OZLocateFileResultNotFound and OZLocateFileResultNotFound, respectively 1 and -1, are used in place of YES and NO, since 0 is reserved for the case where an error occurred.
  • [OZZipReadStream readDataWithBuffer:error:] similarly returns a NSInteger in place of a NSUInteger. Here the special value OZReadStreamResultEndOfFile, corresponding to -1, is used for the end-of-file case, since 0 is again reserved for error occurrence.

License

The library is distributed under the New BSD License.

Version history

Version 1.0.5:

  • Improved support for computation of CRC32

Version 1.0.4:

  • Updated to latest version of MiniZip (as of 13/5/2017)
  • Added unit tests for 32/64 cross compatibility
  • Added unit tests for encryption/decryption with password

Version 1.0.3:

  • Fixed some memory leaks in MiniZip (contributed by @SheffieldKevin)
  • Silenced a warning about shifting a negative value in ZLib (contributed by Martin Winter)
  • Fixed throwing of errors so that it is compatible with Swift 3 (contributed by @andyj-at-aspin)
  • Fixed typos and errors in README (contributed by @deni2s)

Version 1.0.2:

  • Fixed interface for locateFileInZip and readDataWithBuffer in NSError version so that they correctly support Swift error handling.

Version 1.0.1:

  • Fixed compatibility bugs with Swift
  • Added Swift unit tests
  • Merged back GETTING STARTED in README

Version 1.0.0:

  • Added official podspec to distribute via CocoaPods.
  • Added API docs.
  • Added nullability annotations.
  • Refactored DIY tests as unit tests.
  • Added targets for static libraries.
  • Added alternative interfaces with NSError pattern in place of exceptions.
  • Added support for legacy 32-bit zip files.
  • Added class prefix "OZ" to make Objective-Zip a good citizen.
  • Fully ARC-ified (removed ARCHelper)
  • Some code clean-up.

Version 0.8.3:

  • Finally used correctly the 64 bit APIs. Thanks to Nathan Moinvaziri for advicing.
  • Updated test code to zip & unzip up to 5 GB.
  • Added tests with unzip & check of zip files create with Mac OS X 10.8 and Windows 7.

Version 0.8.2:

  • Updated ZLib to 1.2.8
  • Updated MiniZip to Nathan Moinvaziri's Version (thanks Sergio for the suggestions)
  • Added test code to zip & unzip up to (slighlty less than) 4 GB: the library is able to create and expand files up to 4,293,387,000 bytes (compressed); use the test with caution, requires 4 GB of free space and around 10 minutes on the iOS simulator

Version 0.8.1:

  • Added support for ARC through Nick Lockwood's ARC Helper

Version 0.8:

  • Updated ZLib to 1.2.7
  • Updated MiniZip to 1.1
  • Added method to get file name from a ZipFile instance

Version 0.7.3:

  • Fixed memory leak in test app

Version 0.7.2:

  • Added variant of writeFileInZipWithName that accepts also a file date
  • Fixed bug with date handling

Version 0.7.1:

  • Fixed a bug in creation of an encrypted zip file

Version 0.7.0:

  • Initial public beta release

Compatibility

Version 1.0.5 has been tested with iOS up to 10.3 and OS X up to 10.12, but should be compatible with earlier versions too, down to iOS 8.0 and OS X 10.7. Le me know of any issues that should arise.