AblyDeltaCodec

Cocoa VCDiff decoder.

Uses Xdelta version 3, a C library - forked by Ably - for delta compression using VCDIFF/RFC 3284 streams.

Objective-C example

@import AblyDeltaCodec;

NSError *error;
ARTDeltaCodec *codec = [[ARTDeltaCodec alloc] init];
[codec setBase:baseData withId:@"m1"];
NSData *outputData = [codec applyDelta:deltaData deltaId:@"m2" baseId:@"m1" error:&error];

// Output data is an utf-8 string:
NSString *output = [[NSString alloc] initWithData:outputData encoding:NSUTF8StringEncoding];

Swift example

import AblyDeltaCodec

let codec = ARTDeltaCodec()
codec.setBase(baseData, withId: "m1")

do {
    let outputData = try codec.applyDelta(deltaData, deltaId: "m2", baseId: "m1")
    // Output data is an utf-8 string:
    let output = String(data: outputData, encoding: .utf8)
}
catch {
    print(error)
}

API Reference

• class isDelta
• class applyDelta
• setBase
• applyDelta

Stateless

class isDelta

DECLARATION

Objective-C

+(BOOL)isDelta:delta;

Swift

class func isDelta(_ delta: Data) -> Bool

Arguments:

• delta: NSData/Data (is a binary encoded as vcdiff, as specified in RFC 3284.)

Return Value:

Returns a BOOL/Boolean telling if it’s a valid delta or not.

class applyDelta

DECLARATION

Objective-C

+(NSData *)applyDelta:current previous:previous error:error;

Swift

class func applyDelta(_ current: Data, previous: Data) -> Data (throws)

Arguments:

• current: NSData/Data (is the binary encoding of the information needed to transform the source to the target. It is encoded as vcdiff, as specified in RFC 3284.)
• previous: NSData/Data (is the group of bytes to transform into the target.)
• error: NSError (Objective-C only) (is the error object when something goes wrong. It’s nullable so it’s optional.)

Return Value:

Returns a NSData/Data object of the target. It can return nil.

Stateful

setBase

DECLARATION

Objective-C

-(void)setBase:base withId:baseId;

Swift

func setBase(_ base: Data, withId baseId: Data)

Arguments:

• base: NSData/Data (is the group of bytes to transform into the target. This is probably an old, cached version.)
• baseId: NSString/String (is an identifier of the base.)

Return Value:

Returns nothing.

applyDelta

DECLARATION

Objective-C

-(NSData *)applyDelta:delta deltaId:deltaId baseId:baseId error:error;

Swift

func applyDelta(_ delta: Data, deltaId deltaId: String, baseId baseId: String) -> Data (throws)

Arguments:

• delta: NSData/Data (is the binary encoding of the information needed to transform the source to the target. It is encoded as vcdiff, as specified in RFC 3284.)
• deltaId: NSString/String (is an identifier of the delta.)
• baseId : NSString/String (is an identifier of the base used to verify if it matches with the current assigned base.)
• error: NSError (Objective-C only) (is the error object when something goes wrong. It’s nullable so it’s optional.)

Return Value:

Returns a NSData object of the target. It can return nil.

Acknowledgments:

The delta will be the new base.

We use both Xcodeproj and Package.swift

We currently have both these files, each contain their own build configuration/ settings. This means one build may succeed whereas the other may fail. This is required because Carthage and Cocoapods use Xcodeproj but SPM uses the Package.swift.

Release Process

For each release, the following needs to be done:

• Create a new branch release/x.x.x (where x.x.x is the new version number) from the main branch
• Bump the version numbers in AblyDeltaCodec.podspec and in the Xcode project. Commit this.
• Run github_changelog_generator to automate the update of the CHANGELOG. This may require some manual intervention, both in terms of how the command is run and how the change log file is modified. Your mileage may vary:
  • The command you will need to run will look something like this: github_changelog_generator -u ably -p ably-cocoa --since-tag 1.2.5 --output delta.md
  • Using the command above, --output delta.md writes changes made after --since-tag to a new file
  • The contents of that new file (delta.md) then need to be manually inserted at the top of the CHANGELOG.md, changing the “Unreleased” heading and linking with the current version numbers
  • Also ensure that the “Full Changelog” link points to the new version tag instead of the HEAD
  • Commit this change: git add CHANGELOG.md && git commit -m "Update change log."
• Push both commits to origin: git push -u origin release/x.x.x
• Make a pull request against main and await approval of reviewer(s)
• Once approved and/or any additional commits have been added, merge the PR
• Steps to perform before pushing a release tag up:
  • Build the Swift Package locally: Run swift build or open Package.swift and build the library with Xcode. This ensures this library will build for applications using Swift Package Manafer.
  • Build the Xcode Project: open DeltaCodec.xcodeproj and build the library with Xcode. This ensures the library will build for applications using Carthage.
  • Run pod lib lint to validate the Podspec and ensure the library will build for Cocoapods.
  • Warning: Currently, there are 14 warnings related to the xdelta3 submodule. We run pod lib lint --allow-warnings instead.
  • Test the library integration in projects: SPM (in an Xcode project), Cocoapods (in a podfile) and Carthage (in a cartfile).
• If any fixes are needed (e.g. the lint fails with warnings) then either commit them to main branch now if they are simple warning fixes or perhaps consider raising a new PR if they are complex or likely to need review.
• Create a tag for this version number using git tag x.x.x
• Push the tag using git push origin x.x.x
• Release an update for CocoaPods using pod trunk push AblyDeltaCodec.podspec --allow-warnings. Details on this command, as well as instructions for adding other contributors as maintainers, are at Getting setup with Trunk in the CocoaPods Guides
• Add to releases
  • refer to previous releases for release notes format
• Test the integration of the library in a Xcode project using Carthage and CocoaPods using the installation guide