Project site https://github.com/google/google-api-objectivec-client-for-rest
Discussion group http://groups.google.com/group/google-api-objectivec-client
Written by Google, this library is a flexible and efficient Objective-C framework for accessing JSON APIs.
This is the recommended library for accessing JSON-based Google APIs for iOS and Mac OS X applications. The library is compatible with applications built for iOS 7 and later, and Mac OS X 10.9 and later.
To get started with Google APIs and the Objective-C client library, Read the wiki. See BuildingTheLibrary for how to add the library to a Mac or iPhone application project, it covers directly adding sources or using CocoaPods. Study the example applications.
Generated interfaces for Google APIs are in the GeneratedServices folder.
If you have a problem or want a new feature to be included in the library, please join the discussion group. Be sure to include http logs for requests and responses when posting questions. Bugs may also be submitted on the issues list.
Google Data APIs: The much older library for XML-based APIs is still available.
Other useful classes for Mac and iOS developers are available in the Google Toolbox for Mac.