The Readium SDK is an ePub reader SDK of similar scope and capability to Adobe's Reader Mobile SDK (RMSDK). As such, it is designed to implement an ePub3- compliant Reading System, although its initial form will concentrate solely on the core ePub3 specification and fixed-layout metadata, expanding later to cover associated standards such as Page Templates.
The project's aim is to develop a productized, high-performance, cross-platform rendering engine for EPUB 3 content, optimized for use in native applications (mobile/tablet and secondarily desktop systems). Simplistic test applications for Android, iOS, OS X and Windows are part of the SDK, along with unit tests. The SDK is designed to be extensible in various areas, including in support for pluggable encryption and digital signature modules which might be used to implement a form of DRM.
The Readium SDK is at present dual-licensed under both a copyleft license (version 3 of the GNU General Public License) and a commercial-use-enabling Contributor's License. All copyright in the official form of the SDK (that found at http://github.com/readium/readium-sdk/) is assigned to the Readium Foundation. A copy of the GPLv3 can be found in the LICENSE file. A Contributor's License is available from the Readium Foundation directly; see the CONTRIBUTING file for information on how to apply to become an official contributor or sponsor, and to apply for a commercial license.
The SDK consists of three major components, corresponding roughly to Model, View, and Controller. The majority of the ePub3 data handling takes place inside the Model component, which provides parsing and generation of ePub-related data structures. The View component is a browser engine; this engine may be a customized form of WebKit developed as part of this project, but any CSS3-compliant browser engine may be used. The Controller component is a small interface between the data and renderer components; this part is ultimately what Reading System vendors will extend to create their own reading system.
The project is written in cross-platform C++ and JavaScript code, and is designed to be combined with platform-specific glue code and a modern browser rendering engine to instantiate an ePub3-compliant rendering engine within a platform-native Reading System application. The SDK constitutes a capable, high-performance ePub engine, not an overall application (other than per platform test applications).
The general architecture is that the browser engine (headlessly) handles content rendering with some features like pagination being implemented in Readium SDK JavaScript code that has been “injected” into the browser context. The C++ code handles tasks/features that can’t be efficiently performed in JS, such as incrementally fetching (and, as necessary, decrypting/de-obfuscating) resources from an .epub ZIP package. Certain UX affordances are supported within the browser context (e.g. text highlighting) but application-level UX affordances would generally be done in the reading system application via platform-specific code (e.g. Java on Android, Obj-C on iOS). Integration glue is provided in the Readium SDK to facilitate this, and usage illustrated by the test applications (which however are not product-level reading systems). Handling of XML Encryption and Digital Signature files is provided to simplify implementation of DRM or content protection/validation that is compatible with the ePub3 specification.
The Readium SDK is designed to potentially work with multiple browser engines but the #1 priority for initial development is WebKit, including platform-bundled instantiations (e.g. via UIWebView on iOS). #2 priority browser engine will be Trident/IE10 (needed for apps to be able to support screen readers on Windows).