iBooks is an e-book application by Apple Inc. for its iOS and macOS operating systems and devices.
 It was announced in conjunction with the iPad on January 27, 2010, and was released for the iPhone and iPod Touch in mid-2010, as part of the iOS 4 update.
 Initially, iBooks was not pre-loaded onto iOS devices, but users could install it free of charge from the iTunes App Store.
 With the release of iOS 8, it became an integrated app.
 On June 10, 2013, at the Apple Worldwide Developers Conference, Craig Federighi announced that iBooks would also be provided with OS X Mavericks in fall 2013.
The Apple Worldwide Developers Conference (WWDC, also referred to as "dub-dub") is a conference held annually in California by Apple Inc. The event is used by Apple to showcase its new software and technologies for software developers. Attendees can participate in hands-on labs with Apple engineers, and in-depth sessions covering a wide variety of topics.
 WWDC began in 1983 in Monterey, California.
 Until 2007, the number of attendees varied between 2,000 and 4,200; however, during WWDC 2007, Steve Jobs noted that there were more than 5,000 attendees.
 The WWDC events held from 2008 to 2015 were capped, and sold out at 5,000 attendees (5,200 including special attendees).
Files is a file management app developed by Apple Inc. for devices that run iOS 11.
 Discovered as a placeholder title in the App Store just prior to the company's 2017 Worldwide Developers Conference, the app was officially announced at the conference shortly thereafter.
 Files allows users to browse local files stored on their device, as well as files stored in cloud storage services, including iCloud, Dropbox, Google Drive, and OneDrive.
 It allows for the saving, opening and organization of files, including placement into structured folders and sub-folders.
 iPads running iOS 11 will be able to drag-and-drop files between Files and other apps, while iPhone users will be limited to drag-and-drop inside Files itself.
 Further organization can be done through the use of color-coded or custom-named tags, and a persistent search bar allows for finding files inside folders, though not inside other apps.
 A list view enables different sorting options.
 The app offers the exclusive playback of high-quality FLAC audio files, and also offers support for viewing text files, images, "Music Memos", and Zip archives, as well as limited support for video.
iOS is a mobile operating system, developed by Apple Inc. for iPhone, iPad, and iPod Touch.
 Updates for iOS are released through the iTunes software, and, since iOS 5, via over-the-air software updates.
 With the announcement of iOS 5.0 on June 6, 2011, a USB connection to iTunes was no longer needed to activate iOS devices; data synchronization can happen automatically and wirelessly through Apple's iCloud service.
 Major new iOS releases are announced yearly during the Apple Worldwide Developers Conference (WWDC), and are usually released in September of the same year, usually coinciding with the release of new iPhone models.
 The current stable release is iOS 11.0.1.
iOS 5 is the fifth major release of the iOS mobile operating system developed by Apple Inc., being the successor to iOS 4.
 It was announced at the company's Worldwide Developers Conference on June 6, 2011, and was released on October 12, 2011.
 It was succeeded by iOS 6 on September 19, 2012.
The fourth-generation iPad (marketed as iPad with Retina display, colloquially referred to as the iPad 4) is a tablet computer produced and marketed by Apple Inc. Compared to its predecessor, the third-generation iPad, the fourth-generation iPad maintained the Retina Display but featured new and upgraded components such as the Apple A6X chip and the Lightning connector, which was introduced on September 12, 2012.
 It shipped with iOS 6.0, which provides a platform for audio-visual media, including electronic books, periodicals, films, music, computer games, presentations and web content.
 Like the iPad 2 and the third-generation iPad, it has been supported by five major iOS releases, in this case iOS 6, 7, 8, 9, and 10.
 iOS 11, which was released on 19 September 2017, will not have support for the fourth-generation iPad because of hardware limitations.
Google Currents was a social magazine app released by Google on December 8, 2011.
 It was replaced by Google Play Newsstand and is no longer available for download.
 Google Currents was available on Android and iOS devices running iOS 6.
 The application covered a variety of sources and offered a list of featured content.
 Google had partnered with over than 150 publishing partners to offer full-length articles from over 180 editions including CNET, AllThingsD, Forbes, Saveur, PBS, Huffington Post and Fast Company.
 Content is optimized for smartphones and tablets, allowing users to intuitively navigate between words, pictures and video on large as well as small screens.
 The app also worked offline and offered Google+ integration.
iOS 7 is the seventh major release of the iOS mobile operating system developed by Apple Inc., being the successor to iOS 6.
 It was announced at the company's Worldwide Developers Conference on June 10, 2013, and was released on September 18, 2013.
 It was succeeded by iOS 8 on September 17, 2014.
iOS 6 is the sixth major release of the iOS mobile operating system developed by Apple Inc, being the successor to iOS 5.
 It was announced at the company's Worldwide Developers Conference on June 11, 2012, and was released on September 19, 2012.
 It was succeeded by iOS 7 on September 18, 2013.
Apple Wallet (Wallet for short, formerly Passbook) is a mobile app included with the iOS operating system that allows users to store coupons, boarding passes, event tickets, store cards and (starting with iOS 8.1) credit cards, loyalty cards, and debit cards via Apple Pay.
 It was designed by Apple Inc. and was presented at the 2012 Apple Worldwide Developers Conference (WWDC) on June 11, 2012.
 The app first appeared on iOS 6 on September 19, 2012.

Question:  iOS 6 was announced at what conference also referred to as "dub-dub"?
Answer: Apple Worldwide Developers Conference