By Daniel Eran Dilger

Published: 12:00 AM EST (09:00 PM PST)

Apple has been busy all weekend posting updated apps that support iOS 4 and its new features--including multitasking and the soon to be released Retina Display of iPhone 4--to the iTunes App Store.

New iOS 4 savvy titles are tagged with the line "iOS 4.0 tested" in the iPhone App Store app and within iTunes. A search of the web version of iTunes shows over 1,000 titles referencing iOS 4, but the more than 225,000 titles in Apple's App Store library indicate there is lots of work left.

Among the early titles specifically updated to support iOS 4 (and rapidly approved by Apple; many developers are still waiting) is Pandora, which adds Background Audio features that allows the system to continue playing feeds even after the app is dismissed.

The new Loopt adds new support for iOS 4 Background Location with a "live location" feature that updates users' presence information for a set period of time after their last checkin. While using live location, Loopt can send the user a push notification update if one of their Loopt friends happens to show up nearby.

Other popular apps that have recently been updated to support iOS 4 include:

• Dropbox
  
• Evernote
  
• Fandango
  
• FlightTrack (reportedly adds new in-app SMS features and calendar integration)
  
• GoDocs (a GoogleDocs client, adds support for Fast App Switching will shortly add support for "Open In" document handling)
  
• Pastebot
  
• NY Times (adds support for Fast App Switching, was previously broken under iOS)
  
• and Quota.

Pretty simple to update apps for iOS 4

Scott Sykora, the lead developer of TaskerApp, a location based task management tool, has added support for Fast App Switching, Background Location, and the high resolution Retina Display, and is currently waiting for Apple to approve the new update. He described his efforts in a report published by RDM.

"I found the Fast App Switching to be one of the easiest things to get working in iOS4. All it requires is a re-compile against the new libraries and your app will freeze and unfreeze in the background," Sykora noted. Adding support for Background Location updates "was quiet simple," and getting Local Notifications working "was very easy to implement."

Full resolution support for iPhone 4 is "also quite easy if your artwork was created at a higher resolution or using vector artwork," Sykora said. "Basically you just create images with the same filename as the current version with "@2x" added," such as picture@2x.png. "They need to be exactly twice the dimensions but you don't need to change your code at all. The standard bundle image loader automatically checks for a high res version when you're loading the image."

Overall, Sykora said, "I've found the transition to iOS 4 to be very painless. I think a big reason for this is that I develop in Objective C and use the native frameworks whenever I can. Apple has done the work to support developers using their tools but if a developer is using a 3rd party abstraction layer their lives will get quite a bit more complicated. Maybe this is a moot point since most of those have been restricted in the new dev agreement anyway."