Are you one of the many devs who miss Xcode's "Fix and Continue" feature? Apple removed "Fix and Continue" in Xcode 4. This option allowed you to patch your binary with updated code, as you ran and debugged it.
Well, there's good news. Developer John Holdsworth has released Injection for Xcode. It's an OS X application that lets you inject those same code changes into running applications for development and debugging. It works with both OS X and iOS apps, including those running on devices.
