Christian, this chapter has some serious problems. I’m using Xcode 5.1.1 and have testing the code on both my iPad device with iOS 7.1.2 AND with the iPAD simulator targeting iOS 7.0 and 7.1.
First, I tried the experiment suggested at the beginning of the chapter to look for screen flicker when an app is killed. I can’t reproduce what you suggest should be happening so perhaps Apple has changed and doesn’t use the screenshot taken just before an app enters background when it knows that the app is starting fresh from a new launch.
Secondly, after entering all of the suggested code and testing it, it simply does not work on either a real device or the simulator. I even downloaded the solution code from the BNR website and tested it on an iPad and the simulator. Same result … it doesn’t work.
Has somethings changed in iOS that makes this chapter out of date and no longer functional?