对于一个完善的应用来说，需要在UIApplication的各个代理方法里进行相应的处理,特别是前后台切换时一般需要进行一些特定的操作；而对于不同版本的ios系统，其代理方法居然也存在着一些差异，不由得让人大跌眼镜。
IOS4:
// Pressing the home button
Will resign active.
Did enter background.
// Tapping app icon on Springboard
Will enter foreground.
Did become active.
// Pressing the lock button
Will resign active.
// Unlocking the device
Did become active.
IOS5及以后：
// Pressing the home button
Will resign active.
Did enter background.
// Tapping app icon on Springboard
Will enter foreground.
Did become active.
// Pressing the lock button
Will resign active.
Did enter background.
// Unlocking the device
Will enter foreground.
Did become active.可以看到，ios4的锁屏和解锁不会触发didenterbackground方法和willenterforeground方法；而ios5中的didenterbackground方法通过UIApplicationState state = [[UIApplication sharedApplication] applicationState]可以区分是由按home触发还是按锁屏键触发；锁屏时状态为inactive，home键为background.而在willresignactive方法中，两种行为的state均为active。从个人的使用上来看，通过didenterbackground中的state可以很好的在实现上加以区分以实现相异的行为，而在4x系统下就比较麻烦，只能从流程上打主意，不利于实现。