iOS 面试题与答案

问题 41. What are the three significant benefits of guard statement?

The benefits are:

1. There are no nested if let statements
2. An early exit out of the function using return or using break
3. Guard statement safely unwrap optionals. 

问题 42. What is the difference between 'bundle ID' and 'app ID'?

The bundle ID is specified in Xcode, and it defines each App. A single project can have multiple targets and can output multiple apps. Use: it is branded multiple ways and has both free/lite and full/pro versions.

App ID is used to identify one or more apps from a single development team. It is a two-part string with a period(.) separating Team ID and bundle IF search string. The bundle ID search string is supplied by the developer, while Apple supplies the Team ID.  

问题 43. When an app is launched, what are its state transitions like?

Before launch, an app is not said to be running. After a brief transition through the inactive state, it moves to the background or the active state when it is launched. 

问题 44. List class hierarchy of a UIButton until NSObject.

NSObject -> UIResponder -> UIView -> UIControl -> UIButton. 

问题 45. What is the use of application:willFinishLaunchingWithOptions and application:didFinishLaunchingWithOptions?

Both of these methods are present in AppDelegate.swift file and are used to add functionality to the App when the App is going to be launched.