IPhone Developer Interview Questions & Answers

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iPhone OS runs on iPhone and iPod touch devices.
Hardware devices are managed by iPhone OS and provides the technologies needed for implementing native applications on the phone.
The OS ships with several system applications such as Mail, Safari, Phone, which provide standard services to the user.

It is similar to Mac OS X architecture
It acts as an intermediary between the iPhone and iPod hardware an the appearing applications on the screen
The user created applications never interact directly with the appropriate drivers, which protects the user applications from changes to the hardware.

Mac OS 10.5/10.6 iPhone SDK (Software Development Kit 3.0/4.0).
IPhone SDK consists of:

IDE to develop iPhone Apps is XCode(This tool is inbuilt in iPhone SDK)
Interface Builder This is used to design GUI of Apps(Inbuilt feature of iPhone SDK)
Instruments This is used to check any memory leaks in our apps (Inbuilt in SDK)
Simulator This is used to test our apps before deploying into real device.

View controller objects takes care of the presentation of app's content on the screen. A view controller is used to manage a single view along with the collection of subviews. It makes its views visible by installing them in the app's window.

The app delegate object is created by UIApplicationMain function at app launch time. The app delegate object's main job is to handle state transitions within the app.

To change the content of your app, you use a view controller to change the views displayed in the corresponding window. Remember, window itself is never replaced.

Data model objects are app specific objects and store app's content. Apps can also use document objects to manage some or all of their data model objects.

Document objects are not required but are very useful in grouping data that belongs in a single file or file package.

In such case the event will be dispatched to your app delegate for processing.

App delegate is declared as a subclass of UIResponder by Xcode project templates.

Controller object UIApplication is used without subclassing to manage the application event loop. It coordinates other high-level app behaviors.
It works along with the app delegate object which contains app-level logic.

In case system is running low on memory, the system may purge suspended apps without notice.

Background state.

An app is said to be in suspended state when it is still in memory but is not executing any code.

An app enters background state briefly on its way to being suspended.

An app is said to be in active state when it is running in foreground and is receiving events.

During app launching, the system creates a main thread for the app and calls the app's main function on that main thread. The Xcode project's default main function hands over control to the UIKit framework, which takes care of initializing the app before it is run.

An app can get into InActive state when the user locks the screen or the system prompts the user to respond to some event e.g. SMS message, incoming call etc.

An app will be in InActive state if it is running in the foreground but is currently not receiving events. An app stays in InActive state only briefly as it transitions to a different state.

Before the launch of an app, it is said to be in not running state.
When an app is launched, it moves to the active or background state, after transitioning briefly through the inactive state.

On state transitions can be responded to state changes in an appropriate way by calling corresponding methods on app's delegate object.

For example:
application Did Become Active method can be used to prepare to run as the foreground app.
application Did Enter Background method can be used to execute some code when app is running in the background and may be suspended at any time.

application Will Enter Foreground method can be used to execute some code when your app is moving out of the background application Will Terminate method is called when your app is being terminated.

An app is said to be in 'not running' state when:
it is not launched.
it gets terminated by the system during running.

iOS development requires Intel-based Macintosh computer and iOS SDK.

The UIKit framework is used to develop application's user interface for iOS. UIKit framework provides event handling, drawing model, windows, views, and controls specifically designed for a touch screen interface.

UIKit classes should be used only from an application's main thread. Note: The derived classes of UIResponder and the classes which manipulate application's user interface should be used from application's main thread.