iphone Interview Questions part-4

31-What is notification in iOS?

The notification mechanism of Cocoa implements one-to-many broadcast of messages based on the Observer pattern. Objects in a program add themselves or other objects to a list of observers of one or more notifications, each of which is identified by a global string (the notification name). The object that wants to notify other objects—the observed object—creates a notification object and posts it to a notification center. The notification center determines the observers of a particular notification and sends the notification to them via a message. The methods invoked by the notification message must conform to a certain single-parameter signature. The parameter of the method is the notification object, which contains the notification name, the observed object, and a dictionary containing any supplemental information.Posting a notification is a synchronous procedure. The posting object doesn’t regain control until the notification center has broadcast the notification to all observers. For asynchronous behavior, you can put the notification in a notification queue; control returns immediately to the posting object and the notification center broadcasts the notification when it reaches the top of the queue.Regular notifications—that is, those broadcast by the notification center—are intraprocess only. If you want to broadcast notifications to other processes, you can use the istributed notification center and its related API.

32-What is the difference between delegates and notifications?

We can use notifications for a variety of reasons. For example, you could broadcast a notification to change how user-interface elements display information based on a certain event elsewhere in the program. Or you could use notifications as a way to ensure that objects in a document save their state before the document window is closed. The general purpose of notifications is to inform other objects of program events so they can respond appropriately.But objects receiving notifications can react only after the event has occurred. This is a significant difference from delegation. The delegate is given a chance to reject or modify the operation proposed by the delegating object. Observing objects, on the other hand, cannot directly affect an impending operation.

33-What is posing in iOS?

Objective-C permits a class to entirely replace another classwithin an application. The replacing class is said to “pose as” the target class. All messages sent to the target class are then instead received by the posing class. There are some restrictions on which classes can pose:

• A class may only pose as one of its direct or indirect superclasses
• The posing class must not define any new instance variables which are absent from the target class (though it may define or override methods).
• No messages must have been sent to the target class prior to the posing.

Posing, similarly to categories, allows globally augmenting existing classes. Posing permits two features absent from categories:

• A posing class can call overridden methods through super, thus incorporating the implementation of the target class.
• A posing class can override methods defined in categories.

34-What is atomic and nonatomic? Which one is safer? Which one is default?

You can use this attribute to specify that accessor methods are not atomic. (There is no keyword to denote atomic.)

nonatomic

Specifies that accessors are nonatomic. By default, accessors are atomic.

Properties are atomic by default so that synthesized accessors provide robust access to properties in a multithreaded environment—that is, the value returned from the getter or set via the setter is always fully retrieved or set regardless of what other threads are executing concurrently.

If you specify strong, copy, or retain and do not specifynonatomic, then in a reference-counted environment, a synthesized get accessor for an object property uses a lock and retains and autoreleases the returned value—the implementation will be similar to the following:

If you specify nonatomic, a synthesized accessor for an object property simply returns the value directly.

35-Where can you test Apple iPhone apps if you don’t have the device?

iOS Simulator can be used to test mobile applications. Xcode tool that comes along with iOS SDK includes Xcode IDE as well as the iOS Simulator. Xcode also includes all required tools and frameworks for building iOS apps. However, it is strongly recommended to test the app on the real device before publishing it.

36-Which JSON framework is supported by iOS?

SBJson framework is supported by iOS. It is a JSON parser and generator for Objective-C. SBJson provides flexible APIs and additional control that makes JSON handling easier.

37-What are the tools required to develop iOS applications?

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

38- Name the framework that is used to construct application’s user interface for iOS.

A. 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.

39-Name the application thread from where UIKit classes should be used?

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.

40- Which API is used to write test scripts that help in exercising the application’s user interface elements?

UI Automation API is used to automate test procedures. Tests scripts are written in JavaScript to the UI Automation API. This in turn simulates user interaction with the application and returns log information to the host computer.