iOS interview question and answers

[Source : Internet]

Q: Does iOS support multitasking? 
    iOS 4 and above supports multi-tasking and allows apps to remain in the background until they are launched again or until they are terminated.  

Q: 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.

Q: How can we achieve singleton pattern in iOS?

The Singleton design pattern ensures a class only has one instance, and provides a global point of access to it. The class keeps track of its sole instance and ensures that no other instance can be created. Singleton classes are appropriate for situations where it makes sense for a single object to provide access to a global resource.

Several Cocoa framework classes are singletons. They include NSFileManager, NSWorkspace, NSApplication, and, in UIKit, UIApplication. A process is limited to one instance of these classes. When a client asks the class for an instance, it gets a shared instance, which is lazily created upon the first request.

Q: What is Delegation in iOS?

Delegation is a design pattern in which one object sends messages to another object—specified as its delegate—to ask for input or to notify it that an event is occurring. Delegation is often used as an alternative to class inheritance to extend the functionality of reusable objects. For example, before a window changes size, it asks its delegate whether the new size is ok. The delegate replies to the window, telling it that the suggested size is acceptable or suggesting a better size. 

Q: What is delegate pattern in iOS?

Delegation is a mechanism by which a host object embeds a weak reference (weak in the sense that it’s a simple pointer reference, unretained) to another object—its delegate—and periodically sends messages to the delegate when it requires its input for a task. The host object is generally an “off-the-shelf” framework object (such as an NSWindow or NSXMLParserobject) that is seeking to accomplish something, but can only do so in a generic fashion. The delegate, which is almost always an instance of a custom class, acts in coordination with the host object, supplying program-specific behavior at certain points in the task. Thus delegation makes it possible to modify or extend the behavior of another object without the need for subclassing.