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What is GCD in iOS?
GCD basically stands for GRAND Central Dispatch. GCD is a low-level API which allows us for manage multiple concurrent operations. GCD will assist us to increase the responsiveness of our app by deferring COMPUTATIONALLY INTENSIVE tasks to the CONTEXT. It a simpler concurrency model than locks and THREADS to deal with.

Can you write down the FUNCTIONALITY of Accelerometer of an iPhone?
As we all KNOWS the iPhone have feature to respond to MOTION using a built-in accelerometer functionality. The accelerometer automatically detects MOVEMENT and it will changes the display as per the settings. for EXAMPLE when we rotate iPhone from portrait to landscape it will change accordingly.

What is use of Guard statement in iOS?
There are certain conditions when one or more requirements are not going to met. And that case guard statement is used to PASS program control out of that domain. Below are the some of the main advantages of guard statements:-
(1)It will helps in early exit
(2)This will avoids pyramids of doom
(3)We can also SAY its a another way to SAFELY unwrap optionals
Below is the syntax for guard statement:-

guard condition ELSE
{
statements
}

Why DESIGN pattern is very important in iOS development?
It is ONE of the important aspects in iOS development. Design patterns are important in software design because we knows that design patterens are reusable solution to MANY of the common problems. There are some models AVAILABLE that helps in writing CODE in simple way to comprehend and reuse. And one of the most popular design trend in Cocoa are given below:-
(1)Creational:-Singleton
(2)Structural:-Decorator, Adapter, Facade
(3)Behavioral:-Memento, Observer

What is Concurrency and various ways to achieve concurrency in IOS?
Concurrency is one of the fancy WAY to run several tasks at same time. In iOS devices concurrency commonly used when some running in background while doing our user interface sesnitive. For example we downloding some DATA and meanwhile doing some other operations.
Now comes to DIFFERENT ways to achieve concurrency in iOS
(1)Using Operations and OPERATION Queues
(2)Using Grand Central Dispatch(GCD)
(3)using threads

Define various annotations that are available in OBJECTIVE C?
Below are the four annotations available in objective C
(1)Nullable:-Here value can be nil
(2)Null unspecified:-This is the default and bridges to a Swift implicitly-unwrapped optional.
(3)Non-null:-Here value can not be nill and will bridges to regular REFERENCE.
(4)null_resettable:-Here value can not ne nil when we read but we can set it to nil to RESET it.

Define iBeacon in term of iOS
In iOS iBeacon is one of the protocol which is DEVELOPED by APPLE and first it was INTRODUCED at the Apple Worldwide Developers CONFERENCE in year 2013. And iBeacon have feature to enable mobile app to get the signals from beacons in physical world and then it works accordingly. And term iBeacon and Beacon are used offenly interchangeably.
iBeacon is low cost and low energy device which helps to EMITS a bluetooth signal broadcasting a unique ID. This iBeacon technology helps mobile apps to get position on micro-local scale and also deliver hyper contextual content based on the user location.