What is Object mapping that helping Jon parsing?

Ans. : Object mapping is convert to JSON to SomeOtherObject, BussinessModel to PersistenceModel, etc or map fields of JSON to SomeOtherObject fields.

There are many third party library available for same purpose.

1. We use mapping using init method without third party library :

struct User {

  let id: Int

  let name: String

  

  init?(dictionary: [String: Any]) {

    guard let id = dictionary["id"] as? Int else { return nil }

    guard let name = dictionary["user_name"] as? String else { return nil }

    

    self.id = id

    self.name = name

  }

}

do {

  if let userDictionary = try JSONSerialization.jsonObject(with: jsonData, options: []) as? [String: Any] {

    let user = User(dictionary: userDictionary)

    //Do something with user

  }

} catch {

  print(error)

}

2. Lyft’s Mapper Library

3. Using codable protocol  (Without third party)

4. SwiftyJson 

In today's world mostly people using 3rd option for  object mapping.

Do you know SMTP? is it use SOAP or REST?

Ans : 

SMTP : SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used in sending and receiving e-mail.

SOAP and REST both allow you to create your own API. API stands for Application Programming Interface. It makes it possible to transfer data from an application to other applications. An API receives requests and sends back responses through internet protocols such as HTTP, SMTP, and others.

It use SOAP web-service. SOAP is protocol and rules are maintained by World wide web consortium(W3C).

Difference between synchronous and asynchronous calls in Objective-C

Ans : 

Synchronous : This call means task will be executed in order.
Asynchronous : This call means task may or may not be executed in order.

When call is called synchronously, then thread that initiated that operation will be wait to current task to be finished.
When call is called asynchronously, then it will not wait.

If we want to do some task without harassing UI, we can do those tasks in background thread. This goal is to keep free main thread, so it continuously respond UI event. So we can dispatch our task in background state asynchronously.

So for do task in background thread, we will divide in 2 parts.

1. GCD - Grand Central Dispatch. By using GCD, you have to grab one of global background queue or create your own background queue.

// one of the global concurrent background queues
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

// or you could create your own serial background queue:
// dispatch_queue_t queue = dispatch_queue_create("com.iosiqa.app.queuename", 0);

2. Dispatch your task to that queue asynchronously

dispatch_async(queue, ^{
    // task that to be done in background and it may be slow
});

The pattern for operation queues is very similar. Create an operation queue and add operations to that queue.

Lets see example :

Asynchronous call with Multithreading :

// Methods gets called in different thread and does not block the current thread.
[NSURLConnection sendAsynchronousRequest:request
                                   queue:queue
                       completionHandler:
    ^(NSURLResponse *response, NSData *data, NSError *error) {
}];

Synchronous call with Multithreading (not so useful):

//Do something
dispatch_sync(queue, ^{
    //Do something else // work in another queue or thread
});

//Do some more task

Difference between thread-safe and non-thread-safe in iOS

Ans : 

Thread-Unsafe -> If any object allow to modify by more than one thread at the same time.  (non-atomic property is thread-unsafe. Comments are welcomed)

Thread-safe -> If any object not allow to modify by more than one thread at the same time.Immutable objects are generally thread-safe. (atomic property attribute type. Comments are  welcomed)

In general, immutable classes like NSArray, let are thread-safe, while their mutable variants like NSMutableArray,var are thread-unsafe.

What is @escaping and @nonescaping in swfit?

Ans : 

Closure : Closure are self contained blocks of functionality that can be passed around and used in code.

In swift 1.x and 2.x, closure parameter is @escaping by default. It means closure can be escape during function body execution. If don't want to escape, we have to mention @nonescaping as parameter.

In swift 3.x and after, closure parameter is @nonescaping by default.

Life cycle of non-escaping closure

Basically, a non-escape closure can only run the contents inside of it’s body, anything outside of it’s closure cannot be used. A non-escape closure tells the complier that the closure you pass in will be executed within the body of that function and nowhere else. When the function ends, the closure will no longer exist in memory. For example, if we needed to extract any values within our closure to be used outside of it, we may not do so. During the earlier days of Swift, closure parameters were escaping by default. Due to better memory management and optimizations, Swift has changed all closures to be non-escaping by default.

var closuresArray: [() -> Void] = []

func doClosures(completion: () -> Void){

completionHandlers.append(completion)

}

//ERROR!!!

passing non-escaping parameter 'completion' to function expecting an @escaping closure

Here is an example of an non-escaping closure. Here we have an empty array of closure and a function that includes a closure. If we were to append the closure in the function to the array of closure, we cannot do so because it is defaulted to non-escape. One great thing about Xcode is that it will that you that you need an escaping closure and can implement it for you.

Escaping Closure : 

Essentially escaping closure is the opposite of non-escaping closure. An escaping closure grants the ability of the closure to outlive the function and can be stored elsewhere. By using escape closure, the closure will have existence in memory until all of it’s content have been executed. To implement escaping closure, all we have to do is put @escaping in front of our closure. If you are unsure whether your closure needs escaping, no worries, as I’ve said before the complier is smart enough to let you know.

There are several ways when we need to implement an escaping closure. One instance is when we use asynchronous execution. When we are dealing with dispatch queue, the queue will hold onto the closure for you, and when the queue is done completing its work, then it will return back to the closure and complete it. Since dispatch queue is outside of the scope, we need to use escaping closure. Another instance is when we need to store our closure to a global variable, property, or any bit of storage that lives on past the function.

Always remember to use weak self while using a closure.