Will below code be compiled successfully? Explain why?

 Code: 

class MainClass {
    func mainClassFunction() {
        print("mainClassFunction")
    }
}
class SubClass: MainClass {
    func subClassFunction() {
        print("subClassFunction")
    }
}
extension SubClass {
    override func mainClassFunction() {
        print("extension")
    }
}

Answer: Will Not compile.

The provided code will not compile. 

Reason:

In Swift, extensions cannot override methods that are defined in the superclass. Overrides are a feature of subclassing and must be declared within the subclass itself, not in an extension. 

Explanation:

1. The MainClass Definition:

   - The MainClass contains a method mainClassFunction().

2. The SubClass Definition:

   - The SubClass inherits from MainClass and can override mainClassFunction() if needed (but doesn't in this code).

3. The Extension:

   swift

   extension SubClass {

       override func mainClassFunction() {

           print("extension")

       }

   }

   

   Here, the extension attempts to override the mainClassFunction() from MainClass. This results in a compiler error because Swift does not allow method overrides in extensions.

   This restriction exists because extensions are intended to add functionality, not alter or replace existing functionality of the class hierarchy.

Correct Approach:

To override mainClassFunction(), the override must be declared directly within the SubClass:

swift

class SubClass: MainClass {

    override func mainClassFunction() {

        print("extension")

    }

}

Conclusion:

The code will not compile because overriding a method in an extension is not allowed in Swift. Overrides must occur within the subclass itself.

Dependency injection in Swift

 Dependency injection (DI) is a design pattern in software development that promotes the separation of concerns by allowing components to depend on abstractions rather than concrete implementations. In Swift, dependency injection is commonly used to achieve loosely coupled and easily testable code.

There are three main types of dependency injection:

1. Constructor Injection: Dependencies are injected through the class's initializer.

2. Method Injection: Dependencies are injected through methods of the class.

3. Property Injection: Dependencies are injected through properties of the class.

Let's look at an example using constructor injection in Swift:

// Protocol defining the dependency protocol DataService { func fetchData() -> String } // Concrete implementation of the DataService protocol class RemoteDataService: DataService { func fetchData() -> String { return "Data from remote service" } } // Class that depends on DataService through constructor injection class DataManager { let dataService: DataService init(dataService: DataService) { self.dataService = dataService } func processData() -> String { let data = dataService.fetchData() return "Processed: \(data)" } } // Example of using the DataManager with dependency injection let remoteDataService = RemoteDataService() let dataManager = DataManager(dataService: remoteDataService) let result = dataManager.processData() print(result) // Output: Processed: Data from remote service

In this example:

• DataService is a protocol that defines the contract for fetching data.
• RemoteDataService is a concrete implementation of DataService.
• DataManager is a class that depends on DataService through its initializer.

This setup allows you to easily switch the implementation of DataService without modifying DataManager. For example, you could create a LocalDataService implementing DataService and use it with DataManager without changing the DataManager class.

class LocalDataService: DataService { func fetchData() -> String { return "Data from local storage" } } let localDataService = LocalDataService() let dataManagerWithLocalData = DataManager(dataService: localDataService) let resultLocal = dataManagerWithLocalData.processData() print(resultLocal) // Output: Processed: Data from local storage

This flexibility is especially useful for testing, as you can easily substitute real implementations with mock implementations for testing purposes.AI.

Difference between normal function and function ending throw

Function ending with throw can throw error but normal function can not throw error it can just return value.

Lets take example :

1. Custom error enum

enum ErrorsToThrow: Error {

    case fileNotFound

    case fileNotReadable

    case fileSizeIsTooHigh

}

2. Make function which can throw error

func readFiles(path:String) throws  ->String {

        if path == "" {

            throw ErrorsToThrow.fileNotFound

        }

        return "Data from file"

    }

3. Calling readFiles function

   do {

            let dataFromString = try? readFiles(path: "")

            print(dataFromString)

        } catch ErrorsToThrow.fileNotFound {

            print("error generated1")

        } catch ErrorsToThrow.fileNotReadable {

            print("error generated2")

        } catch ErrorsToThrow.fileSizeIsTooHigh {

            print("error generated3")

        } catch {

                print("error")

        }

Now, let's analysis.

1. If in #2, function is not ending with throws can not throw error. But our function can throw error.
2. In #3, we have used try?, so if readFiles function return nil instead of throwing error. So in our case print(dataFromString) statement executed and it will nil means printing nil.

3. If try! is written and if function throw error, then fatal error will be occured and program will be crashed as we ensure that error will not occur by putting ! .
4. So if we want to execute catch statement then we have to use only try . try always used with do-catch.
5. try? and try! does not need do-catch block.

Write a program to define closure which accepts string and return integer.

 Write a closure that accepts string and return length of string(which is integer) :

let simpleClosure:(String) -> (Int) = { name in 

    return name.count

}

let result = simpleClosure("Hello World")

print(result)

What is trailing closure?

 If the last parameter to a function is a closure, Swift lets you use special syntax called trailing closure syntax. Rather than pass in your closure as a parameter, you pass it directly after the function inside braces.

To demonstrate this, here’s our travel() function again. It accepts an action closure so that it can be run between two print() calls:

func travel(action: () -> Void) {

    print("I'm getting ready to go.")

    action()

    print("I arrived!")

}

Because its last parameter is a closure, we can call travel() using trailing closure syntax like this:

travel() {

    print("I'm driving in my car")

}

In fact, because there aren’t any other parameters, we can eliminate the parentheses entirely:

travel {

    print("I'm driving in my car")

}

Trailing closure syntax is extremely common in Swift, so it’s worth getting used to.

Difference between method and function

Methods belong to classes, structs, and enums, whereas functions do not.

Methods always belong to a data type, they have a concept of self that functions do not. This is a special value passed in by Swift, and it refers to whatever instance the method was called on.

Swift uses the same keyword, func, for both functions and methods.

Difference between type method and Instance method?

Type Method: We can call the method using Struct, Class, or Enum name. The method can be static or Class for making such methods. Static method can not be override but class method can be override.

Instance Method: We can call normal method using making instance of strcut or class. This methods are called instance method.

Difference between Swift and Objective C

 

	SWIFT	OBJECTIVE C
	Swift is a general-purpose, high-level programming language that is highly concerned about safety, and performance.	Objective C is a general-purpose language that is considered a superset of C language it was designed with the aim of providing object-oriented capabilities.
	It was developed by Chris Lattner with eventual collaboration with other programmers at Apple.	It was developed by Brad Cox and Tom Love at their company Stepstone.
	It was influenced by Objective C, Rust, Ruby, and Python.	It was influenced by C and Smalltalk.
	Swift first appeared on the year 2014.	Objective C first appeared on the year 1984.
	Swift is a static type.	Objective C is dynamic type.
	Swift is apache licensed open-source project.	Objective C is licensed under General Public License.
	It only has classes.	It has both Structs and classes.
	It was designed for building apps for iOS, Mac, Apple TV, and Apple Watch.	Objective C was designed to be Smalltalk messaging features.
	Swift polymorphism does not exist directly.	Polymorphism in Objective C exists directly in compile time.
	It uses true and false values.	It uses YES and NO values and also BOOl.
	Swift has multiple types of templates than Objective C.	Objective C lacks templates than Swift.

What is KVO?

Key-value observing is the ability for Swift to attach code to variables, so that whenever the variable is changed the code runs. It’s similar to property observers (willSet and didSet ), except KVO is for adding observers outside of the type definition.

KVO isn’t terribly nice in pure Swift code, because it relies on the Objective-C runtime – you need to use @objc classes that inherit from NSObject, then mark each of your properties with @objc dynamic.

For example, we could create a Car class like this:

@objc class Car: NSObject {

    @objc dynamic var name = "BMW"

}

let bmw= Car()

You could then observe that user’s name changing like this:

bmw.observe(\Car.name, options: .new) { car, change in

    print("I'm now called \(car.name)")

}

That asks BMW to watch for new values coming in, then prints the person’s name as soon as the new value is set.

To try it out, just change the car's name to something else:

bmw.name = "Mercedese"

That will print “I’m now called Mercedese.”

Although KVO is unpleasant in pure Swift code, it’s better when working with Apple’s own APIs – they are all automatically both @objc and dynamic because they are written in Objective-C.

However, one warning: even though large parts of UIKit might work with KVO, this is a coincidence rather than a promise – Apple makes no guarantees about UIKit remaining KVO-compatible in the future.

Difference between compact map and Flat map

Compact Map :

Use this method to receive an array of nonoptional values when your transformation produces an optional value.

let scores = ["1", "2", "three", "four", "5"]

let mapped: [Int?] = scores.map { str in Int(str) }
// [1, 2, nil, nil, 5] - Two nil values as "three" and "four" are strings.

let compactMapped: [Int] = scores.compactMap { str in Int(str) } 

// [1, 2, 5] - The nil values for "three" and "four" are filtered out. 

Flat Map :

Use this method to receive a single-level collection when your transformation produces a sequence or collection for each element.

Map vs FlatMap

let scoresByName = ["Henk": [0, 5, 8], "John": [2, 5, 8]]

let mapped = scoresByName.map { $0.value }
// [[0, 5, 8], [2, 5, 8]] - An array of arrays
print(mapped)

let flatMapped = scoresByName.flatMap { $0.value }
// [0, 5, 8, 2, 5, 8] - flattened to only one array

CompactMap vs FlatMap

Used on a sequence and having a transformation returning an optional value, use CompactMap. If not, either map or flatMap should give you the results you need.

When required init method is mandatory? What is that?

Ans :

Write the required modifier before the definition of a class initializer to indicate that every subclass of the class must implement that initializer.

You must also write the required modifier before every subclass implementation of a required initializer, to indicate that the initializer requirement applies to further subclasses in the chain. You do not write the override modifier when overriding a required designated initializer.

“The use of the required modifier ensures that you provide an explicit or inherited implementation of the initializer requirement on all subclasses of the conforming class, such that they also conform to the protocol.”

What is mutating in swift?

Ans :

If you have ever tried using the mutating keyword in your class methods in Swift, the compiler will definitely yell at you because you are doing something wrong.

In swift, classes are reference type whereas structures and enumerations are value types. The properties of value types cannot be modified within its instance methods by default. In order to modify the properties of a value type, you have to use the mutating keyword in the instance method. With this keyword, your method can then have the ability to mutate the values of the properties and write it back to the original structure when the method implementation ends.

Below is a simple implementation of Stack in Swift that illustrates the use of mutating functions.

struct Stack {

     var items = [Int]() // Empty items array

    

    mutating func push(_ item: Int) {

        items.append(item)

    }

    

    mutating func pop() -> Int? {

        if !items.isEmpty {

           return items.removeLast()

        }

        return nil

    }

}

var stack = Stack()

stack.push(4)

stack.push(78)

stack.items // [4, 78]

stack.pop()

stack.items // [4]

You can change the value of items by creating an instance of Stack. But without mutating keyword function can not change property.

Classes are reference type whereas Structures and Enumerations are of a value type in swift. What does that mean is that a class object shares a single instance of the object and passes the same reference if passed to any function or new object whereas the value type is the one which creates a copy of it and passes only the value.

If we try to change any variable inside a class it’s straightforward.

Difference between category in Objective C and extension Swift?

Ans : 

Differences between Objective-C Category and Extension


1. Factor : Source code

Categories provide a way to add methods to a class even if its source code is not available to you. ex: NSString

Extensions are only possible if the source code is available , because compiler compiles the extension & source code at same time.

2. Factor : Instance variable/Properties

Category : Not possible

Extensions : Possible

3. Factor : Accessibility to Inherited classes.

Category — All methods defined inside a category are also available to the inherited class

Extensions — All properties and methods defined inside a class extension are not even available to inherited class.

When to use : 

Use category when you need to want to add any methods to a class whose source code is not available to you OR you want to break down a class into several files depending on its functionality.

Use class extensions when you want to add some required methods to your class outside the main interface file. Also when you want to modify a publicly declared variable in the main interface file .

Difference between Objective-C category and Swift extension :

Mostly Objective-C category and Swift extension are same. 

Diff : 

In Objective-C category, Computed property/variable can not be declared.
In Swift extension, Computed property(not stored property) can be declared.

Difference between class and static and global function

Ans :

In Swift, there are multiple ways to write utility functions.

Static Functions

Static functions are invoked by the class itself, not by an instance. This makes it simple to invoke utility functions without having to manage an object to do that work for you.

class AppUtils {
    static func appUtility() {
    }
}

We can access static function as AppUtils.appUtility()

Static functions can not be overridden.

Class Functions

Class functions (not instance methods) are also static functions but they are dynamically dispatched and can be overridden by subclasses unlike static functions.

class AppUtils{
    class func appUtility(){
        println("In AppUtils")
    }
}
class AppOtherUtils: AppUtils{
    override class func appUtility(){
        println("In AppOtherUtils")
    }
}

We can access them similar to static functions as AppUtils.appUtility() and AppOtherUtils.appUtility().

static is same as class final.

Global Functions

But then you can also just do a stand alone function in Swift which is not within a class and access it anywhere in the project. The global functions can be kept in a separate file that we can import into any project as per requirement.

func appUtility() {
}

We can just access them as appUtility() anywhere in the project. If you have method with same name as of global function, then access global function with MyAppName.appUtility()

In case of static functions, if we access one of the static member, entire class gets loaded in memory. But in case of global function, only that particular function will be loaded in memory.

So, which one is better to use?

It is largly subjective why we pick one over another. Some prefer static method approach as a form of namespacing. For example, using the static method approach also allows us to have a method ClassA.appUtility() and a method named ClassB.appUtility(), which is useful while developing a library or framework.

Global functions are more modular and factor out single tasks for a single function - a good global utility function that does exactly one thing and does it perfectly can also sometimes be abstracted or made generic and used in other context as well.

We can just create a AppUtility.swift file and put all the utility functions in it. Later this file can be used across multiple projects.