What are different type of Queues in GCD?

Ans :

Queue : 

A Queue is a linear structure that follows the First In First Out (FIFO) order. Here we are going to use two types of queue Serial queue and Concurrent queue.

Serial Queue : 

Serial Queue

In the serial queue, only one task runs at a time. Once the first task ends then only the second task will begin. All the task follow the same sequence they have to wait until running task finish.

Create our own serial queue using GCD:

let serialQueue = DispatchQueue(label: "mySerialQueue")

 serialQueue.async {

  // Add your serial task

 }

Download images using serial queue : 

let myArray = [img1, img2, img3, img4, img5, img6]

   

   for i in 0 ..< myArray.count {

     serialQueue.async {

       do {

         let data = try Data(contentsOf: URL(string: myArray[i])!)

         if let image = UIImage(data: data) {

           DispatchQueue.main.async {

             self.imageSerial[i].image = image

           }

         }

       } catch {

         print("error is \(error.localizedDescription)")

       }

     }

   }

Concurrent Queue : 

Concurrent Queue

In the Concurrent queue, multiple tasks can run at the same time. The start time of the task will be the order they are added, means Task 0 start first then Task 1 will start after that and so on. Tasks can finish in any order.

Global queue is example of Concurrent queue.

// Synchronous

DispatchQueue.global().sync {

 // write your code here

}

// Asynchronous

DispatchQueue.global().async {

 // write your code here

}

Let's create own concurrent queue using GCD: 

let concurrentQueue = DispatchQueue(label: "myConcurrentQueue", qos: .default, attributes: .concurrent, autoreleaseFrequency: .inherit, target: nil)

 concurrentQueue.async {

  // Add your concurrent task

 }

Download images using concurrent queue:

let myArray = [img1, img2, img3, img4, img5, img6]

for i in 0 ..< myArray.count {

  concurrentQueue.async {

    do {

      let data = try Data(contentsOf: URL(string: myArray[i])!)

      if let image = UIImage(data: data) {

        DispatchQueue.main.async {

          self.imageConcurrent[i].image = image

        }

      }

    } catch {

      print("error is \(error.localizedDescription)")

    }

  }

}

What is QOS?

Ans : 

QOS - Quality of Services

A quality-of-service (QoS) class categorizes work to be performed on a DispatchQueue. By specifying the quality of a task, you indicate its importance to your app. When scheduling tasks, the system prioritizes those that have higher service classes.

Because higher priority work is performed more quickly and with more resources than lower priority work, it typically requires more energy than lower priority work. Accurately specifying appropriate QoS classes for the work your app performs ensures that your app is responsive and energy efficient.

Levels of Priority

After an app kicks off and starts a runloop on the main thread, one can begin taking advantage of QoS. QoS breaks out priorities into four different groups. Each one corresponds to common tasks one might find themselves coding in their iOS endeavors.

• User Interactive: Work that happens on the main thread, such as animations or drawing operations.
• User Initiated: Work that the user kicks off and should yield immediate results. This work must be completed for the user to continue.
• Utility: Work that may take a bit and doesn’t need to finish right away. Analogous to progress bars and importing data.
• Background: This work isn’t visible to the user. Backups, syncs, indexing, etc.

There are five global queues having quality of service level ranging  from high performance to high energy efficiency. 

1. DispatchQueue.global(qos: .userInteractive) - highest priority
2. DispatchQueue.global(qos: .userInitiated)
3. DispatchQueue.global() // default
4. DispatchQueue.global(qos: .utility)
5. DispatchQueue.global(qos: .background) - lowest priority 

DispatchQueue.global(qos: .userInteractive).async {

 // Event handling task

}

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.

What is delegation pattern?

Ans : 

A design pattern is a generalized way of arranging different parts of code. The main motive behind using a design pattern is to achieve better readability and code reusability.

Delegation is a design pattern that makes a component in code delegate its task(s) to a different component.

In order to make communication possible in two ways, we should use delegate. We can achieve this using delegate-protocol in Objective-C and Swift.

We should use delegate as Weak due to avoid memory leaks.

Read :  Usage of Delegate-Protocol

Write a program for sorting array. Quick sort, merge sort, Heap Sort

Ans : 

Quick Sorting Integer Array - Explanation

func swap<T: Comparable>(leftValue: inout T, rightValue: inout T) {

    (leftValue, rightValue) = (rightValue, leftValue)

}

func partition<T: Comparable>(array: inout [T], startIndex: Int, endIndex: Int) -> Int {

    var q = startIndex

    for index in startIndex..<endIndex {

        if array[index] < array[endIndex] {

            swap(leftValue: &array[q], rightValue: &array[index])

            q += 1

        }

    }

    swap(leftValue: &array[q], rightValue: &array[endIndex])

    return q

}

func quickSort<T: Comparable>(array: inout [T], startIndex: Int, endIndex: Int) {

    // Base case

    if startIndex >= endIndex {

        return

    }

    let placedItemIndex = partition(array: &array, startIndex: startIndex, endIndex: endIndex)

    quickSort(array: &array, startIndex: startIndex, endIndex: placedItemIndex-1)

    quickSort(array: &array, startIndex: placedItemIndex+1, endIndex: endIndex)

}

func quickSort<T: Comparable>(array: inout [T]) {

    quickSort(array: &array, startIndex: 0, endIndex: array.count-1)

}

var numbers = [13, 77, 20, 45, 2, 15, 0, 59, 5, 68, 51, 1, -1, 77]

quickSort(array: &numbers)

Merge Sorting Integer Array - Explanation

func merge<T: Comparable> (array: inout [T], startIndex: Int, middleIndex: Int, endIndex: Int) {

    let leftSubarray = Array(array[startIndex...middleIndex])

    let rightSubarray = Array(array[middleIndex+1...endIndex])

    var index = startIndex

    var leftIndex = 0

    var rightIndex = 0

    while leftIndex < leftSubarray.count && rightIndex < rightSubarray.count {

        if leftSubarray[leftIndex] < rightSubarray[rightIndex] {

            array[index] = leftSubarray[leftIndex]

            leftIndex += 1

        }

        else {

            array[index] = rightSubarray[rightIndex]

            rightIndex += 1

        }

        index += 1

    }

    while leftIndex < leftSubarray.count {

        array[index] = leftSubarray[leftIndex]

        index += 1

        leftIndex += 1

    }

    while rightIndex < rightSubarray.count {

        array[index] = rightSubarray[rightIndex]

        index += 1

        rightIndex += 1

    }

}

func mergeSort<T: Comparable>(array: inout [T], startIndex: Int, endIndex: Int) {

    // Base case

    if startIndex >= endIndex {

        return

    }

    let middleIndex = (startIndex + endIndex) / 2

    mergeSort(array: &array, startIndex: startIndex, endIndex: middleIndex)

    mergeSort(array: &array, startIndex: middleIndex+1, endIndex: endIndex)

    merge(array: &array, startIndex: startIndex, middleIndex: middleIndex, endIndex: endIndex)

}

func mergeSort<T: Comparable>(array: inout [T]) {

    mergeSort(array: &array, startIndex: 0, endIndex: array.count-1)

}

var numbers = [13, 77, 20, 45, 2, 15, 0, 59, 5, 68, 51, 1, -1, 77]

mergeSort(array: &numbers)

Heap Sorting Integer Array - Explanation

extension Heap {

    public mutating func sort() -> [T] {

        for i in stride(from: (nodes.count - 1), through: 1, by: -1) {

            nodes.swapAt(0, i)

            shiftDown(from: 0, until: i)

        }

        return nodes

    }

}

/*

 Sorts an array using a heap.

 Heapsort can be performed in-place, but it is not a stable sort.

 */

public func heapsort<T>(_ a: [T], _ sort: @escaping (T, T) -> Bool) -> [T] {

    let reverseOrder = { i1, i2 in sort(i2, i1) }

    var h = Heap(array: a, sort: reverseOrder)

    return h.sort()

}

//Testing

func testSort() {

    var h1 = Heap(array: [5, 13, 2, 25, 7, 17, 20, 8, 4], sort: >)

    let a1 = h1.sort()

}

// output : [2, 4, 5, 7, 8, 13, 17, 20, 25]