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]

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 pod install and pod update

Ans : 

pod install

This is to be used the first time you want to retrieve the pods for the project, but also every time you edit your Podfile to add, update or remove a pod.

Every time the pod install command is run — and downloads and install new pods — it writes the version it has installed, for each pods, in the Podfile.lock file. This file keeps track of the installed version of each pod and locks those versions.
When you run pod install, it only resolves dependencies for pods that are not already listed in the Podfile.lock.
For pods listed in the Podfile.lock, it downloads the explicit version listed in the Podfile.lock without trying to check if a newer version is available
For pods not listed in the Podfile.lock yet, it searches for the version that matches what is described in the Podfile (like in pod 'MyPod', '~>1.2')

pod outdated

When you run pod outdated, CocoaPods will list all pods which have newer versions than the ones listed in the Podfile.lock (the versions currently installed for each pod). This means that if you run pod update PODNAME on those pods, they will be updated — as long as the new version still matches the restrictions like pod 'MyPod', '~>x.y' set in your Podfile.

pod update

When you run pod update PODNAME, CocoaPods will try to find an updated version of the pod PODNAME, without taking into account the version listed in Podfile.lock. It will update the pod to the latest version possible (as long as it matches the version restrictions in your Podfile).

If you run pod update with no pod name, CocoaPods will update every pod listed in your Podfile to the latest version possible.

How to cancel alamofire api request?

Ans:

You can cancel a single request as below:

1 - First get the request:

let request = Alamofire.SessionManager.default.request(path!, method: .post, parameters: parameters, encoding: JSONEncoding.default, headers: createHeader()).responseJSON { response in
    switch response.result {
    case .success(let data):
        success(data as AnyObject?)
    case .failure(let error) :
        failure(error as NSError)
    }
}

2 - Then, in your viewDidDisappear, just call:

request.cancel()

---

You can cancel all requests as below:

Alamofire.SessionManager.default.session.getTasksWithCompletionHandler { (sessionDataTask, uploadData, downloadData) in
    sessionDataTask.forEach { $0.cancel() }
    uploadData.forEach { $0.cancel() }
    downloadData.forEach { $0.cancel() }
}

According to different alamofire and swift version, above code may change little bit.