How many ways to pass data from one view controller to another view controller?

Ans :

Through following methods, we can pass data from one viewcontroller to another viewcontroller.

1. Using segue (PrepareForSegue) method,
2. Delegate
3. Setting variable

Let's assume we have 2 viewcontroller named VC1 and VC2. We want to pass data from VC1 to VC2 and some times VC2 to VC1. For both cases, we can use segue to pass data. But in general practise, to pass data from VC1 to VC2 (Forward data passing), we use segue and from VC2 to VC1(Backward data passing), we use protocol - delegate way.

1. Using segue (PrepareForSegue) steps :  VC1 --->  VC2

•   Draw segue from VC1 to VC2 and give identifier string 'InputVCToDisplayVC'  to segue.
•   Take variable in VC2.swift as :

           var fullName : String?

•    In VC1, write following code in PrepareForSegue method,

       override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if(segue.identifier == "InputVCToDisplayVC"){
let displayVC = segue.destination as! DisplayViewController
displayVC.fullName = nameTextField.text
        }
        }

•       In VC2, viewDidLoad, we can get  value of fullName and access it.

2. Using delegate steps :    VC2  --->  VC1

Import UIKit

class VC1: UIViewController, VC2Delegate {

       override func viewDidLoad() {

                super.viewDidLoad()

                

       }

       func doSomething(data : String) {

                print(data)

      }

      override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

      if(segue.identifier == "InputVCToDisplayVC"){

      let displayVC = segue.destination as! DisplayViewController

      displayVC.delegate = self

      }

       }

 }

Import UIKit

protocol VC2Delegate {

      func doSomething(data : String) 

}

class VC2 : UIViewController {

        

      var fullName : String?

      weak var delegate : VC2Delegate?

      

      override func viewDidLoad() {

             

     }

     @IBAction func btnPassToVC1_TouchUpInside(_ sender : UIButton) {

              if let delegate = delegate {

                     delegate.doSomething(data: btn.titleLabel.text)       

              }

    }

}

In above, method we make custom protocol and create its object as delegate.We set delegate reference to VC1. So on VC2 button pressed event, VC1 doSomething method will be called.

3. Setting Variable steps :     VC1   --->  VC2

• Take variable in VC2.swift as :  

           var fullName : String?

• On buttonPressed action of VC1, write code as :

           let vc2 : VC2 =  UIStoryboard(name: "Main", bundle:                                                                                                        nil).instantiateViewController(withIdentifier: "VC2_ID") as! VC2

           vc2.fullName = "iOS iQA"

           self.present(vc2, animated: true, completion: nil)

Note : There are also persistent storage, by which we can store data and access that data any where in our project. But this is not correct answer or solution for above question or problem. We can store data in NSUserDefault, PList, Coredata, SQLite, KeyChain Access, File as persistent storage.

    

Difference between Stack and Heap

Ans. 

1. Stack is used for static memory allocation, Heap is used for dynamic memory allocation.

2. Variables allocated on the stack are stored directly into memory and access memory very faster and its allocation dealt with compile time.
  Variable allocated on the heap have their memory allocated at run time and accessing their memory is bit slower.

3. Stack is always reserved in LIFO order, but you can allocate and release any element/block on heap anytime. So this is much complex to say about which block is free or allocated at given time.

4. You can use stack when you know how much data you need to allocate before compile time and they are not too big. You can use heap when you don't know how much data you need to allocate or they are too big.

5. Stack is thread specific and heap is application specific. In multi threaded, each thread has its own stack.

Stack allocation vs Heap allocation (Why stack is faster than heap)

Stack allocation means that assembly just needs to increment stack pointer and that’s it. How ever in case of heap, there is lot more going on. The memory allocator needs to ask kernel for some free page, needs to partition the page properly, with time fragmentation may occur, etc. Thus with one word to say, lot of work. Struct is stored in stack and class is stored in heap.