The Complete Understanding of Swift Delegate and Data Source

Pass data between view controllers and the behind scene.

Motivation

The delegate/data-source pattern exists everywhere within the iOS ecosystem. Most new students/developers simply copy & paste without knowing the behind implementation.

If you’ve wondered how UITableViewDataSource works, you might have come to the right place with BTD.

Introduction

First, the delegate design pattern is used to pass data or communicate between classes or structs. That’s it. The word “delegate” may come across confusing. I will try to use an analogy throughout the tutorial.

Your job is to pass data from FirstVC to SecondVC. Let us use the delegate pattern. First, you need to create a protocol that has a required method with a String parameter.

protocol FirstVCDelegate {
 func passData(data: String)
}

Second, create a class called, FirstVC.

class FirstVC {
 var delegate: FirstVCDelegate?
}

FirstVC has an optional property delegate whose type is FirstVCDelegate. The delegate property will be initialized by SecondVC later. Bear with me for a minute. You will see the magic.

If you aren’t comfortable with protocols, you may go grab some firepower from Intro to Protocol Oriented Programming and revisit.

Last, let us design SecondVC which conforms to FirstVCDelegate.

class SecondVC: FirstVCDelegate {
 func passData(data: String) {
  print("Something happened")
 }
}

Let us create the objects.

let firstVC = FirstVC() 
let secondVC = SecondVC() 

First, we’ve initialized two objects. If you remember correctly, the FirstVC class had an optional property called, delegate. Let us assign the optional property to secondVC.

firstVC.delegate = secondVC  // secondVC = delegate 

Now, the magic happens. You are able to execute the passData method from FirstVC even though the passData method resides in SecondVC.

firstVC.delegate?.passData(data: "a bunch of contracts”)
// "Something happened" 

Analogy

I often describe the delegator/delegate relationship analogous to that of CEO/secretary. delegate/secondVC is the secretary while delegator/firstVC is the CEO. The CEO may have a bunch of contracts and he/she simply tells the secretary to handle them by sending the documents.

firstVC.delegate?.passData(data: "a bunch of contracts”)

Now, let’s use the data passed by the CEO within SecondVC.

class SecondVC: FirstVCDelegate {
 func passData(data: String) {
  print("The CEO gave me \(data)")
 }
}

When the CEO calls the passData method, the secretary automatically calls the method and he/she may use the data a.k.a contacts given from the CEO as shown below.

firstVC.delegate?.passData(data: "a bunch of contracts”)
// "The CEO gave me a bunch of contracts"

Done. You’ve learned how to pass data from FirstVC to SecondVC using the delegate/delegator relationship.

Application

Knowing is not enough. Let us take a look at how the delegate pattern is used within the iOS ecosystem. If you’ve worked with UITableView, you’ve seen,

class BobViewController: UIViewController, UITableViewDelegate {
 override func viewDidLoad() {
  super.viewDidLoad()
  tableView.delegate = self
 }
}

In the code above, there are two objects. One is self which refers to the object of BobViewController and tableView. In this case, the CEO is tableView, and the secretary is self.

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