What is Delegation?
First of all, you should know that Delegation Pattern is not exclusive for iOS world:
In software engineering, the delegation pattern is a design pattern in
object-oriented programming that allows object composition to achieve
the same code reuse as inheritance.
But working with delegation in the iOS world is so common, I assume that you can see many of classes that provide a delegation/datasource for giving the ability to provide properties or behaviors for the used instance. It is one of main mechanisms of how objects talk to each other in CocoaTouch.
Alternatives:
However, delegation is not the only way to let objects talk to each other in iOS, you might want to know that there are:
- NotificationCenter.
- KVO (Key-Value Observing).
- Completion handlers/Callbacks (using closures).
- Target-Action.
Remark: in case if you are interested in comparing between them, you might want to check the following articles:
- Communication Patterns.
- When to Use Delegation, Notification, or Observation in iOS.
- Delegates vs Observers.
When to use Delegation?
So, the question is: “So why should I use delegation instead of those options?”
I will try to make it simple; I would suggest the use of delegation when you have one to one relationship between two objects. Just to make it clearer, the goal of talking a little bit about the NotificationCenter is to try to make sense when delegations are used:
NotificationCenter represents one to many relationship; Simply, it works as: posting (notifying) a notification on a specific event and observing (get notified about) this notification — it could be observed anywhere else; Logically, that’s what one to many relationship means. It is a representation of the Observer Pattern.
How to Apply Delegation?
For the purpose of simplifying, I would mention it as steps:
-
Knowing the requirements: Each delegate has its own rules, listed in the delegate protocol which is a set of method signatures that you should implement for conforming this delegation.
-
Conforming for the delegation: it is simply letting your class to be a delegate, by marking it. For instance:
class ViewController: UIViewController, UITableViewDelegate {}
. -
Connecting the delegate object: Marking your class to be a delegate is not enough, you need to make sure that the object you want to be confirmed by your class to give the required job to your class.
-
Implementing the requirements: Finally, your class have to implement all required methods listed in the delegate protocol.
For Example
Does it sounds a little confusing? What about a real-world example?
Consider the following scenario:
Imagine that you are building an application related to playing audios. Some of the viewControllers should have a view of an audio player. In the simplest case, we assume that it should have a play/pause button and another button for, let’s say, showing a playlist somehow, regardless of how it may look like.
So far so good, the audio player view has its separated UIView
class and .xib
file; it should be added as a subview in any desired viewController.
Now, how can you add functionality to both of the buttons for each viewController? You might think: “Simply, I will add an IBAction
in the view class and that’s it”, at first look, it might sound ok, but after re-thinking a little bit, you will realize that it will not be applicable if you are trying to handle the event of tapping the button at the controller layer; To make it clear, what if each viewController implemented different functionality when tapping the buttons in the audio player view? For example: tapping the playlist in “A” viewController will display a tableView, but tapping it in the “B” viewController will display a picker.
Well, let’s apply Delegation to this issue:
The “#” comments represents the steps of “How to Apply Delegation?” section.
Audio Player View:
// # 1: here is the protocol for creating the delegation
protocol AudioPlayerDelegate: class {
func playPauseDidTap()
func playlistDidTap()
}
class AudioPlayerView: UIView {
//MARK:- IBOutlets
@IBOutlet weak private var btnPlayPause: UIButton!
@IBOutlet weak private var btnPlaylist: UIButton!
// MARK:- Delegate
weak var delegate: AudioPlayerDelegate?
// IBActions
@IBAction private func playPauseTapped(_ sender: AnyObject) {
delegate?.playPauseDidTap()
}
@IBAction private func playlistTapped(_ sender: AnyObject) {
delegate?.playlistDidTap()
}
}
View Controller:
class ViewController: UIViewController {
var audioPlayer: AudioPlayerView?
// MARK:- Life Cycle
override func viewDidLoad() {
super.viewDidLoad()
audioPlayer = AudioPlayerView()
// # 3: the "AudioPlayerView" instance delegate will implemented by my class "ViewController"
audioPlayer?.delegate = self
}
}
// # 2: "ViewController" will implement "AudioPlayerDelegate":
extension ViewController: AudioPlayerDelegate {
// # 4: "ViewController" implements "AudioPlayerDelegate" requirments:
func playPauseDidTap() {
print("play/pause tapped!!")
}
func playlistDidTap() {
// note that is should do a different behavior in each viewController...
print("list tapped!!")
}
}
Quick Tip:
As one of the most popular examples of using delegation is Passing Data Back between View Controllers.