What is the delegation pattern, and why are delegates weak?

#TL;DR

Delegation means "I will tell another object when things happen, or ask it what to do." The contract is a protocol. The delegate property is usually weak so the two objects do not keep each other alive. UIKit uses it everywhere: UITableViewDelegate, UITextFieldDelegate, UIScrollViewDelegate.

#A simple example

protocol DatePickerViewDelegate: AnyObject {
    func datePicker(_ picker: DatePickerView, didSelect date: Date)
    func datePickerShouldAllowPastDates(_ picker: DatePickerView) -> Bool
}

final class DatePickerView: UIView {
    weak var delegate: DatePickerViewDelegate?

    private func userTapped(_ date: Date) {
        let allowPast = delegate?.datePickerShouldAllowPastDates(self) ?? false
        guard allowPast || date >= .now else { return }
        delegate?.datePicker(self, didSelect: date)
    }
}

final class BookingViewController: UIViewController, DatePickerViewDelegate {
    private let picker = DatePickerView()

    override func viewDidLoad() {
        super.viewDidLoad()
        picker.delegate = self
    }

    func datePicker(_ picker: DatePickerView, didSelect date: Date) {
        print("Selected", date)
    }

    func datePickerShouldAllowPastDates(_ picker: DatePickerView) -> Bool { false }
}

Notice two kinds of methods:

• Events: "something happened" (didSelect).
• Questions: "what should I do?" (shouldAllowPastDates).

Passing self (the sender) as the first argument is a UIKit convention. One delegate can then serve many pickers.

#Why weak?

The view controller owns the picker (strong). If the picker also held the controller strongly, neither could be freed.

BookingViewController --strong--> DatePickerView
        ^                               |
        +------------ weak -------------+

weak only works with classes. So the protocol needs AnyObject:

protocol DatePickerViewDelegate: AnyObject { ... }

#Optional delegate methods

Swift protocols have no optional methods. Two options:

// 1. Default implementation in an extension (preferred in Swift)
extension DatePickerViewDelegate {
    func datePickerShouldAllowPastDates(_ picker: DatePickerView) -> Bool { false }
}

// 2. @objc optional (needed for Objective-C interop)
@objc protocol LegacyDelegate {
    @objc optional func didFinish()
}

#Delegate vs closure vs notification

Closure version:

final class DatePickerView: UIView {
    var onSelect: ((Date) -> Void)?
}

picker.onSelect = { [weak self] date in
    self?.save(date)
}

With closures, you must remember [weak self]. With delegates, the weak is in one place.

#Data source is also delegation

UITableViewDataSource asks for data (how many rows, which cell). UITableViewDelegate handles behavior (selection, height). Both are the same pattern.

#Common follow-up questions

• Can a delegate be a struct? Not if the property is weak. Weak needs a class instance.
• What if the delegate is nil? Optional chaining does nothing. Return a safe default for questions.
• Is delegation still used in SwiftUI? Rarely. SwiftUI uses closures, bindings and the environment. You still write delegates in a Coordinator when you wrap UIKit views.
• What about Swift 6? UIKit delegates are @MainActor, so your delegate methods run on the main actor too.