Browse Questions
  • Store tokens securely in the Keychain, attach them to requests via a header, and refresh automatically when a 401 is received.
  • Delegation is a pattern where one object hands off responsibility to another through a protocol, letting a child communicate back to its owner without knowing its concrete type.
  • Use @Binding to pass a two-way connection into the sheet, so changes inside the sheet reflect in the parent.
  • - Inheritance — a class derives behaviour from a parent class ("is-a" relationship) - Composition — a type gets behaviour by holding references to other objects ("has-a" relationship) Swift…

Answer: Delegation is a pattern where one object hands off responsibility to another through a protocol, letting a child communicate back to its owner without knowing its concrete type. UIKit uses it everywhere — UITableViewDelegate, UITextFieldDelegate, UIScrollViewDelegate — and the delegate property is always weak to break the retain cycle that would otherwise form between parent and child.

Code Example:

// 1. Delegation — best for multiple related callbacks
protocol PaymentViewDelegate: AnyObject {   // AnyObject required for weak
    func paymentViewDidTapPay(_ view: PaymentView, amount: Decimal)
    func paymentViewDidCancel(_ view: PaymentView)
}

final class PaymentView: UIView {
    weak var delegate: PaymentViewDelegate?  // weak — parent owns us, we don't own parent

    @objc private func payTapped() {
        delegate?.paymentViewDidTapPay(self, amount: total)
    }
}

// 2. Closure — best for a single callback, keeps call site local
final class TipView: UIView {
    var onPay: ((Decimal) -> Void)?
}

tipView.onPay = { [weak self] amount in       // [weak self] — view retains the closure
    self?.processPayment(amount)
}

// 3. Target-action — the Objective-C control mechanism
payButton.addTarget(self, action: #selector(payTapped), for: .touchUpInside)
// iOS 14+ modern equivalent, closure-based:
payButton.addAction(UIAction { [weak self] _ in self?.processPayment() }, for: .touchUpInside)

Key Points:

  • Use delegation when there are several callbacks, or one needs a return value (tableView(_:numberOfRowsInSection:))
  • Use closures for one-off callbacks — less boilerplate, but always capture self weakly
  • Use target-action only for UIControl subclasses; prefer UIAction on iOS 14+
  • weak var delegate + protocol X: AnyObject is the rule — forgetting weak leaks the view controller

Further reading