Delegate Pattern

The name Delegate comes from the concept of delegating control (giving control to something else). In Swift, a delegate is a controller object with a defined interface that can be used to control or modify the behaviour of another object.

Generalised Example:

Your manager assigns you (developer) a particular task. You do the task and get its output. And then returns it back to the manager. This is a delegation pattern.

Ways to implement delegation pattern:

1. Delegate pattern (with reference):

class Developer {
    var managerReference: Manager?
    
    func developWillAdd(a: Int, b: Int) {
        managerReference?.didFinishedAdding(result: a+b)
    }
}


class Manager {
    var developer: Developer?
    
    init(developer: Developer?) {
        self.developer = developer
        
        // To tell the compiler that output should be returned to this class
        developer?.managerReference = self
    }
    
    func addTwoNumbers(num1: Int, num2: Int) {
        developer?.developWillAdd(a: num1, b: num2)
    }
    
    func didFinishedAdding(result: Int) {
        print(result)
    }
}


let obj = Manager(developer: Developer())
obj.addTwoNumbers(num1: 10, num2: 20)

2. Delegate pattern (with protocol) :

protocol AdditionDelegate: AnyObject {
    func didFinishedAdding(result: Int)
}


class Developer {
    // Basically delegate
    // delegate is initialized with weak keyword here in order to avoid any memory leaks
    weak var managerReference: AdditionDelegate?
    
    func developWillAdd(a: Int, b: Int) {
        managerReference?.didFinishedAdding(result: a+b)
    }
}


class Manager: AdditionDelegate {
    var developer: Developer?
    
    init(developer: Developer?) {
        self.developer = developer
        developer?.managerReference = self
    }
    
    func addTwoNumbers(num1: Int, num2: Int) {
        developer?.developWillAdd(a: num1, b: num2)
    }
    
    func didFinishedAdding(result: Int) {
        print(result)
    }
}


let obj = Manager(developer: Developer())
obj.addTwoNumbers(num1: 10, num2: 20)

3. Delegate pattern (with closures) :

class Developer {
    var didAddNumbers: ((Int) -> Void)?
    
    func developWillAdd(a: Int, b: Int) {
        didAddNumbers!(a+b)
    }
}


class Manager {
    var developer: Developer?
    
    init(developer: Developer?) {
        self.developer = developer
    }
    
    func addTwoNumbers(num1: Int, num2: Int) {
        developer?.developWillAdd(a: num1, b: num2)
        
        developer?.didAddNumbers = { ans in
            print(ans)
        }
    }
}


let obj = Manager(developer: Developer())
obj.addTwoNumbers(num1: 10, num2: 20)