class ClassA {
    let onDone = Delegate<(), Void>()

    func doSomething() {
        // ...
        onDone()
    }
}

class MyClass {
    func askClassAToDoSomething() {
        let a = ClassA()
        a.onDone.delegate(on: self) { (self, _) in
            self.jobDone()
        }
        a.doSomething()
    }

    private func jobDone() {
        print("🎉")
    }
}

protocol ClassADelegate {
    func doSomethingIsDone()
}

class ClassA {
    weak var delegate: ClassADelegate?

    func doSomething() {
        // ...
        delegate?.doSomethingIsDone()
    }
}

class MyClass {
    func askClassAToDoSomething() {
        let a = ClassA()
        a.delegate = self
        a.doSomething()
    }

    private func jobDone() {
        print("🎉")
    }
}

extension MyClass: ClassADelegate {
    func doSomethingIsDone() {
        self.jobDone()
    }
}

class ClassA {
    var onDoneProperty: (() -> Void)?
    //...
}

class MyClass {
    var a: ClassA = ClassA()

    func askClassAToDoSomething() {
        a.onDoneProperty = {
            // Retain cycle!!
            self.jobDone()
        }
    }

class MyClass {
    var a: ClassA = ClassA()

    func askClassAToDoSomething() {
        a.onDoneProperty = { [weak self] in
            guard let self = self else { return }
            self.jobDone()
        }
    }

a.onDone.delegate(on: self) { // This `self` is the delegation target. `onDone` holds a weak ref of it.
    (self, _) in                 // This `self` is a shadowed, non-option type.
    self.jobDone()            // Using of this `self` does not create retain cycle.
}

class DataController {
    let onShouldShowAtIndexPath = Delegate<IndexPath, Bool>()

    func foo() {
        let currentIndexPath: IndexPath = // ...
        let shouldShow: Bool = onShouldShowAtIndexPath(currentIndexPath)
        if shouldShow {
            show()
        }
    }
}

// Caller Side
dataSource.onShouldShowAtIndexPath.delegate(on: self /* : Target */ ) { (self, indexPath) in
    // This block has a type of `(Target, IndexPath) -> Bool`.
    return indexPath.row != 0
}

a.onDone.delegate(on: self) { (_, _) in
    self.jobDone()
}