Demo of weak vs strong capturing in closures

Playground.swift

import Foundation
import PlaygroundSupport
import UIKit

class M {

    var a = ""

    func x() { self.a = "A" }

    func captureStrongly() -> (() -> Void) {
        return { [x] in x()  }
    }

    func captureWeakly() -> (() -> Void) {
        return { [weak self] in self?.x()  }
    }
}

var captureTest: (() -> Void)?
weak var potentiallyCapturedM: M?

// Let's test strong capturing first:

_ = {
    let m = M()
    captureTest = m.captureStrongly()
    potentiallyCapturedM = m
}()

// `false` because scoping x with `[x]` captures
// potentiallyCapturedM via x's reference to `self.a`.
print(potentiallyCapturedM == nil)

// Now let's test weak capturing:

_ = {
    let m = M()
    captureTest = m.captureWeakly()
    potentiallyCapturedM = m
}()

// `true` because potentiallyCapturedM is only
// weakly referenced.
print(potentiallyCapturedM == nil)