playground.swift

import Foundation

class Foo {
    let completionHandler: ([String]) -> Void

    init(completionHandler: @escaping ([String]) -> Void) {
        self.completionHandler = completionHandler
    }
}

class Bar {
    var foo: Foo?
    var result: [String] = []

    deinit {
        print("deinit: \(self)")
    }

    func perform() {
        foo = Foo(completionHandler: { (result: [String]) in
            self.result = result
        })
    }
}

var bar: Bar? = Bar()

bar?.perform()

bar = nil