swift - Swift playground 中的弱引用无法按预期工作

Question

我一直在关注 Playground 中 Intermediate Swift WWDC session 中的弱引用示例。我稍微修改了代码如下:

class Apartment {
    let address: Int

    init(address: Int) {
        self.address = address
    }

    weak var tenant: Person?
}

class Person {
    let name: String

    init(name: String){
        self.name = name
    }

    weak var home: Apartment?

    func moveIn(apt: Apartment) {
        self.home = apt
        apt.tenant = self
    }
}

var renters = ["John Appleseed": Person(name: "John Appleseed")]
var apts = [16: Apartment(address: 16)]

renters["John Appleseed"]!.moveIn(apts[16]!)
renters["John Appleseed"] = nil // memory should be released here

// then apts[16].tenant should be nil
if let tenantName = apts[16]!.tenant?.name {
    // this should only execute if the Person object is still in memory
    println("\(tenantName) lives at apartment number \(apts[16]!.address)")
} else {
    // and this line should execute if the memory is released as we expect
    println("Nobody lives at apartment number \(apts[16]!.address)")
}

// Console output in Playground: John Appleseed lives at apartment number 16
// Console output in standalone app: Nobody lives at apartment number 16

根据我对弱引用的理解，为 Person 实例分配的内存应该在从 renters 字典中删除时释放，因为对它的唯一其他引用是弱引用。但是，如果程序作为独立的命令行应用程序运行与在 Playground 中运行(请参阅评论)，则程序的输出是不同的。

Answer 1

我相信顶层函数 (REPL/playground) 正在保持强大的引用以促进交互行为，并在框架返回时进行清理。此行为消除了交互式环境中的内存泄漏。

我复制了 Viktor 的简单示例并使用了 xcrun swift REPL。

在 REPL 模式下，我将逻辑包装在一个函数中，它按预期工作。如果/当您关心何时清理内存时，我建议将您的逻辑包装在一个函数中。

// declaration of the types
class Person {
   let name: String
   weak var home: Apartment?

  init(pName: String){
      name = pName
  }

}

class Apartment {
    let postalCode: Int

    init(pPostalCode: Int) {
        postalCode = pPostalCode
    }
}

func testArc() {
    // create Person object
    var personJulius: Person = Person(pName: "Julius")

    // create Apartment object
    var apartmentBerlin: Apartment? = Apartment(pPostalCode: 10777)

    // connect Apartment object and Person object
    personJulius.home = apartmentBerlin

    // Set only strong reference of Apartment object to nil
    apartmentBerlin = nil

    // Person object should now have nil as home
    if personJulius.home != nil {
        println("Julius does live in a destroyed apartment")
    } else {
        println("everything as it should")
    }

}

//outputs "everything as it should"
testArc()