类和结构闭包中的Swift可变结构表现不同

Question

我有一个类(A),它有一个struct变量(S).在这个类的一个函数中,我在struct变量上调用一个mutating函数,这个函数需要一个闭包.此闭包的主体检查struct变量的name属性.

结构的变异函数依次调用某个类(B)的函数.这个类的函数再次关闭.在这个闭包的主体中改变结构,即更改name属性,并调用第一个类提供的闭包.

当我们检查struct的name属性时调用第一个类(A)闭包时,它永远不会被更改.

但是在第2步中,如果我使用结构(C)而不是类B,我会看到内部类A的闭包结构实际上已经改变了.以下是代码:

class NetworkingClass {
  func fetchDataOverNetwork(completion:()->()) {
    // Fetch Data from netwrok and finally call the closure
    completion()
  }
}

struct NetworkingStruct {
  func fetchDataOverNetwork(completion:()->()) {
    // Fetch Data from netwrok and finally call the closure
    completion()
  }
}

struct ViewModelStruct {

  /// Initial value
  var data: String = "A"

  /// Mutate itself in a closure called from a struct
  mutating func changeFromStruct(completion:()->()) {
    let networkingStruct = NetworkingStruct()
    networkingStruct.fetchDataOverNetwork {
      self.data = "B"
      completion()
    }
  }

  /// Mutate itself in a closure called from a class
  mutating func changeFromClass(completion:()->()) {
    let networkingClass = NetworkingClass()
    networkingClass.fetchDataOverNetwork {
      self.data = "C"
      completion()
    }
  }
}

class ViewController {
  var viewModel: ViewModelStruct = ViewModelStruct()

  func changeViewModelStruct() {
    print(viewModel.data)

    /// This never changes self.viewModel inside closure, Why Not?
    viewModel.changeFromClass {
      print(self.viewModel.data)
    }

    /// This changes self.viewModel inside/outside closure, Why?
    viewModel.changeFromStruct {
      print(self.viewModel.data)
    }
  }
}

var c = ViewController()
c.changeViewModelStruct()

为什么这种行为不同.我认为区别因素应该是我是使用viewModel的结构还是类.但这取决于Networking是一个类还是结构,它独立于任何ViewController或ViewModel.任何人都可以帮我理解这个吗？

Answer 1

这不是一个解决方案，但通过此代码，我们可以看到 ,ViewController's已viewModel.data针对类和结构情况正确设置。不同的是，viewModel.changeFromClass闭包捕获了一个 stale self.viewModel.data。请特别注意，只有 class 的“3 self”打印是错误的。不是包装它的“2 self”和“4 self”印刷品。

class NetworkingClass {
  func fetchDataOverNetwork(completion:()->()) {
    // Fetch Data from netwrok and finally call the closure
    print("\nclass: \(self)")
    completion()
  }
}

struct NetworkingStruct {
  func fetchDataOverNetwork(completion:()->()) {
    // Fetch Data from netwrok and finally call the closure
    print("\nstruct: \(self)")
    completion()
  }
}

struct ViewModelStruct {

  /// Initial value
  var data: String = "A"

  /// Mutate itself in a closure called from a struct
  mutating func changeFromStruct(completion:()->()) {
    let networkingStruct = NetworkingStruct()
    networkingStruct.fetchDataOverNetwork {
      print("1 \(self)")
      self.data = "B"
      print("2 \(self)")
      completion()
      print("4 \(self)")
    }
  }

  /// Mutate itself in a closure called from a class
  mutating func changeFromClass(completion:()->()) {
    let networkingClass = NetworkingClass()
    networkingClass.fetchDataOverNetwork {
      print("1 \(self)")
      self.data = "C"
      print("2 \(self)")
      completion()
      print("4 \(self)")
    }
  }
}

class ViewController {
  var viewModel: ViewModelStruct = ViewModelStruct()

  func changeViewModelStruct() {
    print(viewModel.data)

    /// This never changes self.viewModel, Why Not?
    viewModel.changeFromClass {
      print("3 \(self.viewModel)")
      print(self.viewModel.data)
    }

    /// This changes self.viewModel, Why?
    viewModel.changeFromStruct {
      print("3 \(self.viewModel)")
      print(self.viewModel.data)
    }
  }
}

var c = ViewController()
c.changeViewModelStruct()