视图关闭后，ObservedObject 视图模型仍在内存中

Question

我在 SwiftUI 和 Combine 的内存管理方面遇到了一些问题。

例如，如果我有一个 NavigationView，然后使用 TextField 导航到详细信息视图，并在 TextField 中输入一个值并点击后退按钮，那么下次我转到该视图时，TextField 具有之前的输入值。

我注意到在关闭详细视图后视图模型仍在内存中，这可能就是 TextField 仍然保留值的原因。

在 UIKit 中，当关闭 ViewController 时，视图模型将被释放，然后在 ViewController 出现时再次创建。这似乎不是这里的情况。

我附上了这个问题的一些最小可重现代码。

import SwiftUI
import Combine

struct ContentView: View {
    var body: some View {
        NavigationView {
            NavigationLink(destination: OtherView()) {
                Text("Press Here")
            }
        }
    }
}

struct OtherView: View {

    @ObservedObject var viewModel = ViewModel()

    var body: some View {
        VStack {
            TextField("Something", text: $viewModel.enteredText)
                .textFieldStyle(RoundedBorderTextFieldStyle())

            Button(action: {
                print("Tap")
            }) {
                Text("Tapping")
            }.disabled(!viewModel.isValid)
        }
    }
}

class ViewModel: ObservableObject {

    @Published var enteredText = ""
    var isValid = false

    var cancellable: AnyCancellable?

    init() {
        cancellable = textValidatedPublisher.receive(on: RunLoop.main)
            .assign(to: \.isValid, on: self)
    }

    deinit {
        cancellable?.cancel()
    }

    var textValidatedPublisher: AnyPublisher<Bool, Never> {
        $enteredText.map {
            [=10=].count > 1
        }.eraseToAnyPublisher()
    }


}

我还注意到，例如，如果我添加另一个视图，比如说在 OtherView 之后添加 SomeOtherView，那么每次我从 OtherView 中键入 TextField 时，都会调用 SomeOtherView 的视图模型中的 deinit。任何人都可以解释为什么会这样吗？

Answer 1

Moreover, I noticed that if I to a change in ContetView and the view is reevaluated, then I will have two ViewModels in memory

由于ViewModel中的交叉引用，这里是固定变体

struct OtherView: View, Constructable {

    @ObservedObject var viewModel = ViewModel()

    var body: some View {
        VStack {
            TextField("Something", text: $viewModel.enteredText)
                .textFieldStyle(RoundedBorderTextFieldStyle())

            Button(action: {
                print("Tap")
            }) {
                Text("Tapping")
            }.disabled(!viewModel.isValid)
        }
        .onDisappear {
            self.viewModel.invalidate()     // << here !!
        }
    }
}

class ViewModel: ObservableObject {

    @Published var enteredText = ""
    var isValid = false

    var cancellable: AnyCancellable?

    init() {
        print("[>>] created")
        cancellable = textValidatedPublisher.receive(on: RunLoop.main)
            .assign(to: \.isValid, on: self)
    }

    func invalidate() {
        cancellable?.cancel()
        cancellable = nil
        print("[<<] invalidated")
    }

    deinit {
//        cancellable?.cancel()     // not here !!!
        print("[x] done")
    }

    var textValidatedPublisher: AnyPublisher<Bool, Never> {
        $enteredText.map {
            [=10=].count > 1
        }.eraseToAnyPublisher()
    }
}

--

更新：

is there a way to instantiate OtherView when navigating?

这是一个解决方案（使用 Xcode 11.4 / iOS 13.4 测试），但这只是事半功倍，因为一旦创建它就会一直存在直到导航 link 重新验证（即在后面它保留在内存中直到下一次导航）

struct ContentView: View {
    var body: some View {
        NavigationView {
            NavigationLink(destination: 
               // create wrapper view with type of view which creation
               // is deferred until navigation
               DeferCreatingView(of: OtherView.self)) {
                Text("Press Here")
            }
        }
    }
}

protocol Constructable {
    init()
}

struct DeferCreatingView<T: View & Constructable>: View {
    var ViewType: T.Type

    init(of type: T.Type) {
        ViewType = type
    }

    var body: some View {
        ViewType.init()     // << create only here
    }
}


struct OtherView: View, Constructable {
    // .. not changed code from first part
}

backup

Answer 2

将导航视图样式 .navigationViewStyle(StackNavigationViewStyle()) 添加到导航视图。它将取消初始化视图模型。请参阅下面您修改后的代码。

import SwiftUI
import Combine

struct ContentView: View {
    var body: some View {
        NavigationView {
            NavigationLink(destination: OtherView()) {
                Text("Press Here")
            }
        }
        .navigationViewStyle(StackNavigationViewStyle())
        // Added navigation style here.
    }
}

struct OtherView: View {

    @ObservedObject var viewModel = ViewModel()

    var body: some View {
        VStack {
            TextField("Something", text: $viewModel.enteredText)
                .textFieldStyle(RoundedBorderTextFieldStyle())

            Button(action: {
                print("Tap")
            }) {
                Text("Tapping")
            }.disabled(!viewModel.isValid)
        }
    }
}

class ViewModel: ObservableObject {

    @Published var enteredText = ""
    var isValid = false

    var cancellable: AnyCancellable?

    init() {
        cancellable = textValidatedPublisher.receive(on: RunLoop.main)
            .assign(to: \.isValid, on: self)
    }

    deinit {
        cancellable?.cancel()
    }

    var textValidatedPublisher: AnyPublisher<Bool, Never> {
        $enteredText.map {
            [=10=].count > 1
        }.eraseToAnyPublisher()
    }


}

视图关闭后，ObservedObject 视图模型仍在内存中

ObservedObject view-model is still in memory after the view is dismissed

ios

swift

swiftui

combine