关闭视图后，ObservedObject视图模型仍在内存中 [英] ObservedObject view-model is still in memory after the view is dismissed

问题描述

我在SwiftUI和Combine中的内存管理遇到了麻烦.

I am having some trouble with memory management in SwiftUI and Combine.

例如，如果我有一个NavigationView，然后导航到带有TextField的详细信息视图，然后在TextField中输入一个值，然后单击后退"按钮，则下次我转到该视图时，TextField具有先前输入的值

For example, if I have a NavigationView and then navigate to a detail view with a TextField, and enter a value in the TextField and tap on the back button, next time I go to that view the TextField has the previously entered value.

我注意到在取消了详细视图后，视图模型仍在内存中，这可能就是TextField仍然保留值的原因.

I noticed that the view-model is still in memory after the detail view is dismissed, and that's probably why the TextField still holds a value.

在UIKit中，当关闭ViewController时，将释放视图模型，然后在显示ViewController时再次创建它.这里似乎不是这种情况.

In UIKit, when dismissing a ViewController, the view-model will be deallocated and then created again when the ViewController is presented. This seems to not be the case here.

我为此问题附了一些最低限度的可复制代码.

I attach some minimum reproductible code for this issue.

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 {
            $0.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，就会调用SomeOtherView的视图模型中的deinit.谁能解释一下为什么会这样吗?

I also noticed that, if for example, I add another view, let's say SomeOtherView after OtherView, then each time I type in the TextField from OtherView, then the deinit from SomeOtherView's view-model is called. Can anyone please also explain why this happens?

推荐答案

此外，我注意到，如果我对ContetView进行了更改并且重新评估了该视图，那么我将在内存中拥有两个ViewModels

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

这是由于ViewModel中的交叉引用，这里是固定的变体

It is due to cross-reference in ViewModel, here is fixed variant

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 {
            $0.count > 1
        }.eraseToAnyPublisher()
    }
}

-

更新:

导航时是否可以实例化OtherView?

is there a way to instantiate OtherView when navigating?

这是一个解决方案(已通过Xcode 11.4/iOS 13.4测试)，但这只是半完成，因为一旦创建，它将一直有效直到导航链接被重新验证(即，背面保留在内存中，直到下一次导航) )

Here is a solution (tested with Xcode 11.4 / iOS 13.4), but this is only half-a-deal, because once created it will be alive until navigation link revalidated (ie. on back it remains in memory until next navigate)

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
}

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