ios - SwiftUI - NavigationView 中的内存泄漏

Question

我正在尝试向模态呈现的 View 导航栏添加一个关闭按钮。但是，解雇后，我的 View 模型 初始化 方法永远不会被调用。我发现问题在于它捕获了 自我 在 导航栏项 的。我不能只通过 weak self在 导航栏项 的 Action ，因为 View 是一个结构，而不是一个类。这是一个有效的问题还是只是缺乏知识？

struct ModalView: View {

    @Environment(\.presentationMode) private var presentation: Binding<PresentationMode>
    @ObservedObject var viewModel: ViewModel

    var body: some View {

        NavigationView {
            Text("Modal is presented")
            .navigationBarItems(leading:
                Button(action: {
                    // works after commenting this line
                    self.presentation.wrappedValue.dismiss()
                }) {
                    Text("close")
                }

            )
        }
    }
}

Answer 1

您不需要在自己的 View 中拆分关闭按钮。您可以通过添加 capture list 来解决此内存泄漏。到 NavigationView 的关闭:这将打破保留您的 viewModel 的引用周期.

您可以在 Playground 中复制/粘贴此示例代码以查看它是否解决了问题(Xcode 11.4.1，iOS Playground)。

import SwiftUI
import PlaygroundSupport

struct ModalView: View {
    @Environment(\.presentationMode) private var presentation
    @ObservedObject var viewModel: ViewModel

    var body: some View {
        // Capturing only the `presentation` property to avoid retaining `self`, since `self` would also retain `viewModel`.
        // Without this capture list (`->` means `retains`):
        // self -> body -> NavigationView -> Button -> action -> self
        // this is a retain cycle, and since `self` also retains `viewModel`, it's never deallocated.
        NavigationView { [presentation] in
            Text("Modal is presented")
                .navigationBarItems(leading: Button(
                    action: {
                        // Using `presentation` without `self`
                        presentation.wrappedValue.dismiss()
                },
                    label: { Text("close") }))
        }
    }
}

class ViewModel: ObservableObject { // << tested view model
    init() {
        print(">> inited")
    }

    deinit {
        print("[x] destroyed")
    }
}

struct TestNavigationMemoryLeak: View {
    @State private var showModal = false
    var body: some View {
        Button("Show") { self.showModal.toggle() }
            .sheet(isPresented: $showModal) { ModalView(viewModel: ViewModel()) }
    }
}

PlaygroundPage.current.needsIndefiniteExecution = true
PlaygroundPage.current.setLiveView(TestNavigationMemoryLeak())