SwiftUI

Question

我正在尝试向以模态方式呈现的视图的导航栏添加一个关闭按钮。但是，在关闭之后，我的视图模型 deinit 方法永远不会被调用。我发现问题出在它在 navigationBarItem 中捕获 self 的地方。我不能只在 navigationBarItem 的操作中传递 weak self，因为 View 是一个结构，而不是 class。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

我推荐设计级解决方案，即。将导航栏项目分解为单独的视图组件会破坏导致泄漏的不需要的循环引用。

测试 Xcode 11.4 / iOS 13.4 - ViewModel 按预期销毁。

完整的测试模块代码如下：

struct CloseBarItem: View { // separated bar item with passed binding
    @Binding var presentation: PresentationMode
    var body: some View {
        Button(action: {
            self.presentation.dismiss()
        }) {
            Text("close")
        }
    }
}

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

    var body: some View {

        NavigationView {
            Text("Modal is presented")
            .navigationBarItems(leading: 
                CloseBarItem(presentation: presentation)) // << decompose
        }
    }
}

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()) }
    }
}

struct TestNavigationMemoryLeak_Previews: PreviewProvider {
    static var previews: some View {
        TestNavigationMemoryLeak()
    }
}

Answer 2

您不需要在自己的视图中拆分关闭按钮。您可以通过向 NavigationView 的闭包添加 capture list 来解决此内存泄漏：这将打破保留 viewModel.

的引用循环

您可以在 playground 中 copy/paste 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())

Answer 3

我的解决方案是

.navigationBarItems(
    trailing: self.filterButton
)

..........................................

var filterButton: some View {
    Button(action: {[weak viewModel] in
        viewModel?.showFilter()
    },label: {
        Image("search-filter-icon").renderingMode(.original)
    })
}

Answer 4

由于 navigationBarItems 并将我的视图模型传递给我用作栏项的视图，我遇到了严重的内存泄漏。

深入研究，我了解到 navigationBarItems is deprecated

我有

        .navigationBarItems(trailing:
            AlbumItemsScreenNavButtons(viewModel: viewModel)
        )

替换为toolbar。

我现在的用法是这样的：

        .toolbar {
            ToolbarItemGroup(placement: .navigationBarTrailing) {
                AlbumItemsScreenNavButtons(viewModel: viewModel)
            }
        }

SwiftUI - NavigationView 中的内存泄漏

SwiftUI - memory leak in NavigationView

memory-leaks

ios

swift

combine