SwiftUI中@State变量attachmentID无法保留值的问题求助

问题根源 #

你使用.sheet(isPresented:)绑定弹窗时，SwiftUI会在视图初始渲染阶段就创建好Sheet的内容，而非弹窗触发时才初始化。所以即便你在按钮点击时更新了attachmentID，Sheet内的AttachmentView拿到的始终是初始值0。

修复方案 #

改用.sheet(item:)绑定弹窗，它会在绑定的可选值变为非nil时触发弹窗，且每次都会重新初始化Sheet内容，确保拿到最新的ID值。具体修改步骤如下：

1. 将attachmentID改为可选Int类型，初始值设为nil：

@State private var attachmentID: Int? = nil

2. 删除原有的showAttachmentSheet变量，按钮点击时直接赋值attachmentID：

Button {
    attachmentID = file.id
    debugPrint(attachmentID)
} label: {
    TextAndImageCell(title: file.filename, imageName: "paperclip")
}

3. 替换原.sheet(isPresented:)为.sheet(item:)，在Sheet中使用解包后的ID：

.sheet(item: $attachmentID) { id in
    AttachmentView(attachmentID: id, ticketID: currentPost!.id)
}

额外建议：代码中currentPost!.id用了强制解包，容易引发崩溃，建议改成可选绑定：

if let currentPost = currentPost {
    // 原List、Sheet代码，ticketID使用currentPost.id
}

完整修改后的代码片段 #

struct PostView: View {
    @State private var attachmentID: Int? = nil
    @State private var showSheet = false // 补充原代码未定义的showSheet
    
    var currentPost: Post?
    
    var body: some View {
        if let currentPost = currentPost {
            List(firstPost.postscontents.reversed(), id:\.id) { post in
                Text(post.contents)

                if !post.attachment.isEmpty {
                    ForEach(post.attachment, id: \.id) { file in
                        HStack{
                            Spacer()
                            Button {
                                attachmentID = file.id
                                debugPrint(attachmentID)
                            } label: {
                                TextAndImageCell(title: file.filename, imageName: "paperclip")
                            }
                            .padding(.horizontal, 10)
                            .padding(.vertical, 4)
                            .background(.gray)
                            .clipShape(Capsule())
                            
                        }
                    }
                }
            }
            .listStyle(.inset)
            .listItemTint(.lightBlue)
        
            .sheet(item: $attachmentID) { id in
                AttachmentView(attachmentID: id, ticketID: currentPost.id)
            }
        }

        Button {
            showSheet = true
        } label: {
            Image(systemName: "plus.message.fill")
                .font(.system(size: 40))
                .padding()
        }
    }
}

// 让Int遵循Identifiable协议，适配sheet(item:)的要求
extension Int: Identifiable {
    public var id: Int { self }
}

若不想让Int遵循Identifiable，也可自定义包装结构体：

struct AttachmentIdentifier: Identifiable {
    let id: Int
}

随后将attachmentID改为@State private var attachmentID: AttachmentIdentifier? = nil，按钮点击时赋值attachmentID = AttachmentIdentifier(id: file.id)，Sheet内使用id.id即可。

内容的提问来源于stack exchange，提问作者BobC