SwiftUI 无法添加 onChange 事件

Question

我有一个显示消息列表的视图。每当添加新消息时，我都想滚动到底部。我正在尝试向 ForEach 添加一个 onChange 事件，但这会破坏我的代码并出现一些奇怪的错误：

Referencing instance method 'onChange(of:perform:)' on 'Array' requires that 'Message' conform to 'Equatable'

有时

The compiler is unable to type-check this expression in reasonable time; try breaking up the expression into distinct sub-expressions

当我删除 onChange 时，一切都符合。

这里是 Message 模型：

struct Message: Identifiable {
    var id = UUID()
    var text: String
    var createdAt: Date = Date()
    var senderID: String
    var seen: Bool
    
    init(dictionary: [String: Any]) {
        self.text = dictionary["text"] as? String ?? ""
        self.senderID = dictionary["senderID"] as? String ?? ""
        self.seen = dictionary["seen"] as? Bool ?? false
    }
}

我做错了什么？

`ForEach

struct MessagesView: View {
    @ObservedObject var messagesViewModel: MessagesViewModel
    
    var body: some View {
        VStack {
            ScrollView(.vertical, showsIndicators: false, content: {
                ScrollViewReader { reader in
                    VStack(spacing: 20) {
                        ForEach(messagesViewModel.request.messages) { message in
                            Text(message.text)
                                .id(message.id)
                        }
                        .onChange(of: messagesViewModel.request.messages) { (value) in
                            reader.scrollTo(value.last?.id)
                        }
                    }
                }
            })
        }
    }
}

Answer 1

如果您的 messages 是 @Published，则以下内容应该有效（正如我在 Make a list scroll to bottom with SwiftUI 中测试的那样）

struct MessagesView: View {
    @ObservedObject var messagesViewModel: MessagesViewModel
    
    var body: some View {
        VStack {
            ScrollView(.vertical, showsIndicators: false, content: {
                ScrollViewReader { reader in
                    VStack(spacing: 20) {
                        ForEach(messagesViewModel.request.messages) { message in
                            Text(message.text)
                                .id(message.id)
                        }
                        .onReceive(messagesViewModel.request.$messages) { (value) in
                           guard !value.isEmpty else { return } 
                           reader.scrollTo(value.last!.id)
                        }
                    }
                }
            })
        }
    }
}

backup

SwiftUI 无法添加 onChange 事件

SwiftUI can not add onChange event

ios

swift

swiftui

xcode12