Swift 6迁移时SwiftData Fetch更新：KeyPath不兼容Sendable问题

我被这个问题困扰一周了，无奈求助。我的项目基于Swift 5，最近把“Strict Concurrency Checking”设置为“Complete”，想逐步迁移到Swift 6同时避免全面崩溃。原本正常工作的SwiftData交互ViewModel出现了警告：在sortBy: [SortDescriptor(\.date)]这一行提示：

Type 'KeyPath<CachedMessage, Date>' does not conform to the 'Sendable' protocol; this is an error in the Swift 6 language mode.

相关代码如下：

import SwiftUI
import SwiftData

@MainActor final class MyViewModel: ObservableObject {
    private(set) var modelContext: ModelContext?
    private var tempMessagesCache: [Message] = []
    @Published var messages: [Message] = []

    func setModelContext(_ modelContext: ModelContext) {
        self.modelContext = modelContext
        self.modelContext!.autosaveEnabled = false
    }

    func fetchCachedMessages(for chatGroupId: String) {
        do {
            let descriptor = FetchDescriptor<CachedMessage>(
                predicate: #Predicate { $0.chatGroupId == chatGroupId }, 
                sortBy: [SortDescriptor(\.date)]
            )
            if let messages = try modelContext?.fetch(descriptor) {
                tempMessagesCache = messages.map({
                    Message(cachedMessage: $0)
                })
            }
        } catch {
            // Handle error
        }
    }
}

struct Message {
    let id: String
    let chatGroupId: String
    let date: Date
    // more metadata here

    // other initializers here

    init(cachedMessage: CachedMessage) {
        self.id = cachedMessage.id
        self.chatGroupId = cachedMessage.chatGroupId
        self.date = cachedMessage.date
        // more metadata here
    }

}

// 我已经尝试让CachedMessage遵循Sendable协议，但警告依然存在。
@Model final class CachedMessage {
    @Attribute(.unique) private(set) var id: String
    private(set) var chatGroupId: String
    private(set) var date: Date
    // more metadata here

    // other initializers here

    init(message: Message) {
        self.id = message.id
        self.chatGroupId = message.chatGroupId
        self.date = message.date
        // more metadata here
    }

}

我已经尝试让CachedMessage遵循Sendable协议，但警告依然存在。请问在Swift 6中，如何通过SwiftData ModelContext以该方式获取数据？

1. 显式确认KeyPath的Sendable一致性 #

问题根源在于Strict Concurrency模式下，编译器无法自动推断KeyPath<CachedMessage, Date>符合Sendable。可以通过显式标注KeyPath的类型来解决：

func fetchCachedMessages(for chatGroupId: String) {
    guard let modelContext = modelContext else { return }
    do {
        // 显式标注KeyPath为Sendable类型
        let dateKeyPath = \.date as Sendable KeyPath<CachedMessage, Date>
        let descriptor = FetchDescriptor<CachedMessage>(
            predicate: #Predicate { $0.chatGroupId == chatGroupId }, 
            sortBy: [SortDescriptor(dateKeyPath)]
        )
        let cachedMessages = try modelContext.fetch(descriptor)
        tempMessagesCache = cachedMessages.map(Message.init)
    } catch {
        print("获取缓存消息失败：\(error)")
    }
}

2. 确保CachedMessage完全符合Sendable #

虽然@Model修饰的类默认应该符合Sendable，但显式声明可以帮助编译器确认一致性，同时检查所有属性是否都是Sendable类型（String、Date均为原生Sendable类型，无需额外处理）：

@Model final class CachedMessage {
    // 现有代码...
}

// 显式添加Sendable协议扩展
extension CachedMessage: Sendable {}

3. 优化ViewModel的线程安全 #

由于ViewModel标记了@MainActor，所有ModelContext操作已限制在主线程执行，符合Sendable要求。可以进一步优化代码避免强制解包，提升安全性：

func setModelContext(_ modelContext: ModelContext) {
    self.modelContext = modelContext
    modelContext.autosaveEnabled = false
}

内容的提问来源于stack exchange，提问作者Ryan Kanno