SwiftUI View和@FetchRequest谓词具有可以更改的变量

Question

我有一个视图，显示团队中使用@Fetchrequest和固定谓词“开发人员”过滤的消息。

struct ChatView: View {

@FetchRequest(
    sortDescriptors: [NSSortDescriptor(keyPath: \Message.createdAt, ascending: true)],
    predicate: NSPredicate(format: "team.name == %@", "Developers"),
    animation: .default) var messages: FetchedResults<Message>

@Environment(\.managedObjectContext)
var viewContext

var body: some View {
    VStack {
        List {
            ForEach(messages, id: \.self) { message in
                VStack(alignment: .leading, spacing: 0) {
                    Text(message.text ?? "Message text Error")
                    Text("Team \(message.team?.name ?? "Team Name Error")").font(.footnote)
                }
            }...

我想使该谓词动态化，以便在用户切换团队时显示该团队的消息。下面的代码给我以下错误

无法在属性初始化程序中使用实例成员'teamName'；属性初始化程序在“自我”可用之前运行

struct ChatView: View {

@Binding var teamName: String

@FetchRequest(
    sortDescriptors: [NSSortDescriptor(keyPath: \Message.createdAt, ascending: true)],
    predicate: NSPredicate(format: "team.name == %@", teamName),
    animation: .default) var messages: FetchedResults<Message>

@Environment(\.managedObjectContext)
var viewContext

...

我可以为此提供一些帮助，但到目前为止我还无法自行解决。

Answer 1

对于 SwiftUI，重要的是 View 结构不会被更改，否则主体将被不必要地调用，在这种情况下@FetchRequest也会访问数据库。SwiftUI 仅使用相等检查视图结构中的更改，如果不相等则调用 body，即，如果任何属性已更改。在 iOS 14 上，即使@FetchRequest使用相同的参数重新创建，也会导致视图结构不同，因此无法通过 SwiftUI 的相等性检查，并导致在正常情况下重新计算主体。@AppStorage并且@SceneStorage也有这个问题，所以我觉得很奇怪，@State大多数人可能先学习的东西没有！无论如何，我们可以使用属性不变的包装器 View 来解决这个问题，这可以阻止 SwiftUI 的差异算法在其轨道上运行：

struct ContentView: View {
    @State var teamName "Team" // source of truth, could also be @AppStorage if would like it to persist between app launches.
    @State var counter = 0
    var body: some View {
        VStack {
            ChatView(teamName:teamName) // its body will only run if teamName is different, so not if counter being changed was the reason for this body to be called.
            Text("Count \(counter)")
        }
    }
}

struct ChatView: View {
    let teamName: String
    var body: some View {
        // ChatList body will be called every time but this ChatView body is only run when there is a new teamName so that's ok.
        ChatList(messages: FetchRequest(sortDescriptors: [NSSortDescriptor(keyPath: \Message.createdAt, ascending: true)], predicate: NSPredicate(format: "team.name = %@", teamName)))
    }
}

struct ChatList : View {
    @FetchRequest var messages: FetchedResults<Message>
    var body: some View {
        ForEach(messages) { message in
             Text("Message at \(message.createdAt!, formatter: messageFormatter)")
        }
    }
}

编辑：有可能使用EquatableView代替包装器来实现相同的事情，View以允许 SwiftUI 对teamName唯一而不是FetchRequestvar进行差异处理。更多信息在这里：https : //swiftwithmajid.com/2020/01/22/optimizing-views-in-swiftui-using-equatableview/

Answer 2

修改后的@FKDev 答案可以工作，因为它会引发错误，我喜欢这个答案，因为它与 SwiftUI 的其余部分保持一致。只需要从 fetch 请求中删除括号。虽然@Antoine Weber 的回答是一样的。

但是我在这两个答案中都遇到了问题，请在下面包含我的答案。这会导致一个奇怪的副作用，其中一些与 fetch 请求无关的行在屏幕上向右移动，然后仅在 fetch 请求数据第一次更改时从左侧返回屏幕。当以默认的 SwiftUI 方式实现获取请求时，不会发生这种情况。

更新： 通过简单地删除获取请求动画参数来修复屏幕外随机行动画的问题。虽然如果你需要那个论点，我不确定一个解决方案。它非常奇怪，因为您期望动画参数仅影响与该获取请求相关的数据。

@Binding var teamName: String

@FetchRequest var messages: FetchedResults<Message>

init() {

    var predicate: NSPredicate?
    // Can do some control flow to change the predicate here
    predicate = NSPredicate(format: "team.name == %@", teamName)

    self._messages = FetchRequest(
    entity: Message.entity(),
    sortDescriptors: [],
    predicate: predicate,
//    animation: .default)
}

Answer 3

另一种可能是：

struct ChatView: View {

@Binding var teamName: String

@FetchRequest() var messages: FetchedResults<Message>

init() {
    let fetchRequest: NSFetchRequest<Message> = Message.fetchRequest()
    fetchRequest.sortDescriptors = NSSortDescriptor(keyPath: \Message.createdAt, ascending: true)
    fetchRequest = NSPredicate(format: "team.name == %@", teamName),
    self._messages = FetchRequest(fetchRequest:fetchRequest, animation: .default)
}

...

Answer 4

有同样的问题，Brad Dillon的评论显示了解决方案：

var predicate:String
var wordsRequest : FetchRequest<Word>
var words : FetchedResults<Word>{wordsRequest.wrappedValue}

    init(predicate:String){
        self.predicate = predicate
        self.wordsRequest = FetchRequest(entity: Word.entity(), sortDescriptors: [], predicate:
            NSPredicate(format: "%K == %@", #keyPath(Word.character),predicate))

    }

在此示例中，您可以在初始化程序中修改谓词。

Answer 5

可能是动态过滤@FetchRequest 的更通用的解决方案。

1?创建自定义DynamicFetchView

import CoreData
import SwiftUI

struct DynamicFetchView<T: NSManagedObject, Content: View>: View {
    let fetchRequest: FetchRequest<T>
    let content: (FetchedResults<T>) -> Content

    var body: some View {
        self.content(fetchRequest.wrappedValue)
    }

    init(predicate: NSPredicate?, sortDescriptors: [NSSortDescriptor], @ViewBuilder content: @escaping (FetchedResults<T>) -> Content) {
        fetchRequest = FetchRequest<T>(entity: T.entity(), sortDescriptors: sortDescriptors, predicate: predicate)
        self.content = content
    }

    init(fetchRequest: NSFetchRequest<T>, @ViewBuilder content: @escaping (FetchedResults<T>) -> Content) {
        self.fetchRequest = FetchRequest<T>(fetchRequest: fetchRequest)
        self.content = content
    }
}

2、如何使用

//our managed object
public class Event: NSManagedObject{
    @NSManaged public var status: String?
    @NSManaged public var createTime: Date?
    ... ...
}

// some view

struct DynamicFetchViewExample: View {
    @State var status: String = "undo"

    var body: some View {
        VStack {
            Button(action: {
                self.status = self.status == "done" ? "undo" : "done"
            }) {
                Text("change status")
                    .padding()
            }

            // use like this
            DynamicFetchView(predicate: NSPredicate(format: "status==%@", self.status as String), sortDescriptors: [NSSortDescriptor(key: "createTime", ascending: true)]) { (events: FetchedResults<Event>) in
                // use you wanted result
                // ...
                HStack {
                    Text(String(events.count))
                    ForEach(events, id: \.self) { event in
                        Text(event.name ?? "")
                    }
                }
            }

            // or this
            DynamicFetchView(fetchRequest: createRequest(status: self.status)) { (events: FetchedResults<Event>) in
                // use you wanted result
                // ...
                HStack {
                    Text(String(events.count))
                    ForEach(events, id: \.self) { event in
                        Text(event.name ?? "")
                    }
                }
            }
        }
    }

    func createRequest(status: String) -> NSFetchRequest<Event> {
        let request = Event.fetchRequest() as! NSFetchRequest<Event>
        request.predicate = NSPredicate(format: "status==%@", status as String)
        // warning: FetchRequest must have a sort descriptor
        request.sortDescriptors = [NSSortDescriptor(key: "createTime", ascending: true)]
        return request
    }
}

通过这种方式，您可以动态更改您的 NSPredicate 或 NSSortDescriptor。

Answer 6

我刚刚遇到了类似的问题，并且发现FetchRequest.Configuration很有帮助。从原始代码中可以看出这一点

@Binding var teamName: String

@FetchRequest(
    sortDescriptors: [NSSortDescriptor(keyPath: \Message.createdAt, ascending: true)],
    predicate: NSPredicate(format: "team.name == %@", "Developers"),
    animation: .default) var messages: FetchedResults<Message>

并且，例如，绑定到 的 TextField teamName，您可以添加一个onChange处理程序来更改 的谓词messages：

TextField("Team Name", text: $teamName)
    .onChange(of: teamName) { newValue in
        messages.nsPredicate = newValue.isEmpty
        ? nil
        : NSPredicate(format: "team.name == %@", newValue)
    }

有关更多信息，请参阅 SwiftUI 的FetchRequest.Configuration的文档，特别是标记为直接设置配置的部分。