Question

我有一个视图，显示团队中的消息，这些消息使用@Fetchrequest和固定谓词'Developers'进行过滤。

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)
                }
            }...

我想使此谓词动态化，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

不能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

有同样的问题，布拉德·狄龙（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 2

可能是用于动态过滤@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，使用方法

//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NSPredicate或NSSortDescriptor。

Answer 3

使用SwiftUI，重要的一点是不要更改View结构，否则将不必要地调用主体，在@FetchRequest的情况下，主体也会命中数据库。 SwiftUI仅使用相等性检查View结构中的更改，如果不相等则调用body。在iOS 14中，即使使用相同的参数重新创建@FetchRequest，它也会导致View结构不同，从而导致SwiftUI的相等性检查失败并导致不必要地重新计算主体。 @AppStorage和@SceneStorage也存在此问题，因此我感到奇怪的是，大多数人可能首先学习的@State却没有！无论如何，我们可以使用具有不变属性的包装视图来解决此问题，这可以停止SwiftUI的差异算法在其轨迹中：

struct ContentView: View {
    @State var teamName "Team"
    @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 {
    var 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)")
        }
    }
}

Answer 4

另一种可能性是：

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 5

修改后的@FKDev答案可以正常工作，因为它会引发错误，我喜欢这个答案，因为它的简洁性和与SwiftUI其余部分的一致性。只需从获取请求中删除括号即可。尽管@Antoine Weber的答案是一样的。

但是我在两个答案上都遇到了问题，请在下面列出。这会导致一个奇怪的副作用，即与获取请求无关的某些行会在屏幕外向右移动，然后仅在第一次获取请求数据发生更改时才从左向屏幕重新显示。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，解决了随机行在屏幕外动画的问题。尽管如果您需要该参数，我不确定是否有解决方案。正如您所期望的那样，animation参数只会影响与该提取请求相关的数据。

@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)
}

SwiftUI View和@FetchRequest谓词具有可以更改的变量

5 个答案: