SwiftUI View 和 @FetchRequest 谓词，带有可以更改的变量

Question

我有一个视图显示团队中使用带有固定谓词“开发人员”的@Fetchrequest 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)
                }
            }...

我想让这个谓词动态化，以便在用户切换团队时显示该团队的消息。下面的代码给了我以下错误

不能在属性初始化器中使用实例成员“teamName”；属性初始化器在“self”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

...

我可以使用一些帮助来解决这个问题，到目前为止我无法自己解决这个问题。

Answer 1

遇到了同样的问题，Brad Dillon 的评论给出了解决方案：

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))

    }

在本例中，您可以修改初始化程序中的谓词。

Answer 2

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 3

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。只需要从获取请求中删除括号。虽然@Antoine Weber 的回答是一样的。

但是我对这两个答案都遇到了问题，请在下面包括我的答案。这会导致一个奇怪的副作用，其中一些与获取请求无关的行仅在获取请求数据第一次更改时才在屏幕右侧显示动画，然后从左侧返回屏幕。当获取请求以默认的 SwiftUI 方式实现时，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，修复了随机行在屏幕外动画的问题。尽管如果您需要该论点，我不确定解决方案。这很奇怪，因为您认为动画参数只会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 4

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 5

对于 SwiftUI，重要的是 View 结构似乎不会被更改，否则 body 将被不必要地调用，在 @FetchRequest 的情况下也会命中数据库。 SwiftUI 仅使用相等来检查 View 结构中的更改，如果不相等，即调用 body，即是否有任何属性已更改。在 iOS 14 上，即使 @FetchRequest 使用相同的参数重新创建，它也会导致 View 结构不同，从而导致 SwiftUI 的相等性检查失败，并导致在正常情况下不会重新计算正文。 @AppStorage 和 @SceneStorage 也有这个问题，所以我觉得奇怪的是大多数人可能首先学习的 @State 没有！无论如何，我们可以使用属性不变的包装 View 来解决这个问题，这可以阻止 SwiftUI 的差异算法：

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：可以使用EquatableView 而不是包装器View 来实现相同的目的，以允许SwiftUI 仅在teamName 而不是FetchRequest var 上进行差异化。更多信息在这里： https://swiftwithmajid.com/2020/01/22/optimizing-views-in-swiftui-using-equatableview/