SwiftUI 中的动态过滤器（谓词）

Question

我正在使用 SwiftUI 和 CoreData 为 iOS 编写应用程序。我正在尝试解决一个问题几天。如何根据用户输入在 SwiftUI 中使用动态变化的谓词制作动态过滤器？

我已经按照这个教程学习了动态过滤器和CoreData：https://www.hackingwithswift.com/quick-start/ios-swiftui/dynamically-filtering-fetchrequest-with-swiftui

经过一些小改动后，我得到了以下代码。 ContentView.swift：

import SwiftUI

struct ContentView: View {

    @Environment(\.managedObjectContext) var moc
    @State var lastNameFilter = "A"

    var body: some View {

        VStack {
            FilteredList(predicate: lastNameFilter)

            Button("Add Examples") {
                let taylor = Singer(context: self.moc)
                taylor.firstName = "Taylor"
                taylor.lastName = "Swift"

                let ed = Singer(context: self.moc)
                ed.firstName = "Ed"
                ed.lastName = "Sheeran"

                let adele = Singer(context: self.moc)
                adele.firstName = "Adele"
                adele.lastName = "Adkins"

                try? self.moc.save()
            }

            Button("Show A") {
                self.lastNameFilter = "A"
            }

            Button("Show S") {
                self.lastNameFilter = "S"
            }
        }

    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

FilteredList.swift：

import CoreData
import SwiftUI

struct FilteredList: View {

    var predicate:String
    var fetchRequest: FetchRequest<Singer>
    var singers: FetchedResults<Singer>{fetchRequest.wrappedValue}

    var body: some View {
        List(singers, id: \.self) { singer in
            Text("\(singer.firstName ?? "Unknown") \(singer.lastName ?? "Unknown")")
        }
    }

    init(predicate: String) {
        self.predicate = predicate
        self.fetchRequest = FetchRequest<Singer>(entity: Singer.entity(), sortDescriptors: [], predicate: NSPredicate(format: "lastName BEGINSWITH %@", predicate))
    }

}

//struct FilteredList_Previews: PreviewProvider {
//    static var previews: some View {
//    }
//}

我还有一个名为 Singer 的实体，该实体有 2 个属性：firstName 和 lastName，它们都是字符串。 上面的示例在模拟器中似乎可以正常工作，但在 Xcode 中使用 Preview 时会导致应用崩溃。

我将不胜感激，例如：

• 指出要更改示例代码的哪一部分以避免预览中的错误
• 在 SwiftUI 中使用动态谓词的另一种方式的简单示例
• 链接到有关 SwiftUI 中的动态过滤器的教程

Answer 1

1. 要使 ContentView 的预览正常工作，您应该编写如下内容：

static var previews: some View {
    let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
    let taylor = Singer(context: context)
    taylor.firstName = "Taylor"
    taylor.lastName = "Swift"
    let ed = Singer(context: context)
    ed.firstName = "Ed"
    ed.lastName = "Sheeran"
    let adele = Singer(context: context)
    adele.firstName = "Adele"
    adele.lastName = "Adkins"
    return ContentView().environment(\.managedObjectContext, context)
}

1. 您可以实现更通用的 FilteredList 类型，其中您只提供列表的谓词（可选的排序描述符）。示例：

struct FilteredList<T: NSManagedObject, Content: View>: View {
    var fetchRequest: FetchRequest<T>
    var items: FetchedResults<T> { fetchRequest.wrappedValue }

    let content: (T) -> Content

    var body: some View {
        List(items, id: \.self) { item in
            self.content(item)
        }
    }

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

将此新的FilteredList 类型与@State private var predicate: NSPredicate? 一起使用，而不是@State var lastNameFilter = "A"。当需要新过滤时，只需将此私有 @State 属性设置为新谓词，列表就会相应更新。

具体用法是：

FilteredList(predicate: predicate) { (singer: Singer) in
    Text("\(singer.firstName ?? "") \(singer.lastName ?? "")")
}

FilteredList 预览：

static var previews: some View {
    FilteredList(predicate: nil) { (singer: Singer) in
        Text("\(singer.firstName ?? "") \(singer.lastName ?? "")")
    }.environment(\.managedObjectContext, (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext)
}

Answer 2

这是另一种方式：

import CoreData
import SwiftUI

struct FilteredList: View {

    @FetchRequest var singers: FetchedResults<Singer>

    init(lastName: String) {
        _singers = FetchRequest(sortDescriptors: [], predicate: NSPredicate(format: "lastName BEGINSWITH %@", lastName))
    }

    var body: some View {
        List(singers) { singer in
            Text("\(singer.firstName ?? "Unknown") \(singer.lastName ?? "Unknown")")
        }
    }
}

请注意，每次 View 初始化且 FetchRequest 初始化时，都会命中数据库，因此您可能需要将 fetch 提升到超级视图中。