core-data - 使用 FetchRequest 时如何在 PreviewProvider 中显示示例数据

Question

我有一个这样的 SwiftUI View :

import SwiftUI

struct ReView: View {
    @Environment(\.managedObjectContext) var moc
    @FetchRequest(
        entity: Re.entity(),
        sortDescriptors: [
            NSSortDescriptor(keyPath: \Re.name, ascending: false)
        ]
    ) var entities: FetchedResults<Re>
    
    var body: some View {
        NavigationView {
            List(entities, id: \.self) { entity in
                Text(entity.name ?? "Unknown")
            }
        }
    }
}

struct ReView_Previews: PreviewProvider {
    static var previews: some View {
            Group {
               
                ReView()
                    .environment(\.managedObjectContext, PersistentCloudKitContainer.persistentContainer.viewContext)
            }
        }
}

如何在预览中显示一些示例数据？

Answer 1

这是经过调整的方法(早期在 https://stackoverflow.com/a/61495490/12299030 中提出)，已使用 Xcode 12/iOS 14 进行测试
这个想法是将显式 View +模型与模型提供者(在这种情况下为云数据库)分开，因此可以设计和测试(预览)具有本地或动态构建的模拟模型(不涉及大量云连接)的 View

struct ReView: View {
    @Environment(\.managedObjectContext) var moc
    @FetchRequest(
        entity: Re.entity(),
        sortDescriptors: [
            NSSortDescriptor(keyPath: \Re.name, ascending: false)
        ]
    ) var entities: FetchedResults<Re>
    
    var body: some View {
        ReEntitiesView(entities: entities)
    }
}

struct ReEntitiesView<Results:RandomAccessCollection>: View where Results.Element == Re {
    let entities: Results

    var body: some View {
        NavigationView {
            List(entities, id: \.self) { entity in
                Text(entity.name ?? "Unknown")
            }
        }
    }
}

struct ReView_Previews: PreviewProvider {
    static let entity = NSManagedObjectModel.mergedModel(from: nil)?.entitiesByName["Re"]

    static var previews: some View {
       let object = Re(entity: entity!, insertInto: nil)
       object.name = "Test Name"

       return ReEntitiesView(entities: [object])
   }
}