使用FetchRequest时如何在PreviewProvider中显示示例数据 [英] How to show sample data in PreviewProvider when using FetchRequest

问题描述

我有一个这样的SwiftUI视图：

I have an SwiftUI View like this:

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？

What can I do to show some sample data in my preview?

推荐答案

这是一种适应性方法（在 https://stackoverflow.com/a/61495490中早有提出/ 12299030 ），已在Xcode 12 / iOS 14上进行了测试

Here is adapted approach (early proposed in https://stackoverflow.com/a/61495490/12299030), tested with Xcode 12 / iOS 14

该想法是将显式视图+模型与模型提供程序（在本例中为云数据库）分开，因此vi可以设计和制造经过测试（预览）的具有本地或动态构建的模拟模型（不涉及大量的云连接）

The idea is to separate explicit view + model from model provider (in this case cloud database), so view could be designed & tested (previewed) having local or dynamically constructed mock model (not involving heavy cloud connection)

truct 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])
   }
}

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