PreviewProvider和ObservedObject属性

Question

如何为类型为CDObject的CoreData对象设置属性，它有一个名为name: String的属性

我现在的问题是，我不知道如何在PreviewProvider中设置name属性

代码如下：

struct MainView: View {
    @ObservedObject var obj: CDObject
    
    var body: some View {
        Text("Hello, World!")
    }
}

struct MainView_Previews: PreviewProvider {
    static var previews: some View {
        MainView(obj: CDObject())
    }
}

在将其传递给视图之前，我想做一些类似的事情：

let itm = CDObject()
itm.name = "Hello"

Answer 1

如果您在使用CoreData启动新项目时使用Xcode附带的标准PersistenceController，只需添加下面的方法，这样当您在预览中运行时，Xcode就会返回.preview container。

    public static func previewAware() -> PersistenceController{
        //Identifies if XCode is running for previews
        if ProcessInfo.processInfo.environment["XCODE_RUNNING_FOR_PREVIEWS"] == "1"{
            return PersistenceController.preview
        }else{
            return PersistenceController.shared
        }
    }

至于其余的部分，你可以使用类似这样的东西。

import SwiftUI
import CoreData

struct SamplePreviewView: View {
    @ObservedObject var item: Item
    var body: some View {
        Text(item.timestamp?.description ?? "nil")
    }
}
struct SamplePreviewView_Previews: PreviewProvider {
    static let svc = CoreDataPersistenceService()
    static var previews: some View {
        SamplePreviewView(item: svc.addSample())
    }
}

class CoreDataPersistenceService: NSObject {

    var persistenceController: PersistenceController

    init(isTest: Bool = false) {
        if isTest{
            self.persistenceController = PersistenceController.preview
        }else{
            self.persistenceController = PersistenceController.previewAware()
        }
        super.init()
    }
    func addSample() -> Item {
        let object = createObject()
        object.timestamp = Date()
        return object
    }
    //MARK: CRUD methods
    func createObject() -> Item {
        let result = Item.init(context: persistenceController.container.viewContext)
        return result
    }
}