SwiftUI预览画布和核心数据 [英] SwiftUI Preview canvas and Core Data

问题描述

预览画布崩溃了，但是在模拟器中一切正常。我假设它与@ObservedObject和@Fetchrequest有关...

Preview canvas is is crashing but in simulator everything working fine. I assuming it related to @ObservedObject and @Fetchrequest...

在这里使用CoreData预览ContentView

不起作用

 import SwiftUI
 import CoreData

struct TemplateEditor: View {

@Environment(\.managedObjectContext) var managedObjectContext

@FetchRequest(
    entity: GlobalPlaceholders.entity(),
    sortDescriptors: [
        NSSortDescriptor(keyPath: \GlobalPlaceholders.category, ascending: false),
    ]
) var placeholders: FetchedResults<GlobalPlaceholders>


@ObservedObject var documentTemplate: Templates
@State private var documentTemplateDraft = DocumentTemplateDraft()
@Binding var editing: Bool


var body: some View {

    VStack(){
        HStack(){
            cancelButton
            Spacer()
            saveButton
        }.padding()
        addButton
        ForEach(placeholders)  {placeholder in
            Text(placeholder.name)
        }
        TextField("Title", text: $documentTemplateDraft.title)

        TextField("Body", text: $documentTemplateDraft.body)
            .padding()
            .frame(width: 100, height:400)
        Spacer()
    }

...




}
struct TemplateEditor_Previews: PreviewProvider {
    static var previews: some View {

    let managedObjectContext = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
    let request = NSFetchRequest<NSFetchRequestResult>(entityName: "Templates")
    request.sortDescriptors = [NSSortDescriptor(keyPath: \Templates.created, ascending: false)]
    let documentTemplate = try! managedObjectContext.fetch(request).first as! Templates


    return TemplateEditor(documentTemplate: documentTemplate, editing: .constant(true)).environment(\.managedObjectContext, managedObjectContext).environmentObject(documentTemplate)

    }
}

预期会生成预览

推荐答案

如果没有数据，我不确定您的try行是否可以工作。

I'm not sure if your try line will work if there is no data.

let documentTemplate = try! managedObjectContext.fetch(request).first as! Templates

为了使我的工作正常，我创建了一个要使用的测试项目。像这样：

To get mine to work I created a test Item to use. Like this:

struct DetailView_Previews: PreviewProvider {
    static var previews: some View {
        let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
//Test data
        let newEvent = Event.init(context: context)
        newEvent.timestamp = Date()
        return DetailView(event: newEvent).environment(\.managedObjectContext, context)
    }
}

我还注意到，我需要较早的托管CoreData视图的tabView中的.environment（.managedObjectContext，context）代码。否则预览将失败。

I've also noticed that I needed the .environment(.managedObjectContext, context) code in an earlier tabView that hosted the CoreData views or the preview would fail.

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