Question

我有一个SwiftUI预览有时无法正常工作的问题（但没有给我任何错误，只是一个空白的Canvas）。我已经缩小了问题的范围-当我将fetchRequest与init一起使用时，它不起作用。但是我不知道下一步该怎么做。

预览使用以下代码：

import SwiftUI

struct ListView: View {

    var fetchRequest = FetchRequest<NPBooking>(entity: NPBooking.entity(), sortDescriptors: [])
    var bookings: FetchedResults<NPBooking> { fetchRequest.wrappedValue }

    var body: some View {
        ForEach(bookings, id: \.self) { booking in
            Text("item")
        }
    }

}

struct ListView_Previews: PreviewProvider {
    static var previews: some View {
        let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
        //Test data
        let testBooking = NPBooking.init(context: context)
        testBooking.date = Date()
        testBooking.name = "name"
        return ListView().environment(\.managedObjectContext, context)
    }
}

预览不适用于以下代码：

import SwiftUI

struct ListView: View {

    var fetchRequest: FetchRequest<NPBooking>
    var bookings: FetchedResults<NPBooking> { fetchRequest.wrappedValue }

    var body: some View {
        ForEach(bookings, id: \.self) { booking in
            Text("item")
        }
    }

    init(startDateOfMonth: Date) {
        fetchRequest = FetchRequest<NPBooking>(entity: NPBooking.entity(), sortDescriptors: [
            NSSortDescriptor(keyPath: \NPBooking.date, ascending: true)
        ], predicate: NSPredicate(format: "date >= %@", startDateOfMonth as NSDate))
    }

}

struct ListView_Previews: PreviewProvider {
    static var previews: some View {
        let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
        //Test data
        let testBooking = NPBooking.init(context: context)
        testBooking.date = Date()
        testBooking.name = "name"
        return ListView(startDateOfMonth: Date()).environment(\.managedObjectContext, context)
    }
}

我想我需要为此init或fetchRequest或这两者添加一些测试数据？尝试了几件事，但我无法使其正常工作。

Answer 1

如果您将预览中的通话更改为

，则该代码可以正常工作

 ListView(startDateOfMonth: Date().addingTimeInterval(-86400 * 30)).environment(\.managedObjectContext, context)

此行将您的startDateOfMonth变量设置为今天前30天

SwiftUI预览不适用于FetchRequest和Init

1 个答案: