ios - SwiftUI Preview 不适用于 FetchRequest 和 Init

Question

我遇到 SwiftUI Preview 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。但我不知道下一步该怎么做。

预览适用于此代码:

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)
    }
}

预览不适用于此代码:

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)
    }
}

我想我需要为这个 init 或 fetchRequest 或两者添加一些测试数据？尝试了一些东西但我无法让它工作。

Answer 1

如果您将预览中的调用更改为

，则代码可以正常工作

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

您的 startDateOfMonth var 通过此行设置为今天之前 30 天