我遇到了 SwiftUI Preview 有时无法工作的问题(但没有给我任何错误,只是一个空白的画布)。
我已经缩小了问题范围 - 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做什么。
预览适用于以下代码:
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 或两者添加一些测试数据?尝试了一些东西,但我无法使其发挥作用。
如果您将预览中的呼叫更改为
ListView(startDateOfMonth: Date().addingTimeInterval(-86400 * 30)).environment(\.managedObjectContext, context)
您的 startDateOfMonth 变量通过以下行设置为今天之前的 30 天
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