GroupBy 在 SwiftUI 中使用 @FetchRequest

Question

我有一个 SwiftUI 视图，如下所示：

@FetchRequest(
    entity: Message(),
    sortDescriptors: [NSSortDescriptor(keyPath: \Message.date, ascending: false)]
) var allMessages: FetchedResults< Message.date >

var body: some View {
    NavigationView {
        List(self.allMessages) { message in
            Text(message.text)
        }
        .navigationBarTitle(Text("Last Messages"))
    }
}

我希望 FetchRequest 仅返回每个用户的最后一条消息。我需要使用分组依据。有办法使用吗@FetchRequest？如果没有，你会怎么做？

Answer 1

一步步：

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1. 创建 NSFetchRequest
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2. 根据您选择的条件指定要过滤\n的 NSPredicate
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3. 将 resultType 设置为“.DictionaryResultType”（“Group By”必需）
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4. 设置要包含在提取中的属性。
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5. 设置分组依据的属性
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6. 创建一个 NSSortDescriptor 以按名称降序排序
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7. 最后，您可以使用初始值设定项“@FetchRequest(fetchRequest:”) 创建@FetchRequest
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class TODOItem: NSManagedObject, Identifiable {\n    @NSManaged public var name: String?\n    @NSManaged public var notes: String?\n}\n\nextension TODOItem {\n    static func allTODOItem() -> NSFetchRequest<TODOItem> {\n        //1\xc2\xba\n        let fetchRequest = NSFetchRequest<TODOItem>(entityName: "TODOItem")\n        //2\xc2\xba\n        //fetchRequest.predicate = NSPredicate(format: "", ...)\n        //3\xc2\xba\n        fetchRequest.resultType = .dictionaryResultType\n        //4\xc2\xba\n        // To get computed props you can use NSExpression (for example the sum of something)\n        fetchRequest.propertiesToFetch = ["name", "notes"]\n        //5\xc2\xba\n        fetchRequest.propertiesToFetch = ["name"]\n        //6\xc2\xba\n        let sort = NSSortDescriptor(key: "name", ascending: false)\n        fetchRequest.sortDescriptors = [sort]\n\n        return fetchRequest\n    }\n}\n\nstruct Home : View {\n    @FetchRequest(fetchRequest: TODOItem.allTODOItem()) var items: FetchedResults<TODOItem>\n\n    var body: some View {\n        Text("Home")\n    }\n}\n

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