由于您正在为dayLogs使用@Query，因此已经获取了值。
如果你想用一些 predicate 来获取值，最好使用

@Query(filter: Predicate, sort: KeyPath, order: SortOrder, animation: Animation)

或者：

@Query(<descriptor: FetchDescriptor>, animation: Animation)

如果要在获取数据后进行筛选，请使用简单的.filter()

我试过这个逻辑。我觉得挺好的。

extension Collection where Element == DayLog {

    /// Today Log filter.
    func todayLog() -> [DayLog] {
        let currentDate = Date()

        let from = currentDate.startOfDay
        let to = currentDate.endOfDay

        let todayPredicate = #Predicate<DayLog> { from < $0.startDate && $0.startDate < to }

        do {
            let result = try filter(todayPredicate)
            return result

        } catch let error {
            print("Failed to filter today record from DayLog. Error: \(error.localizedDescription)")
        }
        return []
    }
}

对于Date函数：

extension Date {

    var startOfDay: Date {
        return Calendar.current.startOfDay(for: self)
    }

    var endOfDay: Date {
        var components = DateComponents()
        components.day = 1
        components.second = -1
        return Calendar.current.date(byAdding: components, to: startOfDay)!
    }
}

调用方式：

let todayLog = self.dayLogs.todayLog().last