Question

我遇到了一个我很难理解的奇怪问题。我编写了一些代码来创建一个可调用的observable。它编译得很好，但是一旦我为它指定了一个调度程序，它就会改变返回类型并且不会编译。

这是没有subscribeOn（编译）的代码：

/**
 * Gets all the room bookings for the specified day
 */
override fun getRoomBookingsForDay(date: Date): Observable<Collection<Model.RoomBooking>> =
        Observable.fromCallable {
            Realm.getDefaultInstance().use { realm ->
                // Get all the bookings that begin and end within the specified date
                val dbRoomBookings =
                        realm.where(DBRoomBooking::class.java)
                                .greaterThan("timeFromUtc", date)
                                .lessThan("timeToUtc", date)
                                .findAllSorted("timeFromUtc")

                if (dbRoomBookings.isEmpty()) {
                    emptyList()
                } else {
                    dbRoomBookings.asSequence().map { dbRoomBooking ->
                        makeRoomBookingModel(dbRoomBooking)
                    }.filterNotNull().toList()
                }
            }
        }

以下是带有subscribeOn的代码（它没有编译）：

/**
 * Gets all the room bookings for the specified day
 */
override fun getRoomBookingsForDay(date: Date): Observable<Collection<Model.RoomBooking>> =
        Observable.fromCallable {
            Realm.getDefaultInstance().use { realm ->
                // Get all the bookings that begin and end within the specified date
                val dbRoomBookings =
                        realm.where(DBRoomBooking::class.java)
                                .greaterThan("timeFromUtc", date)
                                .lessThan("timeToUtc", date)
                                .findAllSorted("timeFromUtc")

                if (dbRoomBookings.isEmpty()) {
                    emptyList()
                } else {
                    dbRoomBookings.asSequence().map { dbRoomBooking ->
                        makeRoomBookingModel(dbRoomBooking)
                    }.filterNotNull().toList()
                }
            }
        }.subscribeOn(AndroidRealmSchedulers.realmThread())

编译时错误消息是：

Type mismatch.
Required: Observable<Collection<Model.RoomBooking>>
Found: Observable<List<Model.RoomBooking>!>!

当然，指定调度程序不应该更改正在返回的类型？有什么想法吗？

Answer 1

我认为您必须使用return-type定义协方差：

override fun getRoomBookingsForDay(date: Date): Observable<out Collection<Model.RoomBooking>>

使用out T与Java中的? extends T相同。

或者，您可以确保仅使用Collection。在您的第一个示例中，Observable.fromCallable()的返回类型由fun的返回类型推断，而第二个示例中的Callable的返回类型推断出Observable.fromCallable<Collections<Model.RoomBooking>> { ... }的返回类型。所以直接申报：

bundle.putString("Types", types.toString());

添加subscribeOn（）正在改变observable的返回类型

1 个答案: