How to implement different states of object in Kotlin? - kotlin

Question

What's a compact and simple way to implement variants of object going through different states?
The complication is that for every state a new properties added to the same object.
A task executed and follow through states Created / Waiting / InProgress possible way would be
to define following classes
class Task(
val type: String,
val make_request: () -> Void
)
class TaskWaitingExecution(
val task: Task,
val retry_count: Int
)
class TaskInProgress(
val task: TaskWaitingExecution,
val request_id: Int
) {
val timer = bon.timer()
}
And progress with steps
val task = Task("some_request', {})
val waiting = TaskWaitingExecution(task, 1)
val in_progress = TaskInProgress(waiting, 10)
But it's not ideal as accessing say type property would end up in
in_progress.task.task.type
Sadly there's no way to use implements by inhreitance implement PreviousTask by task
as it's not working for classes.
Is there a better way to express such logic?

Answer 1

Turn it inside out and put the status as a property of the Task:
class Task(
val type: String,
val make_request: () -> Void,
var status: TaskStatus = null,
)
To store the status-specific fields, a sealed class is perfect:
sealed class TaskStatus;
class WaitingExecution : TaskStatus (
val retry_count: Int
)
class InProgress : TaskStatus (
val request_id: Int
) {
val timer = bon.timer()
}
Usage:
val task = Task("some_request', {})
task.status = WaitingExecution(1)
task.status = InProgress(10)
You can do it with val status as well, but then you'd need to create a new Task object each time its status changes.
It's been a while since I last Kotlin'ed so there might be syntax errors, but I hope the idea is clear.