I have an interesting use case I m looking to support I m cu

Question

I have an interesting use case I m looking to support I m currently using a MutableStateFlow of a single data class to represent the UI state I have several video controllers that control several videos Each one exposes the current timestamp of the video as a Flow lt Double gt The user can select one video and the seekbar should set its position based on the timestamp of the selected video My current solution is to collect the state flow get the current controller and collect the position inside of that I can then update the state based on the collected value Is there a better way to handle this

Landry Norris · Answer

general structure state is a MutableStateFlow that holds among other properties a list of video controllers state value controllers and a state value currentController and I need to collect state value currentController positionFlow which is a Flow lt Double gt even if I change state value currentController

Michael Marshall · Answer

I think some extra code snippets might help here

Landry Norris · Answer

Sure I have a class that looks like `data class UIState val controllers List lt Controller gt currentController Controller = controllers firstOrNull ` and a class like ```class Controller val positionFlow Flow lt Double gt = MutableStateflow 0 0 ``` Whenever the position changes I call positionFlow update value In my code I have `val state = MutableStateFlow UIState ` I want to collect the currentController s positionFlow but I want it to update whenever I change currentController

Landry Norris · Answer

Right now I have ```state collect currentState gt currentState currentController positionFlow collect position gt Code that uses position ```

Landry Norris · Answer

I m sure there s some more clear way to do this without nesting collect calls If I add other flows to the controller I d like the solution to scale well

Landry Norris · Answer

I m also considering moving the controllers list outside of the state since it isn t really UI state but I m thinking I should keep the currentController part of the state class

Michael Marshall · Answer

If the Controllers are actually just data classes then it s probably okay to leave them in the UI state If they interact with lifecycle and UI events then I d move them out

Michael Marshall · Answer

currentController could probably just be an index of the list so you re not emitting the same controller from two places in the UIState

Michael Marshall · Answer

Are you looking for something like this ``` fun reactToControllerAndPositionChanges I assume you have some scope val scope = CoroutineScope Dispatchers Default Old way scope launch state collect currentState gt currentState currentController positionFlow collect position gt positionCode position New way state flatMapLatest state gt state currentController positionFlow emptyFlow onEach positionCode launchIn scope private fun positionCode position Double TODO ```

Michael Marshall · Answer

If the UI only shows one video at a time and doesn t need to know about the whole list of Controllers which at this point you could call `ControllerState` you could instead just use `val state = MutableStateFlow ControllerState ` and switch out the controllers info in the business logic behind the scenes instead

Michael Marshall · Answer

^In this case you could just have `data class ControllerState val position Double ` and update the whole `state` whenever the position changes

Landry Norris · Answer

It will show multiple videos side by side but the position should only respond to changes in the selected video s controller I ll definitely look at using flatmap

Landry Norris · Answer

I guess the best I could describe what I m looking for is being able to state combine state selected positionFlow but of course the combine method wouldn t be able to update what it s combining with automatically as state selected changes

Michael Marshall · Answer

Yeah combine is for transforming the results from multiple flows flatmap definitely makes more sense here In your original code I believe you might have been creating memory leaks by collecting a new flow each time the current controller changed but never cancelling the old ones Flatmap handles that for you

Landry Norris · Answer

That makes sense I need to study up on the more complex flow operators

Landry Norris · Answer

Using flatMap seems to work perfectly but I m noticing my Compose UI takes about a second to react when I select a new video

Michael Marshall · Answer

Could it be that the point flows don t emit for a second so the change doesn t happen until then You could try using a StateFlow for the point flows in that case

Landry Norris · Answer

The positionFlow shouldn t be updating its value when a new video is selected The positionFlow does use a MutableStateFlow under the hood

Landry Norris · Answer

From my understanding when I update state it will trigger onEach Is this correct That should be what causes the positionCode to run

Landry Norris · Answer

Now that I think about it I know Compose Debug mode is supposed to run a bit slower I m having CI build a production version now I m wondering if that s the cause

Michael Marshall · Answer

When you update the screen the chosen position flow will change When that position flow emits onEach will run

Michael Marshall · Answer

It could be debug causing it

Landry Norris · Answer

Just added logs when I call state update and when onEach runs The difference is 0 002 seconds so I think it s safe to say the delay in rendering is Compose not the flatMap Thank you for helping me with this

Michael Marshall · Answer

All good relaxed