Not update model with saveViewState flag

[SerhiiStrila]

Seems to be an issue with updating a model with saveViewState flag.
I attach a link to the project to reproduce the bug.
https://github.com/SerhiiStrila/EpoxyIssue

[elihart]

@SerhiiStrila Thanks for providing a project to reproduce, I will look into it soon.

Can you describe the issue in more detail first, though? I don't understand the issue completely

[SerhiiStrila]

@elihart Sure

Step for reproduce bug:

1. Set for model saveViewState flag
2. Load some data to controller
3. Scroll to any position, all ok
4. Tap on button and update data in controller
   Result:
   Cell in recycler view updated to cell with error, but the error text of input field not showed.
   Seems, that after update the controller data, view trys to restore state from bundle with ignoring of new params or maybe my code has a mistake and all ok.

[elihart]

@SerhiiStrila Saved state is not intended to compete with values set via model props. It doesn't make sense to have a value set via both a prop and saved state. This generally works fine, like with textview selection indexes - the model doesn't need to control that.

In the case of error text it makes sense for the model to control it - the issue you're seeing happens because saved state is applied after the model data is bound to the view. An easy fix would be to swap that order. I think that would make more sense, but it could also break existing behavior for people so I will have to think about how best to handle it.

[SerhiiStrila]

@elihart Thanks for your answer! Could you please explain where I can change order of applying saved state data for my case?

[elihart]

Unfortunately the order is not customizable, that is the problem. you can see it in BaseEpoxyAdapter#onBindViewHolder. I need to think about the best way to support changing this order without breaking existing behavior

[elihart]

After consideration, I don't think it is possible to reverse the order in the general case. The model data needs to be bound before saved state is applied, otherwise cases like restoring a carousel position, or an edit text cursor position, would not work.

I think a simple solution may be for Epoxy to just not restore saved state if the data has changed. ie restore state when the view is scrolled back on screen, but not when it is already on screen and it changes. I can experiment with that - it would be a simple code change, but I also want to make sure it doesn't break any use cases.

[SerhiiStrila]

@elihart are there any changes on this issue?

[elihart]

Sorry, I haven't had time to spend on this. I will try to work on it in the next week.

[elihart]

With #367 I have changed state to not be reapplied when a view is updated. I think this will help your case.

However, you should really avoid having a model prop that conflicts with saved state. It is possible for them to get out of sync and cause weird issues - this would happen if your model prop is updated while the model is offscreen, when it is scrolled on screen the previous saved state won't include the model update.

In your case all of the state should probably be stored in the model instead of using view state.

[SerhiiStrila]

I see. @elihart Thank you for your time and work!