Instance state not saved when app is killed by OS

[LouisCAD]

What is instance state, and why it exists

On Android, an Activity can be killed at any time by the system. This happens usually when Android needs memory when your Activity is not in the foreground, or because of a non-handled configuration change, such as a locale change.
To avoid the user having to restart what he did from scratch when Android killed the Activity, the system calls onSaveInstanceState(…) when the Activity is paused, where the app is supposed to save it's data in a Bundle, and passes the saved bundle in both onCreate(…) and onRestoreInstanceState(…) when the task is resumed if the activity has been killed by the system.

The issue about it in flutter

In the flutter apps I tried (Flutter Gallery, and the base project with the FAB tap counter), if I open enough apps to make Android kill the flutter app's Activity, all the state is lost when I come back to the flutter activity (while not having remove the task from recents).

Steps to Reproduce

1. Install Flutter Gallery
2. Open the device's settings, and in developer options, switch "Don't keep activities" on. (see the screenshot)
   
   This will allow to simulate when Android kills Activities because it lacks memory and you're not in the foreground.
3. Open the Flutter Gallery app and go anywhere other than the main screen.
4. Go to the launcher by pressing the device's home button.
5. Press the overview button and return to Flutter Gallery. Here's the bug.

What's expected: The app is in the same state that where we left off, with untouched UI.
What happens: The activity is restarted from scratch, losing all the UI state, even really really long forms.

[eseidelGoogle]

#3427 is also likely related.

[LouisCAD]

@eseidelGoogle That's right. In which format could the flutter Activity state be saved, if it can be at all as of the current version?

[Hixie]

Right now we don't do anything to save anything.

The framework itself has very little state worth saving -- it's all animation and stuff like that -- so it may be that we always leave this up to the app to do. We should probably expose it at the Dart level though. (Right now it's only exposed at the Java level.)

[LouisCAD]

@Hixie On Android, all framework Views have their state automatically saved and restored by the system, which only requires the developer to save manually the non UI part of the instance state. Shouldn't flutter work the same way for all UI widgets to prevent developers from having to write all the boilerplate each time to save where the user was, the scroll position, the enabled state of some button, the state of the previous screen and so on…?

[Hixie]

In Flutter, there's very little framework state to save. For example, the enabled state of a button is not state, it's input provided by the application. The current route history is stored in the framework, but not stored in a state that the framework can rebuild (since it's all instances of objects provided by the application code). The scroll position is about the only thing we could actually store (and we do save that in a store currently, just not one that survives the app).

But in any case we should definitely do better than today.

[LouisCAD]

As an experienced Android Developer, I'd like to take part to the conversation with iOS developers too when it'll be discussed so flutter can be the perfect framework to build iOS and Android apps.
I think the persistence ability of the framework may be important to save data, preferences, cache data and states in the most developer friendly possible way.

[Takhion]

Whoa I didn't realise this was the case? This is actually critical, and it's worth mentioning that on devices with less memory the process could be killed quite easily!

What about saving the PageStore (or related) as a byte stream through serialization.dart in onSaveInstanceState?

[LouisCAD]

@Takhion Could you link "PageStore" you're mentioning? I'm interested in trying to workaround this issue since it seems it won't be fixed any soon (31st december of 2029 is a liiittle far IMHO)

[Takhion]

@LouisCAD sure it's here: https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/widgets/page_storage.dart

I think you'll need to save more than that though: at least the current route(s) and maybe some state?
What do you think @Hixie?

[Hixie]

We don't currently do anything to make this easy. We haven't studied this problem in detail yet. For now I recommend storing the information you want to persist manually, and applying it afresh when the app is restored.