Add rememberRetainedSaveable

[alexvanyo]

Adds a rememberRetainedSaveable variant that participates in both RetainedStateRegistry and SaveableStateRegistry restoration.

The logic is the following upon rememberRetainedSaveable entering composition:

• consume from both RetainedStateRegistry and SaveableStateRegistry, if available
• if the retained value is available, use that
• otherwise, if the saveable restored value is available, use that
• otherwise, re-initialize the value

The RememberObserver behavior is duplicated from the rememberRetained behavior.

[ZacSweers]

Can we do this? #722 (comment)

[alexvanyo]

Pushed up two commits for comparison:

7872adb replaces the existing rememberRetained implementation with rememberRetainedSaveable, but keeps the rememberRetainedSaveable function names.

8f784f0 effectively adds a saver parameter to rememberRetained. Unlike rememberSaveable, which uses an autoSaver() by default, the default value for rememberRetained is a Saver({ null }, { null }), which is effectively a "never save" Saver.

[stagg]

Think the rememberRetained that takes the saver is a good place to start. If we can put the new one under an experimental api to iron it out without impacting the existing rememberRetained that'd be great.

Not a huge fan of rememberRetainedSaveable as a name, bit of a mouthful. Randomly sort of like keep as a name for all this is doing now?

[ZacSweers]

I like keep, but feel like remember is an important convention to use too. Not sure what other options there are, rememberKept doesn't have the same ring

[ZacSweers]

Had some stale comments that I apparently never sent!

[ZacSweers]

Can you elaborate a bit on this? I think I'm learning something new here heh

[alexvanyo]

Updated - this matches the overloads of rememberSaveable, where it can be convenient to just write the Saver for the thing inside of mutableStateOf, instead of the whole MutableState

[ZacSweers]

TIL!

[ZacSweers]

One nit around a param name, otherwise I think with the nullable Saver change this can be good to go!