Evaluate replacing R2RTLViewPager and R2ViewPager with ViewPager2

[stevenzeck]

I don't know the history of the usage of R2RTLViewPager, was it used just for RTL support? ViewPager2 supports RTL natively.

Migration guide: https://developer.android.com/training/animation/vp2-migration
Jetpack library reference: https://developer.android.com/jetpack/androidx/releases/viewpager2
Class docs: https://developer.android.com/reference/androidx/viewpager2/widget/ViewPager2

I had looked at this before but had personal things come up, so had to stop. Once readium/r2-navigator-kotlin#148 is merged I can take another look.

[mickael-menu]

Thanks, such contribution is most welcome! And yes, as far as I know R2RTLViewPager was here for the RTL support.

Right now, I'm focused on releasing the 2.0.0-alpha.1, but once this is behind us, I'll take a look at readium/r2-navigator-kotlin#148 first thing.

[stevenzeck]

Initial investigation

• ViewPager2 would eliminate the need for R2RTLViewPager and R2ViewPager
• ViewPager2 is final so can't be extended
• addOnPageChangeListener is replaced by registerOnPageChangeCallback
• VIewPager2 has a layoutDirection attribute that accepts a View.LAYOUT_DIRECTION_ int. Thinking of adding these to the ReadingProgression enum class
• FragmentStatePagerAdapter is replaced by FragmentStateAdapter

I replaced all of these locally and am trying to get through errors and how best to replace some of the old ViewPager code (like setPrimaryItem). Right now when I swipe to page, it goes from chapter to chapter instead of page to page. Clicking the edge pages properly.

[mickael-menu]

@stevenzeck This sounds great, thanks for taking a look into this.

VIewPager2 has a layoutDirection attribute that accepts a View.LAYOUT_DIRECTION_ int. Thinking of adding these to the ReadingProgression enum class

For ReadingProgression we're limited to what is specified in RWPM, so: left to right, right to left, top to bottom, bottom to top and auto. So what do you need to add? I think that we won't use LAYOUT_DIRECTION_INHERIT and LAYOUT_DIRECTION_LOCALE.

[stevenzeck]

I was hesitant at first to put the logic that determines which one to use in EpubNavigatorFragment. But now I don't think it's horrible to just put it there.

I pushed a first round to the testapp and navigator repositories.