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Nested scrolling works partially (as per http://b/122818889). Let's discuss if we need full support and if so make sure it works.

Hi!

What is 'partially' exactly?
How do I see it?
Thanks!

As of now:
- Nesting scroll views with a scroll direction perpendicular to the ViewPager2's orientation inside ViewPager2 works
- Nesting scroll views with a scroll direction parallel to the ViewPager2's orientation inside ViewPager2 does not work

Horizontal ViewPager2 not correctly working into a vertical RecyclerView
Set a setNestedScrollingEnabled to the RecyclerView into the ViewPager2 (across reflection) resolves the problem

ageevvalentin@gmail.com, could you provide a sample app? I'd like to learn more about the circumstances that cause the problem; putting a clean ViewPager2 inside a clean RecyclerView seems to work fine, so there must be other factors involved.

Added a patch for androidx-master-dev that demos a horizontal ViewPager2 inside a vertical RecyclerView.

Verified that it works correctly on a:
- Nexus 4 emulator with API 17
- Nexus 5X emulator with API 28
- Pixel 2 device with API 29

To reproduce:
- Check out the Android Jetpack source (at commit 256899f482ff85cddfb050f37550be7b5ec927ef) (see steps in [1])
- Apply the patch (`git apply 0001-Add-sample-where-a-horizontal-ViewPager2-is-nested-i.patch`)
- Build and install viewpager2's integration-tests app: `./gradlew viewpager2:integration-tests:testapp:installDebug`
- Run it

Closing the issue for now, but please reopen if you have a minimal reproduction app

[1] https://android.googlesource.com/platform/frameworks/support/+/256899f482ff85cddfb050f37550be7b5ec927ef

Re-opening as requested by a few users. Narrowed down the focus to nested scrolling elements with the same scroll direction.

Any progress on this? (only talking about the "same scroll direction" issue)

I searched a lot for workarounds on Google and SO, but it seems no one really found a suitable solution yet.
The thing is that all other VerticalViewPagers out there have other problems that are similarly annoying. Most of them revolve around touch issues like "What's a click and what's a swipe" or "what's the fling threshold to snap to the next page".

So if someone knows of a working sample of VerticalViewPagers, I'd be happy to see a link :)

Hi, we're unlikely to address the "same scroll direction" issue before 1.0-Stable as it's likely going to require API changes (so v1.1+).

One possible option for VerticalViewPager with nested scrolling in the same scroll direction is using a RecyclerView implementing NestedScrollingParent + PagerSnapHelper to get snapping.
You'd then have to implement the NestedScrollingParent/Child contract between parent/child RecyclerView instances.

Cool, thanks for this quick answer - Both pieces of information are highly appreciated (rough estimate of tackling this as well as giving hint on how to do it).
I will try implementing what you suggested and in case it is readable code, I will post it here as sample project or somthing similar.

That'd be great, much appreciated!

So...I was too stupid to understand the back and forth and up and down of NestedScrollingParent and NestedScrollingChild/NestedScrollingChild2 and NestedScrollingParentHelper and NestedScrollingChildHelper
But I figured out another way to achieve the same goal. I attached a demo project with the following setup:

1. A parent RecyclerView with PagerSnapHelper:
This vertical RecyclerView has two items/pages.
The upper page is just a simple TextView.
The lower page contains the nested RecyclerView.
Both pages are Fragments if that is of any concern.

2. A PagingLinearLayoutManager:
This is a LinearLayoutManager with the capability to enable/disable scrolling.
It is used for the parent RecyclerView.

3. A OnItemTouchListener:
This is also used for the parent RecyclerView.
It saved the initial y coordinate on ACTION_DOWN
During ACTION_MOVE event, it decides whether the PagingLinearLayoutManager is allowed to scroll (i.e. to switch between pages)

The important parts of the code are commented. Hope this helps someone else!

Same promble

There is a  hook for this
https://medium.com/@al.e.shevelev/how-to-reduce-scroll-sensitivity-of-viewpager2-widget-87797ad02414

I'm a bit skeptical about the suggested solution from comment#15 that modifies the touch slop.

We're working on a solution that'll work reliably with ViewPager2 1.0. See https://r.android.com/1131083.
Not finalized at the time of writing, but might help some people already. Notice NestedScrollableHostLayout wrapping [1] a scrollable child.


[1] https://r.android.com/c/platform/frameworks/support/+/1131083/4/viewpager2/integration-tests/testapp/src/main/res/layout/item_horizontal_rvs.xml#55

Project: platform/frameworks/support
Branch: androidx-master-dev

commit 1316417bc87741ea9f739a453eb9e792512c9651
Author: Jelle Fresen <jellefresen@google.com>
Date:   Tue Oct 01 16:58:57 2019

    Add sample of nested scrolling in ViewPager2
   
    This shows how nested scrolling can be achieved in ViewPager2. Every
    child on a page that scrolls in the same direction as ViewPager2 must be
    wrapped by a NestedScrollableHostLayout, and one must make sure that the
    scrollable child implements canScrollHorizontally/Vertically in order
    for this to work.
   
    Bug: 123006042
    Test: Manual, run ViewPager2 demo app
    Change-Id: I79e24b7668cf7ff5c05c350ea3ab9134dd74b3a4

M       viewpager2/integration-tests/testapp/src/main/AndroidManifest.xml
M       viewpager2/integration-tests/testapp/src/main/java/androidx/viewpager2/integration/testapp/BrowseActivity.kt
A       viewpager2/integration-tests/testapp/src/main/java/androidx/viewpager2/integration/testapp/NestedScrollableHost.kt
A       viewpager2/integration-tests/testapp/src/main/java/androidx/viewpager2/integration/testapp/ParallelNestedScrollingActivity.kt
A       viewpager2/integration-tests/testapp/src/main/res/layout/item_nested_recyclerviews.xml
M       viewpager2/integration-tests/testapp/src/main/res/values/colors.xml
M       viewpager2/integration-tests/testapp/src/main/res/values/strings.xml

https://android-review.googlesource.com/1131083
https://goto.google.com/android-sha1/1316417bc87741ea9f739a453eb9e792512c9651

We have now merged a working example of ViewPager2 with nested scrollable elements in both: same and orthogonal orientation. See the sample under: https://goo.gle/viewpager2-sample
Notice how the scrollable children of ViewPager2 are wrapped in NestedScrollableHost -- optional for orthogonal elements, required for same-orientation elements.