Vertical Android ViewPager not working on Android 9 (Pie)

[ferrannp]

Swiping for a short distance and fast should change the page. This is working fine in my Pixel 2 (Android 10) and in Android the emulator. However, we tried a couple of devices where is not working correctly:

• Samsung Galaxy Note 9 (Android 9)
• Xiaomi MI MIX 2 (Android 9)

This is the behavior on for the vertical ViewPager:

Basically the user needs to scroll all the way down or up (manually) to trigger a change of page. With the horizontal ViewPager (recorded on the same phone), it works just as expected:

Would love to get some help from you @devvit if you know what might be causing this.

[devvit]

@ferrannp, What about this ?
I haven't these devices, only tested on my 5 devices(4.4 - 9.0) :(

[ferrannp]

Thanks for the reference @devvit! I'll be trying it tomorrow.

[ferrannp]

Mmm for now still that did not really help @devvit. That just seems to play with the threshold the page needs to be moved to swipe... But I think the problem here is different. If you drag very small distance but fast (with power), it should swipe to the next page. Still investigating... By the way, I was able to reproduce it on an emulator (Note 9 Android 9) using https://developer.samsung.com/remotetestlab/. The only drawback is that every time you want to try something you need to build the production apk.

[ferrannp]

Ok, I can reproduce the issue with the Android emulator (good to find a solution) on API level 28 (Android P). It seems to work before and after but not on Android P (most popular nowadays).

[devvit]

@ferrannp, Make ViewPager snap with shorter drag

My reflection solution, you can change the value of mMinimumVelocity and mFlingDistance.

    public void setOrientation(boolean vertical) {
        mVertical = vertical;
        if (!mVertical) return;

        // Make page transit vertical
        setPageTransformer(true, new VerticalPageTransformer());

        // Nested scroll issue, follow the link
        // https://stackoverflow.com/questions/46828920/vertical-viewpager-with-horizontalscrollview-inside-fragment
        mGestureDetector = new GestureDetector(getContext(), new GestureDetector.SimpleOnGestureListener() {
            @Override
            public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {
                return Math.abs(distanceY) > Math.abs(distanceX);
            }
        });

        // reflection
        try {
            Field mFlingDistance = ViewPager.class.getDeclaredField("mFlingDistance");
            mFlingDistance.setAccessible(true);
            mFlingDistance.setInt(this, 10);

            Field mMinimumVelocity = ViewPager.class.getDeclaredField("mMinimumVelocity");
            mMinimumVelocity.setAccessible(true);
            mMinimumVelocity.setInt(this, 5);
        } catch (Exception e) {
            Log.e("###", e.getMessage());
        }
    }

Could you try straya's answer? I'm not sure that.

[ferrannp]

@devvit I found a a perfect working solution modifying the source code. https://android.googlesource.com/platform/frameworks/support/+/5b614a46f6ffb3e9ca5ab6321c12412550a4e13a/viewpager/src/main/java/androidx/viewpager/widget/ViewPager.java#2251. Instead of getXVelocity, if pager is vertical would be getYVelocity.

Can we achieve this by reflection without copying the whole source code?

[devvit]

Looks impossible 😓, just change mMinimumVelocity.
Or maybe we commit a PR to android master dev 😅

[ferrannp]

Or I'll fork the internal code here (it will not change anyway as they are with ViewPager2 now). I'll prepare a PR.

[devvit]

Yes, fork it, freeze it, until we migrate to VP2

[troZee]

we can close it, when #139 will be merged.

[todorone]

@troZee @ferrannp Is there any workaround to use vertical ViewPager now? I was so excited about native feel viewpager on react native, but found out that it's broken on Android...😿

PS: Thanks for working on it!