Issue scrolling the list/recyclview down

[Ayoola Ajebeku]

Whenever I scroll down, the header scrolls and becomes visible; but I have to release and scroll down again for the scrolling to continue.

How do I change this behavior such that with one scroll down/drag down the header will show while the list will still continue to scroll down.

[Dimitry]

Hi!
If I got you right you mean something like that:

If it looks like your issue, than you somehow misused CanScrollVerticallyDelegate.
In this gif It always returns false, which is equal to setting ScrollableLayout.setCanScrollVerticallyDeletegate to NULL or never setting it at all

[Ayoola Ajebeku]

Thanks so much.

Please how do I reduce the slidedown speed of the header? I want it to slide down with the same speed as the list fling speed.

[Dimitry]

Set scroll friction via code scrollableLayout.setFriction(float friction) or via xml attribute app:scrollable_friction="0.075".
Try playing with values in the sample application, it has seek bar with current scroll friction indication. For my own taste something between 0.05 & 0.075 feels smooth

[Ayoola Ajebeku]

Thanks.

[Terence]

@noties somehow recyclerView is always null in canScrollVertically method in my Fragment. Do you have any idea why this might be the case?

[Dimitry]

@squeeish Well, it might be two things: first check if you have RecyclerView in your layout file, second check the id of the RecyclerView (most likely you miss typed it)

[Terence]

@noties it's definitely in my layout file and the id is correct as I'm setting layout managers and adapters to it. I've 4 tabs in a ViewPager with Android Design Support Library's TabLayout as the tab headers. The header is a RelativeLayout.

When I debug, I can see that the CanScrollVertically chain is there (delegate, adapter, fragment) but in all 4 fragments' CanScrollVertically method, the recyclerview variable is always null.

[Terence]

Ok I've solved it. Turns out it was my FragmentPagerAdapter returning new instances of the fragments in getItem. 😄

[Terence]

@noties Could you help me out?

1. Go to developer options and check "Do not keep activities".
2. Start the sample app and click "Inside Fragment".
3. Scroll up and down. The header only shows when the top of the list is reached.
4. Press the home button, then open the sample app again.
5. Scroll up and down. CanScrollVerticallyDelegate seems to always return false this time.

Do you have any idea why this might be the case? This behaviour is not seen in MainActivity. If you are in debug mode, canScrollVertically method in RecyclerViewFragment thinks that mRecyclerView is null, which isn't supposed to be the case.

EDIT: Fixed. mRecyclerView has to be static. Same for mListView in ListViewFragment.

[Dimitry]

Hi! Nice catch!

It happens because we pass to the ViewPagerAdapter newly created fragments, but the previous ones are still attached to the FragmentManager. Thus, our newly created fragments won't be added, won't o through onCreateView() and that's why they will always return false from canScrollVertically method. So, to work around this issue, we must do something like this:

private List<BaseFragment> fragments() {

    final FragmentManager manager = getChildFragmentManager();

    final BaseFragment list;
    {
        final Fragment fragment = manager.findFragmentByTag(ListViewFragment.TAG);
        if (fragment == null) {
            list = ListViewFragment.newInstance(0x80FF0000);
        } else {
            list = (ListViewFragment) fragment;
        }
    }

    final BaseFragment recycler;
    {
        final Fragment fragment = manager.findFragmentByTag(RecyclerViewFragment.TAG);
        if (fragment == null) {
            recycler = RecyclerViewFragment.newInstance(0x8000FF00);
        } else {
            recycler = (RecyclerViewFragment) fragment;
        }
    }

    return Arrays.asList(list, recycler);
}

And then:

final ViewPagerAdapter adapter = new ViewPagerAdapter(
        getChildFragmentManager(),
        getResources(),
        fragments()
);

This solution might work if you have not much fragments. Otherwise it's better to create some kind of helper.

[Dimitry]

I will update sample with the next library release

[Terence]

@noties Thanks! I knew it was something to do with ViewPagerAdapter but for the life of me I couldn't figure out why. Your explanation is great, thanks! Looking forward to the next library release.