Android RecyclerView plus ViewPager

Question

I have a ViewPager that utilizes a RecyclerView for each page and shares ViewItem rows across pages. Accordingly I share a single RecyclerViewPool between them. However, the ViewPager loads each RecyclerView whether or not it is the page on screen. Is there a way to indicate to the RecyclerView that all of its items are offscreen and force its views to be returned to the Recycler?

My sense is that subclassing LinearLayoutManager and overriding its onLayoutChildren method is the way to go, but I don't have much experience with LayoutManager and would like some guidance.

ViewPager creates the adjacent page before it is visible because it needs to show two adjacent pages as the user swipes between them. So the views really are required, even though for a time they are offscreen. Pages that are not adjacent to the current page are not created, and are destroyed. — dsh, Feb 24 '17 at 20:27
For some reason, no matter what I set `setOffscreenPageLimit` to, the ViewPager always loads 6 pages. My previous solution was to put the `RecyclerView`s into `ViewStub`s and inflate the adjacent pages when a center page was scrolled to. That works pretty well actually, but I thought maybe I could a little more speed out of this. — NewEndian, Feb 24 '17 at 20:57
Interesting. The app I am working with has one ViewPager with 3 tabs. I was not aware of the setOffscreenPageLimit method, but I did observe that the current and adjacent tabs exist and the other one does not as the tabs are changed. (I recently had print statements in my custom view to watch/debug the lifecycle of it; that is how I know when it was detached from the window; at one point we had debug prints in the lifecycle of the fragments as we worked on them) — dsh, Feb 28 '17 at 21:41

score 0 · Accepted Answer · answered Feb 24 '17 at 21:08

So here is a subclass of LinearLayoutManager that operates the way I described:

public class PageVisibleLinearLayoutManager extends LinearLayoutManager {
    public PageVisibleLinearLayoutManager(Context context) {
        super(context);
    }

    public PageVisibleLinearLayoutManager(Context context, int orientation, boolean reverseLayout) {
        super(context, orientation, reverseLayout);
    }

    public PageVisibleLinearLayoutManager(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
    }

    private boolean pageVisible = true;

    void setPageVisible(boolean pageVisible) {
        boolean change = (this.pageVisible != pageVisible);
        this.pageVisible = pageVisible;
        if(change) requestLayout();
    }

    @Override
    public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
        if(pageVisible) {
            super.onLayoutChildren(recycler, state);
        } else {
            removeAndRecycleAllViews(recycler);
        }
    }
}

It works nicely and gives up its views if requested. As dsh mentioned, it is important to mark adjacent pages as being on screen (and I really don't know why setOffscreenPageLimit doesn't limit the number of pages loaded as expected). My previous solution was to use ViewStub and inflate a page only when it was on screen or adjacent. The layout manager method is slightly faster upon initial turning to an unloaded page, but ViewStub has the advantage of pages staying in memory once loaded (making subsequent scrolling more smooth), so I decided to stick with that.

Thank you all. Next question...

Android RecyclerView plus ViewPager

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