IssueTracker

Component used: LinearLayoutManager

Version used: 1.2.1

Devices/Android versions reproduced on: OnePlus 7 Pro - Android 11

This is an issue I cannot locally reproduce, I got notified of this via Google Play's Android Vitals feature (hence the provided device and android version) and I'm convinced that I'm using the API correctly and it should be safe to use in this particular case.

This is the stacktrace I got:

java.lang.NullPointerException: 
  at androidx.recyclerview.widget.RecyclerView$LayoutManager.detachViewAt (RecyclerView.java)
  at androidx.recyclerview.widget.RecyclerView$LayoutManager.scrapOrRecycleView (RecyclerView.java)
  at androidx.recyclerview.widget.RecyclerView$LayoutManager.detachAndScrapViewAt (RecyclerView.java)
  at com.roiex.twitchemotes4whatsapp.packlist.preview.TruncatingLinearLayoutManager.onLayoutChildren (TruncatingLinearLayoutManager.java)
  at androidx.recyclerview.widget.RecyclerView.s (RecyclerView.java:66)
  at androidx.recyclerview.widget.RecyclerView.onMeasure (RecyclerView.java)
  at android.view.View.layout (View.java:23029)
  at android.view.ViewGroup.layout (ViewGroup.java:6419)
  at android.widget.LinearLayout.setChildFrame (LinearLayout.java:1829)
  at android.widget.LinearLayout.layoutHorizontal (LinearLayout.java:1818)
  at android.widget.LinearLayout.onLayout (LinearLayout.java:1584)
  at android.view.View.layout (View.java:23042)
  at android.view.ViewGroup.layout (ViewGroup.java:6419)
  at android.widget.LinearLayout.setChildFrame (LinearLayout.java:1829)
  at android.widget.LinearLayout.layoutVertical (LinearLayout.java:1673)
  at android.widget.LinearLayout.onLayout (LinearLayout.java:1582)
  at android.view.View.layout (View.java:23042)
  at android.view.ViewGroup.layout (ViewGroup.java:6419)
  at android.widget.FrameLayout.layoutChildren (FrameLayout.java:332)
  at android.widget.FrameLayout.onLayout (FrameLayout.java:270)
  at android.view.View.layout (View.java:23042)
  at android.view.ViewGroup.layout (ViewGroup.java:6419)
  at androidx.recyclerview.widget.RecyclerView$LayoutManager.layoutDecoratedWithMargins (RecyclerView.java:32)
  at androidx.recyclerview.widget.LinearLayoutManager.layoutChunk (LinearLayoutManager.java)
  at androidx.recyclerview.widget.LinearLayoutManager.fill (LinearLayoutManager.java)
  at androidx.recyclerview.widget.LinearLayoutManager.onLayoutChildren (LinearLayoutManager.java)
  at androidx.recyclerview.widget.RecyclerView.s (RecyclerView.java:66)
  at androidx.recyclerview.widget.RecyclerView.dispatchLayout (RecyclerView.java)
  at androidx.recyclerview.widget.RecyclerView.onLayout (RecyclerView.java)
  at android.view.View.layout (View.java:23042)
  at android.view.ViewGroup.layout (ViewGroup.java:6419)
  at com.google.android.material.appbar.HeaderScrollingViewBehavior.layoutChild (HeaderScrollingViewBehavior.java)
  at com.google.android.material.appbar.ViewOffsetBehavior.h (ViewOffsetBehavior.java)
  at androidx.coordinatorlayout.widget.CoordinatorLayout.onLayout (CoordinatorLayout.java)
  at android.view.View.layout (View.java:23042)
  at android.view.ViewGroup.layout (ViewGroup.java:6419)
  at android.widget.FrameLayout.layoutChildren (FrameLayout.java:332)
  at android.widget.FrameLayout.onLayout (FrameLayout.java:270)
  at android.view.View.layout (View.java:23042)
  at android.view.ViewGroup.layout (ViewGroup.java:6419)
  at android.widget.LinearLayout.setChildFrame (LinearLayout.java:1829)
  at android.widget.LinearLayout.layoutVertical (LinearLayout.java:1673)
  at android.widget.LinearLayout.onLayout (LinearLayout.java:1582)
  at android.view.View.layout (View.java:23042)
  at android.view.ViewGroup.layout (ViewGroup.java:6419)
  at android.widget.FrameLayout.layoutChildren (FrameLayout.java:332)
  at android.widget.FrameLayout.onLayout (FrameLayout.java:270)
  at android.view.View.layout (View.java:23042)
  at android.view.ViewGroup.layout (ViewGroup.java:6419)
  at android.widget.LinearLayout.setChildFrame (LinearLayout.java:1829)
  at android.widget.LinearLayout.layoutVertical (LinearLayout.java:1673)
  at android.widget.LinearLayout.onLayout (LinearLayout.java:1582)
  at android.view.View.layout (View.java:23042)
  at android.view.ViewGroup.layout (ViewGroup.java:6419)
  at android.widget.FrameLayout.layoutChildren (FrameLayout.java:332)
  at android.widget.FrameLayout.onLayout (FrameLayout.java:270)
  at com.android.internal.policy.DecorView.onLayout (DecorView.java:820)
  at android.view.View.layout (View.java:23042)
  at android.view.ViewGroup.layout (ViewGroup.java:6419)
  at android.view.ViewRootImpl.performLayout (ViewRootImpl.java:3786)
  at android.view.ViewRootImpl.performTraversals (ViewRootImpl.java:3227)
  at android.view.ViewRootImpl.doTraversal (ViewRootImpl.java:2182)
  at android.view.ViewRootImpl$TraversalRunnable.run (ViewRootImpl.java:8730)
  at android.view.Choreographer$CallbackRecord.run (Choreographer.java:1352)
  at android.view.Choreographer.doCallbacks (Choreographer.java:1149)
  at android.view.Choreographer.doFrame (Choreographer.java:1049)
  at android.view.Choreographer$FrameHandler.handleMessage (Choreographer.java:1275)
  at android.os.Handler.dispatchMessage (Handler.java:106)
  at android.os.Looper.loop (Looper.java:233)
  at android.app.ActivityThread.main (ActivityThread.java:8010)
  at java.lang.reflect.Method.invoke (Native Method)
  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run (RuntimeInit.java:631)
  at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:978)

As you can see the exception occurs as a result to the call of LinearLayoutManager#detachAndScrapViewAt(i, recycler) where both the recycler and the index are completely valid (to my knowledge), so the call should complete without any exception.

I implemented a custom LinearLayoutManager that is not scrollable and hides any elements that are partially visible to provide a "preview feature" like this:

package com.roiex.twitchemotes4whatsapp.packlist.preview;

import android.content.Context;
import android.util.AttributeSet;

import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

public class TruncatingLinearLayoutManager extends LinearLayoutManager {

    public TruncatingLinearLayoutManager(Context context) {
        super(context);
    }

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

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

    @Override
    public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
        super.onLayoutChildren(recycler, state);
        int start = findLastCompletelyVisibleItemPosition();
        int stop = findLastVisibleItemPosition();
        for (int i = start + 1; i <= stop; i++) {
            detachAndScrapViewAt(i, recycler);
        }
    }

    @Override
    public boolean canScrollHorizontally() {
        return false;
    }

    @Override
    public boolean canScrollVertically() {
        return false;
    }
}

I think my code in onLayoutChildrenshould be completely valid under any circumstances and that's why I don't understand why the exception is thrown in the first place. I had a look at the source code for this particular stacktrace and it seems like the Stacktrace is a little bit inaccurate, most likelydue to minification and obfuscation as part of the build process