一个ViewPager+Fragment嵌入RecyclerView的结构，进入下层activity返回的时候切换页面就会崩溃，报错信息如下

 java.lang.NullPointerException
     at androidx.recyclerview.widget.StaggeredGridLayoutManager.recycleFromEnd(StaggeredGridLayoutManager.java:1954)
     at androidx.recyclerview.widget.StaggeredGridLayoutManager.recycle(StaggeredGridLayoutManager.java:1788)
     at androidx.recyclerview.widget.StaggeredGridLayoutManager.fill(StaggeredGridLayoutManager.java:1707)
     at androidx.recyclerview.widget.StaggeredGridLayoutManager.scrollBy(StaggeredGridLayoutManager.java:2180)
     at androidx.recyclerview.widget.StaggeredGridLayoutManager.scrollVerticallyBy(StaggeredGridLayoutManager.java:2039)
     at androidx.recyclerview.widget.RecyclerView.scrollStep(RecyclerView.java:1855)
     at androidx.recyclerview.widget.RecyclerView$ViewFlinger.run(RecyclerView.java:5281)
     at android.os.Handler.handleCallback(Handler.java:733)
     at android.os.Handler.dispatchMessage(Handler.java:95)
     at android.os.Looper.loop(Looper.java:136)
     at android.app.ActivityThread.main(ActivityThread.java:5039)
     at java.lang.reflect.Method.invokeNative(Native Method)
     at java.lang.reflect.Method.invoke(Method.java:515)
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:861)
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:677)
     at dalvik.system.NativeStart.main(Native Method)

看报错堆栈信息，错误不是项目的代码直接奔溃，而是触发了系统的bug，是RecyclerView在detached from window之后仍然进行滚动造成的，由于RecyclerView的scrollStep无法重写，所以要从StaggeredGridLayoutManager入手，而它的scrollVerticallyBy是public的，因此只要在detached状态下直接return 0;就行了，具体代码如下

class MyStaggeredGridLayoutManager extends StaggeredGridLayoutManager {

    private boolean mViewDetached = false;

    public void setViewDetached(boolean viewDetached) {
        this.mViewDetached = viewDetached;
    }

    @Override
    public int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) {
        if (mViewDetached)
            return 0;
        return super.scrollVerticallyBy(dy, recycler, state);
    }

}

同时修改RecyclerView代码控制detached状态

MyStaggeredGridLayoutManager mLayoutManager;

@Override
protected void onDetachedFromWindow() {
    if (mLayoutManager != null) {
        mLayoutManager.setViewDetached(true);
    }
    super.onDetachedFromWindow();
}

@Override
protected void onAttachedToWindow() {
    super.onAttachedToWindow();
    if (mLayoutManager != null) {
        mLayoutManager.setViewDetached(false);
    }
}