How to tell RecyclerView to start at specific item position

Question

I want my RecyclerView with LinearLayoutManager to show up with scroll position at specific item after adapter got updated. (not first/last position) Means the at first (re-)lay

Answer 1

I found a solution myself:

I extended the LayoutManager:

  class MyLayoutManager extends LinearLayoutManager {

    private int mPendingTargetPos = -1;
    private int mPendingPosOffset = -1;

    @Override
    public void onLayoutChildren(Recycler recycler, State state) {
        if (mPendingTargetPos != -1 && state.getItemCount() > 0) {
            /*
            Data is present now, we can set the real scroll position
            */
            scrollToPositionWithOffset(mPendingTargetPos, mPendingPosOffset);
            mPendingTargetPos = -1;
            mPendingPosOffset = -1;
        }
        super.onLayoutChildren(recycler, state);
    }

    @Override
    public void onRestoreInstanceState(Parcelable state) {
        /*
        May be needed depending on your implementation.

        Ignore target start position if InstanceState is available (page existed before already, keep position that user scrolled to)
         */
        mPendingTargetPos = -1;
        mPendingPosOffset = -1;
        super.onRestoreInstanceState(state);
    }

    /**
     * Sets a start position that will be used as soon as data is available.
     * May be used if your Adapter starts with itemCount=0 (async data loading) but you need to
     * set the start position already at this time. As soon as itemCount > 0,
     * it will set the scrollPosition, so that given itemPosition is visible.
     * @param position
     * @param offset
     */
    public void setTargetStartPos(int position, int offset) {
        mPendingTargetPos = position;
        mPendingPosOffset = offset;
    }
}

It stores may target position. If onLayoutChildren is called by RecyclerView, it checks if itemCount is already > 0. If true, it calls scrollToPositionWithOffset().

So I can tell immediately what position should be visible, but it will not be told to LayoutManager before position exists in Adapter.