Work in progress RecyclerView decoration to pin header

HeaderDecor.java

@Beta
private class HeaderDecor extends RecyclerView.ItemDecoration {
  private RecyclerView.AdapterDataObserver adapterDataObserver;
  private RecyclerView.ViewHolder headerViewHolder;
  private int headerViewType;
  private final Rect tmpBounds = new Rect();

  @Override public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
    if (headerViewHolder == null) {
      headerViewHolder = obtainViewHolder(parent);
    }

    c.save();

    final View view = headerViewHolder.itemView;
    view.getDrawingRect(tmpBounds);

    if (tmpBounds.isEmpty()) {
      layoutHeaderIfMissedLayoutPass(parent);
    }

    c.clipRect(tmpBounds);

    view.draw(c);
    c.restore();
  }

  void bindHeaderHolder() {
    if (headerViewHolder == null) {
      return;
    }

    final ViewGroup parentView = (ViewGroup) headerViewHolder.itemView;

    if (parentView != null && parentView instanceof RecyclerView) {
      RecyclerView parent = (RecyclerView) parentView;
      //noinspection unchecked
      parent.getAdapter().onBindViewHolder(headerViewHolder, 0);
      parent.invalidateItemDecorations();
    }
  }

  private RecyclerView.ViewHolder obtainViewHolder(RecyclerView parent) {
    final RecyclerView.Adapter adapter = parent.getAdapter();
    if (adapterDataObserver == null) {
      parent.setViewCacheExtension(new HeaderViewCaching());
      adapter.registerAdapterDataObserver(adapterDataObserver = new HeaderObserver());
    }
    RecyclerView.ViewHolder headerViewHolder = parent.findViewHolderForAdapterPosition(0);
    // Means that it was recycled by framework
    if (headerViewHolder == null) {
      final int viewType = headerViewType = adapter.getItemViewType(0);

      // Try to obtain it from Recycled pool
      headerViewHolder = parent.getRecycledViewPool().getRecycledView(viewType);

      if (headerViewHolder == null) {
        headerViewHolder = adapter.onCreateViewHolder(parent, viewType);
        Timber.i("New header was created");
      } else {
        Timber.i("Header is taken from RecycledViewPool");
      }
    } else {
      Timber.i("Header is attached to RecyclerView");
    }

    // noinspection unchecked
    adapter.onBindViewHolder(headerViewHolder, 0);
    return headerViewHolder;
  }

  void layoutHeaderIfMissedLayoutPass(RecyclerView parent) {
    if (headerViewHolder == null) {
      return;
    }

    //reset from previous settings
    tmpBounds.setEmpty();

    final View itemView = headerViewHolder.itemView;

    // mark View invisible for RecyclerView to skip overdrawing
    itemView.setVisibility(View.INVISIBLE);
    // If View is still not measured
    if (!itemView.getGlobalVisibleRect(tmpBounds)) {
      Timber.d("View is not laid out, measuring it");

      // Take care of view offsets
      final ViewGroup.LayoutParams lp = itemView.getLayoutParams();
      if (lp instanceof ViewGroup.MarginLayoutParams) {
        final ViewGroup.MarginLayoutParams mlp = (ViewGroup.MarginLayoutParams) lp;
        tmpBounds.left = mlp.leftMargin;
        tmpBounds.top = mlp.topMargin;
        tmpBounds.right = mlp.rightMargin;
        tmpBounds.bottom = mlp.bottomMargin;
      }

      // and measure it all
      itemView.measure(makeSpec(parent.getWidth()), makeSpec(parent.getHeight()));
      itemView.layout(tmpBounds.left, tmpBounds.top, tmpBounds.right, tmpBounds.bottom);
    }
  }

  private static int makeSpec(int size) {
    return View.MeasureSpec.makeMeasureSpec(size, View.MeasureSpec.UNSPECIFIED);
  }

  final class HeaderObserver extends RecyclerView.AdapterDataObserver {
    @Override public void onItemRangeChanged(int positionStart, int itemCount) {
      if (positionStart == 0) {
        bindHeaderHolder();
      }
    }
  }

  final class HeaderViewCaching extends RecyclerView.ViewCacheExtension {
    @Override public View getViewForPositionAndType(RecyclerView.Recycler recycler, int position,
        int viewType) {
      if (headerViewHolder != null && viewType == headerViewType) {
        return headerViewHolder.itemView;
      }
      return null;
    }
  }
}