angularfire从firebase获取对象的长度

Question

我坚持这个。 我在离子2中使用了角火2，我需要得到一个物体的长度：

private ArrayList<Food> filterFood = new ArrayList<Food>();

private ItemFilter mFilter = new ItemFilter();

    public Filter getFilter() {
    return mFilter;
}

private class ItemFilter extends Filter {
    @Override
    protected FilterResults performFiltering(CharSequence constraint) {
        FilterResults results = new FilterResults();

        if (constraint == null || constraint.length() == 0) {

            results.values = food;
            results.count = food.size();
        }
        else {

            ArrayList<Food> filterFood = new ArrayList<Food>();

            for (Food txtPlace : food) {
                 if (txtPlace.getName().toUpperCase().startsWith(constraint.toString().toUpperCase()))
                    filterFood.add(txtPlace);
            }

            results.values = filterFood;
            results.count = filterFood.size();

        }
        return results;
    }

    @Override
    protected void publishResults(CharSequence charSequence, FilterResults filterResults) {
        if (filterResults.count == 0)
            notifyDataSetInvalidated();
        else {
            food = (ArrayList<Food>) filterResults.values;
            notifyDataSetChanged();
        }
    }
}

如果对象存在，如何知道它是否存在？

感谢。