RecyclerView添加头部和尾部并解决no

原理：封装基类RecyclerView.Adapter，名字为WrapAdapter；功能性的Adapter名字叫RealAdapter；在WrapAdapter的回调中封装头和尾View，并且调用RealAdapter的相应回调。实现功能。

主要属性：

    private final T mRealAdapter;    private boolean isStaggeredGrid;    private static final int BASE_HEADER_VIEW_TYPE = -1 << 10;    private static final int BASE_FOOTER_VIEW_TYPE = -1 << 11;    private ArrayList<FixedViewInfo> mHeaderViewInfos = new ArrayList<>();    private ArrayList<FixedViewInfo> mFooterViewInfos = new ArrayList<>();

主要功能：

1：添加头部和尾部

  /**     * Adds a header view     *     * @param view     */    public void addHeaderView(View view) {        if (null == view) {            throw new IllegalArgumentException("the view to add must not be null!");        }        final FixedViewInfo info = new FixedViewInfo();        info.view = view;        info.viewType = BASE_HEADER_VIEW_TYPE + mHeaderViewInfos.size();        mHeaderViewInfos.add(info);        notifyDataSetChanged();    }    /**     * Adds a footer view     *     * @param view     */    public void addFooterView(View view, boolean reverse) {        if (null == view) {            throw new IllegalArgumentException("the view to add must not be null!");        }        final FixedViewInfo info = new FixedViewInfo();        info.view = view;        info.viewType = BASE_FOOTER_VIEW_TYPE + mFooterViewInfos.size();        mFooterViewInfos.add(info);        if (reverse) {            for (int i = 0; i < mFooterViewInfos.size(); i++) {                FixedViewInfo fixedViewInfo = mFooterViewInfos.get(i);                fixedViewInfo.viewType = BASE_FOOTER_VIEW_TYPE + mFooterViewInfos.size() - i - 1;            }        }        notifyDataSetChanged();    }

2：功能逻辑代码

 @Override    public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int viewType) {        if (isHeader(viewType)) {            int whichHeader = Math.abs(viewType - BASE_HEADER_VIEW_TYPE);            View headerView = mHeaderViewInfos.get(whichHeader).view;            return createHeaderFooterViewHolder(headerView);        } else if (isFooter(viewType)) {            int whichFooter = Math.abs(viewType - BASE_FOOTER_VIEW_TYPE);            View footerView = mFooterViewInfos.get(whichFooter).view;            return createHeaderFooterViewHolder(footerView);        } else {            return mRealAdapter.onCreateViewHolder(viewGroup, viewType);        }    }    private RecyclerView.ViewHolder createHeaderFooterViewHolder(View view) {        if (isStaggeredGrid) {            StaggeredGridLayoutManager.LayoutParams params = new StaggeredGridLayoutManager.LayoutParams(                    StaggeredGridLayoutManager.LayoutParams.MATCH_PARENT, StaggeredGridLayoutManager.LayoutParams.WRAP_CONTENT);            params.setFullSpan(true);            view.setLayoutParams(params);        }        return new RecyclerView.ViewHolder(view) {        };    }    @SuppressWarnings("unchecked")    @Override    public void onBindViewHolder(RecyclerView.ViewHolder viewHolder, int position) {        if (position < mHeaderViewInfos.size()) {            // Headers don't need anything special        } else if (position < mHeaderViewInfos.size() + mRealAdapter.getItemCount()) {            // This is a real position, not a header or footer. Bind it.            mRealAdapter.onBindViewHolder(viewHolder, position - mHeaderViewInfos.size());        } else {            // Footers don't need anything special        }    }    @Override    public int getItemCount() {        return mHeaderViewInfos.size() + mRealAdapter.getItemCount() + mFooterViewInfos.size();    }    @Override    public int getItemViewType(int position) {        if (isHeaderPosition(position)) {            return mHeaderViewInfos.get(position).viewType;        } else if (isFooterPosition(position)) {            return mFooterViewInfos.get(position - mHeaderViewInfos.size()                    - mRealAdapter.getItemCount()).viewType;        } else {            return mRealAdapter.getItemViewType(position - mHeaderViewInfos.size());        }    }

以上代码参考于zwenkai的博客，地址：http://blog.csdn.net/xuehuayous/article/details/50498345;
这样做可以方便，简洁的实现RecylerView添加头部和尾部的功能，但是有一个问题：
当用自己写的adapter.notifyDataSetChanged()时，无效果。
因为setAdapter时用的时WrapAdapter，它成为了观察者。
源码：

这样就可以简介的实现RecylerView添加头部和尾部的功能。这样做会误导我们：比如我们notify时，都是用自己的adapter.nitify的，这样封装后，notify无效。

原因：因为RecylerView设置adapter用的WrapAdapter，所以无效。你可以直接用WrapAdapter的对象notify。

如果想要用自己的adapter通知，则需要在注册称为被观察者，代码如下：

private RecyclerView.AdapterDataObserver mDataObserver = new RecyclerView.AdapterDataObserver() {        @Override        public void onChanged() {            notifyDataSetChanged();        }        @Override        public void onItemRangeInserted(int positionStart, int itemCount) {            notifyItemRangeInserted(positionStart, itemCount);        }        @Override        public void onItemRangeChanged(int positionStart, int itemCount) {            notifyItemRangeChanged(positionStart, itemCount);        }        @Override        public void onItemRangeChanged(int positionStart, int itemCount, Object payload) {            notifyItemRangeChanged(positionStart, itemCount, payload);        }        @Override        public void onItemRangeRemoved(int positionStart, int itemCount) {            notifyItemRangeRemoved(positionStart, itemCount);        }        @Override        public void onItemRangeMoved(int fromPosition, int toPosition, int itemCount) {            notifyItemMoved(fromPosition, toPosition);        }    };

realAdapter.notify时，调用wrapAdaper的notify。