MVVM开发模式中RecyclerView使用SwipRefreshLayout实现下拉刷新，自定义上拉加载更多
- 1.RecyclerView设置滑动监听，当前可见页面滑动到底部时(newState == RecyclerView.SCROLL_STATE_IDLE)， 根据adapter返回的item类型显示foot（加载更多控件），此时去更新加载数据，如果有新数据，显示正常的item； 如果没有新的数据，显示没有更多数据。具体见代码中的注释，比较详细
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