项目中在使用ViewPager的时候，一般都要在界面滑动的时候做一些事情，android中有个专门的状态回调接口OnPageChangeListener。
/**
* Callback interface for responding to changing state of the selected page.
*/
public interface OnPageChangeListener {
/**
* This method will be invoked when the current page is scrolled, either as part
* of a programmatically initiated smooth scroll or a user initiated touch scroll.
*
* @param position Position index of the first page currently being displayed.
* Page position+1 will be visible if positionOffset is nonzero.
* @param positionOffset Value from [0, 1) indicating the offset from the page at position.
* @param positionOffsetPixels Value in pixels indicating the offset from position.
*/
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels);
/**
* This method will be invoked when a new page becomes selected. Animation is not
* necessarily complete.
*
* @param position Position index of the new selected page.
*/
public void onPageSelected(int position);
/**
* Called when the scroll state changes. Useful for discovering when the user
* begins dragging, when the pager is automatically settling to the current page,
* or when it is fully stopped/idle.
*
* @param state The new scroll state.
* @see ViewPager#SCROLL_STATE_IDLE
* @see ViewPager#SCROLL_STATE_DRAGGING
* @see ViewPager#SCROLL_STATE_SETTLING
*/
public void onPageScrollStateChanged(int state);
}
SDK中关于ViewPager OnPageChangeListener 的介绍如上.
public void onPageScrolled ( int position, float positionOffset , int positionOffsetPixels) ;
position:当前页码
positionOffset 页面偏移量（百分比） 当往右滑动时递增，往左滑动时递减。
positionOffsetPixels 页面像素偏移量 当往右滑动时递增，往左滑动时递减。
public void onPageSelected(int position)
position:当前选中的页码
public void onPageScrollStateChanged(int state)
state:滑动时，页面的状态。在ViewPager.class中定义了三个状态，分布为：
/**
* Indicates that the pager is in an idle, settled state. The current page
* is fully in view and no animation is in progress.
*/
public static final int SCROLL_STATE_IDLE = 0;
/**
* Indicates that the pager is currently being dragged by the user.
*/
public static final int SCROLL_STATE_DRAGGING = 1;
/**
* Indicates that the pager is in the process of settling to a final position.
*/
public static final int SCROLL_STATE_SETTLING = 2;
SCROLL_STATE_IDLE : 值为0，表示当前页已经选定。
SCROLL_STATE_DRAGGING: 值为1，表示当前页面正在拖动。
SCROLL_STATE_SETTING: 值为2，表示当前页面正在拖动中，还没有到选定状态。
在项目使用中，可以在onPageScrolled中得到页面的偏移量来设置滑动距离。
在onPageStateChanged中，通过页面的状态来进行处理相关的逻辑。