Viewpager删除item(数据源发生改变)没有即时更新

最近使用viewpager的时候遇到一个问题，viewpager设置过pagerAdapter之后，当需要修改viewpager的数据时,我这里是删除item，使用pagerAdapter.notifyDataSetChanged方法似乎并没有完全生效。
我的Adapter如下

public class ViewPagerAdapter extends PagerAdapter {

    private Context context;
    private List<String> imgList; //图片的数据源

    public ViewPagerAdapter(Context context, List<String> imgList) {
        this.context = context;
        this.imgList = imgList;
    }

    @Override
    public int getCount() {
        return imgList.size();
    }

    //判断当前的View 和 我们想要的Object(值为View) 是否一样;返回 true/false
    @Override
    public boolean isViewFromObject(View view, Object object) {
        return (view == object);
    }

    //instantiateItem()：将当前view添加到ViewGroup中，并返回当前View
    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        View itemView = getItemView(R.layout.view_pager_img);
        PhotoView imageView = (PhotoView) itemView.findViewById(R.id.img_iv);
        Glide.with(context).load(imgList.get(position)).into(imageView);
        container.addView(itemView);
        return itemView;
    }

    //destroyItem()：删除当前的View;  
    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        container.removeView((View) object);
    }

    private View getItemView(int layoutId) {
        View itemView = LayoutInflater.from(this.context).inflate(layoutId, null, false);
        return itemView;
    }
}

后来在stackoverflow找到答案
http://stackoverflow.com/questions/8060904/add-delete-pages-to-viewpager-dynamically

原来是因为这个PagerAdapter中有个默认方法getItemPosition();我们在Adapter中重写这个方法，如下

/**
     * Called when the host view is attempting to determine if an item's position
     * has changed. Returns {@link #POSITION_UNCHANGED} if the position of the given
     * item has not changed or {@link #POSITION_NONE} if the item is no longer present
     * in the adapter.
     *
     * <p>The default implementation assumes that items will never
     * change position and always returns {@link #POSITION_UNCHANGED}.
     *
     * @param object Object representing an item, previously returned by a call to
     *               {@link #instantiateItem(View, int)}.
     * @return object's new position index from [0, {@link #getCount()}),
     *         {@link #POSITION_UNCHANGED} if the object's position has not changed,
     *         or {@link #POSITION_NONE} if the item is no longer present.
     */
    public int getItemPosition(Object object) {
        return POSITION_UNCHANGED;
    }

大概意思是这样的：该方法默认的返回值是POSITION_UNCHANGED，也就是说即使你删了list中的一个item，positon任然没有改变。如果把返回值改成#POSITION_NONE之后会更新数据源了，所以解决办法如下。

public class ViewPagerAdapter extends PagerAdapter {

    private Context context;
    private List<String> imgList; //图片的数据源

    public ViewPagerAdapter(Context context, List<String> imgList) {
        this.context = context;
        this.imgList = imgList;
    }

    //解决ViewPager数据源改变时，刷新无效的解决办法
    @Override
    public int getItemPosition(Object object) {
        return POSITION_NONE;
    }


   

    @Override
    public int getCount() {
        return imgList.size();
    }

    //判断当前的View 和 我们想要的Object(值为View) 是否一样;返回 true/false
    @Override
    public boolean isViewFromObject(View view, Object object) {
        return (view == object);
    }

    //instantiateItem()：将当前view添加到ViewGroup中，并返回当前View
    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        View itemView = getItemView(R.layout.view_pager_img);
        PhotoView imageView = (PhotoView) itemView.findViewById(R.id.img_iv);
        Glide.with(context).load(imgList.get(position)).into(imageView);
        container.addView(itemView);
        return itemView;
    }

    //destroyItem()：删除当前的View;  
    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        container.removeView((View) object);
    }

    private View getItemView(int layoutId) {
        View itemView = LayoutInflater.from(this.context).inflate(layoutId, null, false);
        return itemView;
    }
}