ViewPager+PagerAdapter局部刷新

ViewPager+PagerAdapter局部刷新

ViewPager如果想要刷新页面，按照正常的操作，直接调用adapter 的notifyDataSetChanged方法，结果发现并没有刷新；
经过查看了notifyDataSetChanged方法源码，发现了一处代码能说明为什么没有更新

 for (int i = 0; i < mItems.size(); i++) {
            final ItemInfo ii = mItems.get(i);
            //获取View的状态
            final int newPos = mAdapter.getItemPosition(ii.object);
			//不刷新
            if (newPos == PagerAdapter.POSITION_UNCHANGED) {
                continue;
            }
			//刷新
            if (newPos == PagerAdapter.POSITION_NONE) {
                mItems.remove(i);
                i--;

                if (!isUpdating) {
                    mAdapter.startUpdate(this);
                    isUpdating = true;
                }

                mAdapter.destroyItem(this, ii.position, ii.object);
                needPopulate = true;

                if (mCurItem == ii.position) {
                    // Keep the current item in the valid range
                    newCurrItem = Math.max(0, Math.min(mCurItem, adapterCount - 1));
                    needPopulate = true;
                }
                continue;
            }

从上面代码可以看出ViewPager是否刷新的关键在PagerAdapter的getItemPosition中返回的状态，根据官方API

Called when the host view is attempting to determine if an item’s position has changed. Returns POSITION_UNCHANGED if the position of the given item has not changed or POSITION_NONE if the item is no longer present in the adapter.

当返回POSITION_UNCHANGED时不会更新页面布局，返回POSITION_NONE重新刷新页面

 @Override
    public int getItemPosition(@NonNull Object object) {
    			//强制刷新所有页面
                return POSITION_NONE;
    }

但有时候只需要做局部刷新时，比如只刷新当前显示的页面，这时候只需要重写setPrimaryItem（），获取当前View

    @Override
    public void setPrimaryItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
        mCurrentView= (View) object;
        super.setPrimaryItem(container, position, object);
    }

然后在getItemPosition（）判断要刷新的View是否为当前的View，返回相应的状态即可

 @Override
    public int getItemPosition(@NonNull Object object) {
        if (mCurrentView==object) {
            return POSITION_NONE;
        }else{
            return POSITION_UNCHANGED;
        }
    }