ViewPager调用notifyDataSetChanged() 刷新问题解决方案

一、问题来由

ViewPager控件很大程度上满足了开发者开发页面左右移动切换的功能，使用非常方便。但是使用中发现，在删除或者修改数据的时候，PagerAdapter无法像BaseAdapter那样仅通过notifyDataSetChanged方法通知刷新View。有人提出一种解决方案：给Viewpager重新设置一遍适配器adapter，来达到刷新数据的目的。但是这种方法在大多数情况下，是存在问题的。

二、问题分析

为什么调用数据更新的方法，Viewpager却没有更新呢，我们跟进该方法的源代码看一下。

/**
 * This method should be called by the application if the data backing this adapter has changed
 * and associated views should update.
 */
public void notifyDataSetChanged() {
    mObservable.notifyChanged();
}

注释里说到，当附加在适配器上的数据发生变化时，应该调用该方法刷新数据。该方法调用了一个mObservable .notifyChanged()；

/**
 * Invokes {@link DataSetObserver#onChanged} on each observer.
 * Called when the contents of the data set have changed.  The recipient
 * will obtain the new contents the next time it queries the data set.
 */
public void notifyChanged() {
    synchronized(mObservers ) {
         // since onChanged() is implemented by the app, it could do anything, including
         // removing itself from {@link mObservers} - and that could cause problems if
         // an iterator is used on the ArrayList {@link mObservers}.
         // to avoid such problems, just march thru the list in the reverse order.
         ; i >= ; i--) {
            mObservers.get(i).onChanged();
         }
    }
}

这都不是重点，重点我们来看这个mObservers的类型是一个抽象类DataSetObserver，里面只有两个未实现的方法，都有谁使用了这个抽象类呢，快捷键 ctrl + alt + H ,在众多的调用者当中，我们发现了Viewpager的身影。进入Viewpager，我们终于找到了Viewpager中控制数据变更的重点方法dataSetChanged ，这个方法如下：

      void dataSetChanged () {
        // This method only gets called if our observer is attached, so mAdapter is non-null.

        boolean needPopulate = mItems .size() < mOffscreenPageLimit * 2 + 1 &&
                mItems.size() < mAdapter.getCount();
        int newCurrItem = mCurItem ;

        boolean isUpdating = false;
        for (int i = 0; i < mItems.size(); i++) {
            final ItemInfo ii = mItems .get(i);
            final int newPos = mAdapter.getItemPosition(ii.object );

            if (newPos == PagerAdapter.POSITION_UNCHANGED ) {
                continue;
            }

            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, mAdapter.getCount() - 1));
                    needPopulate = true;
                }
                continue;
            }

            if (ii.position != newPos) {
                if (ii.position == mCurItem ) {
                    // Our current item changed position. Follow it.
                    newCurrItem = newPos;
                }

                ii. position = newPos;
                needPopulate = true;
            }
        }

        if (isUpdating) {
            mAdapter.finishUpdate( this);
        }

        Collections. sort(mItems, COMPARATOR);

        if (needPopulate) {
            // Reset our known page widths; populate will recompute them.
            final int childCount = getChildCount();
            for (int i = 0; i < childCount; i++) {
                final View child = getChildAt(i);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                if (!lp.isDecor ) {
                    lp. widthFactor = 0.f;
                }
            }

            setCurrentItemInternal(newCurrItem, false, true);
            requestLayout();
        }
    }

重点看这样一行代码：

final int newPos = mAdapter.getItemPosition(ii.object);
if (newPos == PagerAdapter.POSITION_UNCHANGED) {
      continue ;
}

到这里我们就找到了解决这个问题的核心方法：getItemPosition()。官方对getItemPosition()的解释是：

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 orPOSITION_NONE if the item is no longer present in the adapter.

The default implementation assumes that items will never change position and always returns POSITION_UNCHANGED.

意思是如果item的位置如果没有发生变化，则返回POSITION_UNCHANGED。如果返回了POSITION_NONE，表示该位置的item已经不存在了。默认的实现是假设item的位置永远不会发生变化，而返回POSITION_UNCHANGED

三、解决方案

根据上面的分析，我们可以尝试着修改适配器的写法，覆盖getItemPosition()方法，当调用notifyDataSetChanged时，让getItemPosition方法人为的返回POSITION_NONE，从而达到强制Viewpager重绘所有item的目的。

@Override
public int getItemPosition(Object object) {
    return POSITION_NONE;
}

参考资料：https://www.cnblogs.com/cheneasternsun/p/6017012.html