Android -- 处理ViewPager的notifyDataSetChanged无刷新

Viewpager在调用notifyDataSetChanged()时，界面无刷新

Viewpager在调用notifyDataSetChanged()时，界面无刷新，它确实影响我们功能的实现了。可能选择为Viewpager重新设置一遍适配器adapter，达到刷新的目的。但是这种方法在大多数情况下，是有问题的。

查看方法

super.notifyDataSetChanged()调用的是PagerAdapter.notifyDataSetChanged()

/**
     * 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()；

我们继续跟进这个方法，进入DataSetObservable类中，发现这样一段代码：

/**
     * 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.
            for (int i = mObservers .size() - 1; i >= 0; 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，其中我们发现了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 ;
   }

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 returnsPOSITION_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

解决方案

所以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，从而达到强迫viewpager重绘所有item的目的。

class SearchAdapter extends PagerAdapter {

     private int mChildCount = 0;

     @Override
     public void notifyDataSetChanged() {
           mChildCount = getCount();
           super.notifyDataSetChanged();
     }

     @Override
     public int getItemPosition(Object object)   {
           if ( mChildCount > 0) {
           mChildCount --;
           return POSITION_NONE;
           }
           return super.getItemPosition(object);
     }

}

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