Adapter.notifyDataSetChanged与ListView.Post()刷新问题

二哈镇楼

笔者在实际开发中碰到的问题，在这里记录一下

描述一下 碰见的问题：在一个listview页面中，onResume()回来，请求完数据后，对adapter进行notifyDataSetChanged后，需要对item进行ui操作。这时候首先页面展现的时候，ListView 会getView()一遍，请求完数据notifyDataSetChanged后，又会getView(),这时候直接进行UI操作的话 在主线程中 会先执行UI操作，然后在进行getView ，就会导致我的进行UI操作后的item 又被重新getView了一遍。

图1

notifyDataSetChanged 异步机制

首先这个机制问题，不是直接在主线程中去更新UI，来看一下里面的代码

/**
 * Notifies the attached observers that the underlying data has been changed
 * and any View reflecting the data set should refresh itself.
 */
public void notifyDataSetChanged() {
    mDataSetObservable.notifyChanged();
}
***
继续进入 DataSetObservable：
***
 /**
 * 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();
        }
    }
}

会调用观察者的onChanged()方法，它的实现方法在DataSetObservable的子类中AdapterDataSetObserver实现。而AdapterDataSetObserver则是在listview.setadapter 时候将AdapterDataSetObserver创建并且绑定，可以看一下setAdapter()方法

/**
 * Sets the data behind this ListView.
 *
 * The adapter passed to this method may be wrapped by a {@link WrapperListAdapter},
 * depending on the ListView features currently in use. For instance, adding
 * headers and/or footers will cause the adapter to be wrapped.
 *
 * @param adapter The ListAdapter which is responsible for maintaining the
 *        data backing this list and for producing a view to represent an
 *        item in that data set.
 *
 * @see #getAdapter()
 */
@Override
public void setAdapter(ListAdapter adapter) {
    if (mAdapter != null && mDataSetObserver != null) {
        mAdapter.unregisterDataSetObserver(mDataSetObserver);
    }

    resetList();
    mRecycler.clear();

    if (mHeaderViewInfos.size() > 0 || mFooterViewInfos.size() > 0) {
        mAdapter = new HeaderViewListAdapter(mHeaderViewInfos, mFooterViewInfos, adapter);
    } else {
        mAdapter = adapter;
    }

    mOldSelectedPosition = INVALID_POSITION;
    mOldSelectedRowId = INVALID_ROW_ID;

    // AbsListView#setAdapter will update choice mode states.
    super.setAdapter(adapter);

    if (mAdapter != null) {
        mAreAllItemsSelectable = mAdapter.areAllItemsEnabled();
        mOldItemCount = mItemCount;
        mItemCount = mAdapter.getCount();
        checkFocus();

        mDataSetObserver = new AdapterDataSetObserver();
        mAdapter.registerDataSetObserver(mDataSetObserver);

        mRecycler.setViewTypeCount(mAdapter.getViewTypeCount());

        int position;
        if (mStackFromBottom) {
            position = lookForSelectablePosition(mItemCount - 1, false);
        } else {
            position = lookForSelectablePosition(0, true);
        }
        setSelectedPositionInt(position);
        setNextSelectedPositionInt(position);

        if (mItemCount == 0) {
            // Nothing selected
            checkSelectionChanged();
        }
    } else {
        mAreAllItemsSelectable = true;
        checkFocus();
        // Nothing selected
        checkSelectionChanged();
    }

    requestLayout();
}

再看下AdapterDataSetObserver 中onChanged的实现

class AdapterDataSetObserver extends DataSetObserver {

        private Parcelable mInstanceState = null;

        @Override
        public void onChanged() {
            mDataChanged = true;
            mOldItemCount = mItemCount;
            mItemCount = getAdapter().getCount();

            // Detect the case where a cursor that was previously invalidated has
            // been repopulated with new data.
            if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null
                    && mOldItemCount == 0 && mItemCount > 0) {
                AdapterView.this.onRestoreInstanceState(mInstanceState);
                mInstanceState = null;
            } else {
                rememberSyncState();
            }
            checkFocus();
            requestLayout();
        }

        @Override
        public void onInvalidated() {
            mDataChanged = true;

            if (AdapterView.this.getAdapter().hasStableIds()) {
                // Remember the current state for the case where our hosting activity is being
                // stopped and later restarted
                mInstanceState = AdapterView.this.onSaveInstanceState();
            }

            // Data is invalid so we should reset our state
            mOldItemCount = mItemCount;
            mItemCount = 0;
            mSelectedPosition = INVALID_POSITION;
            mSelectedRowId = INVALID_ROW_ID;
            mNextSelectedPosition = INVALID_POSITION;
            mNextSelectedRowId = INVALID_ROW_ID;
            mNeedSync = false;

            checkFocus();
            requestLayout();
        }

这时候，才会调用view的requestLayout()放在主线程中重绘页面。所以notifyDataSetChanged调用的时候是异步调用观察者里的方法，然后push到主线程里去刷新UI。因此
notifyDataSetChanged之后直接对UI进行操作的话，view的UI操作会在绘制队列，才导致问题的关键

那如何解决问题呢

都只要view中有个方法Post(),将任务push到主线程队列中，意思也是将任务添加到消息队列中，保证在UI线程执行。从本质上说，它还是依赖于以Handler、Looper、MessageQueue、Message为基础的异步消息处理机制。大家一定多碰到过，在onCreate()里面去获取一个view的高宽度时候，往往得到的数值是0；也都知道原因，view要经过onMeasure、onLayout和onDraw三个过程，在onCreate()时候明显，view 还没有到onLayout，也就没有宽度。这时候只要进行view.Post(),将计算高宽的方法放置post里面，这样获取的就没问题了。可以看下post的源码：

public boolean post(Runnable action) {    
  final AttachInfo attachInfo = mAttachInfo;
  if (attachInfo != null) {       
  return attachInfo.mHandler.post(action);
  }    
// Assume that post will succeed later       
  ViewRootImpl.getRunQueue().post(action); 
  return true;
}

原理就在view.Post()是将任务添加到这个view绘制结束后的消息队列中，保证了这个view绘制的优先性。受到了这个启发，我认为listview的adapter.ontifyDataSetchanged方法也是这样的道理，亲测将刷新UI操作放在listview.post()中进行操作，这时候打印出来的日志，就是我UI的操作在getview完成之后进行了。为了防止是因为异步的先后行，特的将getview次数增加到10次

图2

最后完美解决问题