listview 列表项中的子项变为空 p.e.当视图回收

Question

按钮视图的可见性应该改变onClick，但是当视图回收时，所述视图出现空。似乎是以下项目可见性的状态与列表中的先前状态不同时。这是相关的Java：

class OpenChavrutaAdapter extends ArrayAdapter<HostSessionData> {


private Context mContext;
private static final String LOG_TAG = 
OpenChavrutaAdapter.class.getSimpleName();
HostSessionData hostSessionItemData;

public OpenChavrutaAdapter(Context context, ArrayList<HostSessionData> 
hostSessionArrayList) {
    super(context, 0, hostSessionArrayList);
    this.mContext = context;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {

    //get data item for position
    final ViewHolder viewHolder;
    View listItemView = convertView;
    Boolean hostListItemView = true;
    String userId = UserDetails.getmUserId();
    final HostSessionData hostLookup = getItem(position);
    final String hostId = hostLookup.getmHostId();


    if (listItemView == null) {
        viewHolder = new ViewHolder();

        //sets correct listItemView based on Caller's Context
        if (hostId.equals(userId) && mContext == mainActivityContext) {
            hostListItemView = true;
            listItemView = 
LayoutInflater.from(
getContext()).inflate(R.layout.hosting_chavrutas_list_item, parent, false); 

} else {
    //adapter called from HostSelect.class
    listItemView = 
LayoutInflater.from(getContext()).inflate(R.layout.open_host_list_item, 
parent, false);
    hostListItemView = false;
}           
//cache the viewHoslder object inside the fresh view
        listItemView.setTag(viewHolder);

    } else {
        //view is already been populated
        hostListItemView = false;
        viewHolder = (ViewHolder) listItemView.getTag();
    }
    final HostSessionData hostSessionDatas = getItem(position);

    if (hostListItemView) {
        String idOfConfirmedUser = hostLookup.getmConfirmed();
        String request1NullCheck = hostLookup.getMchavrutaRequest1();
        String request2NullCheck = hostLookup.getMchavrutaRequest2();
        String request3NullCheck = hostLookup.getMchavrutaRequest3();

下面是尝试 setVisibility 时抛出 NPE 的情况！！似乎仅当“pendingRequest1”按钮线性布局视图处于与填充的前一个视图不同的状态时才会发生。请...一些见解！

        if (request1NullCheck != null && 
hostLookup.getMchavrutaRequest1().length() > 5) {
            String chavrutaRequestIdOne = hostLookup.getMchavrutaRequest1();
            viewHolder.pendingRequest_1.setVisibility(View.VISIBLE);

            if( idOfConfirmedUser.equals(chavrutaRequestIdOne)){  
viewHolder.confirmRequest_1.setBackgroundColor(Color.parseColor("#10ef2e"));
            }
        }else{viewHolder.pendingRequest_1.setVisibility(View.GONE);}
        notifyDataSetChanged();
    }
    return listItemView;
}

Answer 1

问题是您使用了两种不同的布局。当视图被回收时，它有可能是两者中的任何一个，而您不知道会得到哪一个。当 convertView 不为 null 时，会跳过使视图膨胀的代码，因此是 NPE。

好消息是有一种简单的方法可以使用 BaseAdapter 类解决这个问题。

您需要在适配器中覆盖的方法是 getItemViewType 和 getViewTypeCount。

这将帮助您的适配器知道如何将空视图传递给您的 getView 方法以进行回收。