如何更新ExpandableListAdapter之外的ExpandableList项

Question

我有一个ExpandableListAdapter，我正在使用像这样的持有者填充孩子

public View getChildView(final int groupPosition, final int childPosition, boolean isLastChild,
                             View convertView, ViewGroup parent) {

    View v = convertView;

    ChildElement e= new ChildElement();
    e.childPosition = childPosition;

    if (convertView == null) {

        LayoutInflater mInflater = (LayoutInflater) this._context
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);

        convertView = mInflater.inflate(R.layout.main_list_item, null);


        holder = new ViewHolder();
        holder.itemname = (TextView) convertView.findViewById(R.id.image_icon);
        holder.Save_check = (CheckBox) convertView.findViewById(R.id.checkBox);
        ////holder.Save_check.setTag(childPosition);

        holder.Remarks = (EditText) convertView.findViewById(R.id.remarks);
        holder.Qty = (EditText) convertView.findViewById(R.id.qty);

        holder.Date = (EditText) convertView.findViewById(R.id.date);
        ////holder.Date.setTag(childPosition);
        holder.Time = (EditText) convertView.findViewById(R.id.time);
        ////holder.Time.setTag(childPosition);
        holder.itemname = (TextView) convertView.findViewById(R.id.image_icon);

        holder.Date.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {

                int getDPPostion = (Integer) holder.Date.getTag();
                String DP = getChild(groupPosition, childPosition).toString();
                long SP = getChildId(groupPosition, childPosition);
                GP = groupPosition; CP = childPosition;
                showDialog(DATE_DIALOG_ID);
                globalPosition = holder.ref;
            }
        });

        ////convertView.setTag(holder);
    }
    else {
        holder = (ViewHolder) convertView.getTag();
    }

    holder.itemname.setText(getChild(groupPosition, childPosition).toString().toLowerCase());

    return convertView;

}

并且在适配器外面我就像这样的日期列表器

private DatePickerDialog.OnDateSetListener datePickerListener = new DatePickerDialog.OnDateSetListener() {
    // when dialog box is closed, below method will be called.
    public void onDateSet(DatePicker view, int selectedYear,
                          int selectedMonth, int selectedDay) {
        year = selectedYear;
        month = selectedMonth;
        day = selectedDay;
        ///final Calendar c = Calendar.getInstance();
        //final ViewHolder holder;
        //int getPostion = (Integer) view.getTag();
        String date = (month + 1)+"/"+year;
        holder = new ViewHolder();
        holder.Date.setText(date);
   };

和一个对话框

@Override
protected Dialog onCreateDialog(int id) {

    switch (id) {
        case DATE_DIALOG_ID:
            // set date picker as current date
            return new DatePickerDialog(MenuScreen.this, datePickerListener, year, month,
                    day);
        case TIME_DIALOG_ID:
            // set time picker as current time
            return new TimePickerDialog(MenuScreen.this, timePickerListener, hour, minute,
                    false);
    }
    return null;
}

但它给出了空指针异常。