Java 如何从对话框中的日期选择器获取日期

我有一个代码，当你点击一个文本视图时，它会打开一个对话框，其中有一个来自另一个布局的日期选择器，我想在点击save按钮时获取日期，但返回null。下面是我尝试过的代码：

fromDate.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            LayoutInflater factory = LayoutInflater.from(getActivity());
            final View textEntryView = factory.inflate(R.layout.calendar_view, null);
            final AlertDialog.Builder alert = new AlertDialog.Builder(getActivity());
            alert.setTitle("Pick a date")
                    .setView(textEntryView)
                    .setPositiveButton("save",
                            new DialogInterface.OnClickListener() {
                                public void onClick(DialogInterface dialog, int whichButton) {
                                    day = calendar.getDayOfMonth();
                                    month = calendar.getMonth();
                                    year = calendar.getYear();
                                    fromDate.setText(day+"/"+month+"/"+year);
                                }
                            })
                    .setNegativeButton("Cancel",
                            new DialogInterface.OnClickListener() {
                                public void onClick(DialogInterface dialog,
                                                    int whichButton) {
                                }
                            });
            alert.show();

fromDate是文本视图

日、月、年是整数

日历是日期选择器id

有人知道为什么以及如何解决这个问题吗

fromDate.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
        Calendar mcurrentDate = Calendar.getInstance();
        final int mYear = mcurrentDate.get(Calendar.YEAR);
        final int mMonth = mcurrentDate.get(Calendar.MONTH);
        final int mDay = mcurrentDate.get(Calendar.DAY_OF_MONTH);


    DatePickerDialog mDatePicker = new DatePickerDialog(SearchResultClass.this, R.style.MyDialogTheme,
            new DatePickerDialog.OnDateSetListener() {
                public void onDateSet(DatePicker datepicker, int selectedyear, int selectedmonth, int selectedday) {

                    fromDate.setText(selectedyear + "-" + month + "-" + selectedday);   

               }
            }, mYear, mMonth, mDay);
 }
  mDatePicker.setTitle(getString(R.string.datepicker_title2));
  mDatePicker.show();
});

并将此主题添加到styles.xml中

 <style name="MyDialogTheme">
    <item name="colorPrimary">@color/colorPrimary</item>
    <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
    <item name="colorAccent">@color/colorPrimary</item>
</style>

@颜色/原色
@颜色/原色暗
@颜色/原色